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Feather colours can be produced by the organization of feather microstructure, by pigmentation (mainly due to melanins and carotenoids) or both. The expression of feather colours is usually influenced by testosterone (T) levels through several interacting mechanisms. First, high T levels can negatively affect bird condition and hence the expression of condition‐dependent feather colours (e.g. carotenoid‐based colours). Secondly, high levels of T can slow moult progression, which in turn may result in brighter colours, as moult speed has been shown to be negatively correlated with the expression of feather colours. Thirdly, T can affect the bioavailability of pigments that are involved in feather colours. The effect of experimental manipulation of T levels may therefore influence, either positively or negatively, feather colour expression according to the relative importance of these mechanisms and the type of coloration involved. We experimentally investigated whether plumage coloration is affected by T in yearling Eurasian Blue Tits Cyanistes caeruleus. We implanted 11 males and 11 females with T (T‐birds) at the onset of their post‐juvenile moult and compared the spectral reflectance of their structural ultraviolet (UV)/blue colour (crown feathers and upper lesser wing‐coverts) and carotenoid‐based yellow colour (breast feathers) with those of control birds (C‐birds) that were sham‐implanted (12 males, 16 females). At the implant date, all the right lesser wing‐coverts were plucked in both experimental groups to test the effect of T on feathers grown exclusively during our treatment. After 40 days, the implants were removed. Two weeks before removing the implants, the mean T plasma levels were assayed (average 3.21 ng/mL in T‐birds and 0.33 ng/mL in C‐birds). In T‐birds, body moult progression was suspended, and moult was resumed 33 days after implant removal. Moult duration from implant date, excluding suspension, did not differ between treatments and controls. T had a negative effect on UV coloration only in plumage that started growing exclusively during the treatment (right lesser wing‐coverts), and not in other UV/blue plumage. In contrast, the carotenoid‐based yellow coloration of the breast increased in T‐birds, suggesting an immediate effect of T on the mobilization of carotenoids available for ornamentation. Despite sexual dichromatism, T treatment had similar effects in the two sexes. Our results highlight the importance of considering the effect of T on moult speed and possibly moult suspension in the interpretation of the results of studies based on T manipulation. [ABSTRACT FROM AUTHOR]

In many animal species, including most birds, parental care is performed by both parents, which has important implications for mate choice (good parent hypothesis) and parental investment strategies. Partitioning the variance in measures of parental care into heritable and nonheritable components is important to understand the evolvability of parental investment and its potential role in mate choice. We employed an automated system to monitor provisioning behavior at 817 blue tit nests over 10 years (totaling ~3 million visits). Daily provisioning rates of males and females were moderately repeatable between years (Radj = 0.16 and 0.15, respectively), which was almost entirely explained by additive genetic effects. While this degree of heritability is sufficient for parental investment to respond to selection, we argue that the modest level of repeatability provides limited potential for a "provisioning phenotype" to be used as a criterion in mate choice. Daily visit rates were positively correlated between pair members, but after accounting for shared environmental factors, this relationship became clearly negative, thereby providing support for models of partial compensation. Visit rates also differed substantially between years, and between days within a year. Thus, it is important to account for these variables when comparing the parental investment between individuals. Our results highlight the interplay between genetic, social, and environmental influences on provisioning behavior. [ABSTRACT FROM AUTHOR]

Many factors may affect the diversity and abundance of arthropods in their natural habitats including changes in temperature and humidity. Sometimes, large arthropods remain undetected in unsuspected habitats such as bird nests. Here, we explore the effects of an experiment modifying temperature and relative humidity inside the nest of a troglodyte bird species, the blue tit, Cyanistes caeruleus (L.) (Passeriformes: Paridae), on the abundance of dermestid beetles (Coleoptera: Dermestidae) inhabiting the nests. Ten taxa were identified, six at the species level and four—based on larval stages—at the genus level. Most of the dermestids found in the larval stage were nidicolous dermestids of the genus Anthrenus. Specimens of necrobiont dermestids (those that feed on carcasses in both the larval and adult stages) were found in lower numbers, and almost always in adult stages. The abundance of dermestid larvae found in blue tit nests where the temperature or the humidity were experimentally increased was significantly higher than in the control nests. In addition, adult necrobiont dermestids of genus Dermestes appeared more frequently in those nests where corpses of blue tit nestlings were found. [ABSTRACT FROM AUTHOR]

Urbanisation has been increasing worldwide in recent decades, driving environmental change and exerting novel selective pressures on wildlife. Phenotypic differences between urban and rural individuals have been widely documented in several taxa. However, the extent to which urbanisation impacts mating strategies is less known. Here, we investigated extra‐pair paternity variation in great tits (Parus major) and blue tits (Cyanistes caeruleus) breeding in nestboxes set in a gradient of urbanisation in Warsaw, Poland, over three breeding seasons. Urbanisation was quantified as the amount of light pollution, noise pollution, impervious surface area (ISA) and tree cover within a 100‐m radius around each nestbox. We obtained genotypes for 1213 great tits at 7344 SNP markers and for 1299 blue tits at 9366 SNP markers with a genotyping‐by‐sequencing method, and inferred extra‐pair paternity by computing a genomewide relatedness matrix. We report higher extra‐pair paternity in blue tits breeding in more urbanised areas, for example, with higher light pollution and ISA, and lower tree cover. However, no such trend was found in great tits. Late‐stage survival of individual nestlings in both species was not associated with paternity or urbanisation proxies, thus we were not able to detect fitness benefits or drawbacks of being an extra‐pair offspring in relation to urbanisation. Our results contribute to the growing body of knowledge reporting on the effects of urbanisation on avian ecology and behaviour, and confirm species‐specific and population‐specific patterns of extra‐pair paternity variation. [ABSTRACT FROM AUTHOR]

In many bird species, nestling begging signals play a key role in the interaction between parents and their offspring during development. The information conveyed by begging calls can be disrupted by anthropogenic noise, which is one of the major threats to biodiversity in increasingly urbanized landscapes. Here, we describe the developmental change in acoustic structure of begging calls in nestling Eurasian Blue Tits Cyanistes caeruleus; begging calls are pure‐tone, low‐frequency, soft calls during the first days of development and gradually turn into white‐noise, hiss‐like, powerful calls of broadband frequency. This strong developmental variation highlights the importance of an extended sampling scheme in developmental studies. Furthermore, we pinpoint two phases where begging calls could be most vulnerable to masking by anthropogenic noise. First, during early development, begging calls are very soft and low‐pitched, closer to high‐intensity noise bands of traffic noise. Secondly, around day 11, begging calls show reduced tonality, which implies higher degradation, and relatively low amplitude, which implies reduced signal range. We encourage future research to describe acoustic development of begging calls in other species, to provide a robust foundation that will make noise mitigation policies more effective. [ABSTRACT FROM AUTHOR]

Nestlings normally exhibit a mix of traits that attract parental care, such as postural and vocal begging and carotenoid‐based mouth colouration. These signals are hypothesised to be signs of nestling needs (vocal begging) and quality (mouth colouration). Therefore, we hypothesised that broods, where nestlings beg for less time and display more saturated carotenoid‐based mouth colouration, would have lower nestling mortality. We tested these predictions in two wild blue tit (Cyanistes caeruleus) populations. The breeding success (here defined as the proportion of eggs that produced fledglings) was related to nestling mouth flanges' carotenoid‐based colour saturation. This means that blue tits that raised nestlings with more coloured flanges had a higher within‐population breeding success. Time spent in vocal begging, by contrast, was not associated with breeding success. Hence, our findings reveal that some communication signals displayed by nestlings (carotenoid‐based colouration) predict breeding success, in our study mostly reflecting the proportion of eggs that hatched, while begging intensity does not, probably because the former reflects a better maternal pre‐laying condition and nestling physiological status in the mid‐term. [ABSTRACT FROM AUTHOR]

Most of our knowledge of secondary hole-nesting birds comes from populations breeding in human-provided nestboxes, yet these might differ from natural cavities in a number of parameters, including internal dimensions or microclimate, leading to differences in reproductive ecology. Here, we demonstrate differences in plumage coloration, an important visual signal of individual quality, in Blue Tit (Cyanistes caeruleus) and Great Tit (Parus major) nestlings raised in natural cavities and nestboxes. For this study, we collected feather samples over two breeding seasons and applied reflectance spectrophotometry to calculate color metrics. Carotenoid chroma was higher in nestbox-reared Blue Tits, whereas brightness was higher in nestbox-reared Great Tits (with a similar tendency for Blue Tits). The latter result might be explained by the better nutritional condition of Great Tit nestlings raised in nestboxes. Furthermore, we found no evidence for preference of adults expressing more elaborate plumage towards a specific cavity type in either species. Consequently, we assume that differences in nestling plumage reported here are driven by rearing conditions (nestboxes vs. natural cavities) and not by differences in plumage-based parental quality. Our study adds to the growing body of evidence confirming that anthropogenic environmental modifiers, such as nestboxes, might influence avian physiology and the resulting phenotype. [ABSTRACT FROM AUTHOR]

Nest construction behaviour is widespread among bird species. Causes for the occurrence and great variation in nest characteristics between and within populations are still poorly understood. In this study, we present results concerning nest parameters of Great Tits Parus major and Blue Tits Cyanistes caeruleus over a period of 11 breeding seasons (2012–2022) in two structurally and floristically contrasting study areas in central Poland. The nest depth differed significantly between young (yearlings) and older females (≥ 2 years after hatching) in Blue Tits (but not in Great Tits): younger females tended to build deeper nests. The size and mass of the nest were very variable in both tit species. Blue Tits tended to build deeper and heavier nests than Great Tits. The depth of the nest was positively correlated with nest mass in Great and Blue Tits. In Great Tits, the nest mass (but not nest depth) differed between study sites, with nests being heavier in the forest and the nest depth differed between years; in Blue Tits, there was an interaction between year and study site that affected nest depth. The nest mass (but not depth) from the same nestbox tended to be consistently similar within species in Great Tits and Blue Tits. We argue that the large amount of variation in nest parameters between and within populations of both tit species may be a multi-factorial result of female age, breeding area, breeding season, individual predispositions and the size of the nestbox/natural hole. [ABSTRACT FROM AUTHOR]

Genetic differentiation between populations inhabiting ecologically different habitats might appear because of limited dispersal and gene flow, which may lead to patterns of phenotypic divergence and local adaptation. In this study, we use dispersal, genotypic (24 microsatellite loci) and phenotypic (body size and clutch size) data to analyse patterns of genetic structuring and phenotypic divergence in a blue tit (Cyanistes caeruleus) population inhabiting a continuous and heterogeneous woodland along a valley. The two slopes of the valley differ in their forest formations and environmental conditions. Findings showed that most blue tits reproduced within their natal slope. Accordingly, microsatellite analyses revealed that populations of blue tits established in the two slopes show subtle genetic differentiation. The two genetic populations diverged in clutch size, exceeding the level of differentiation expected based on genetic drift, hence suggesting divergent selection (or other processes promoting divergence) on this life-history trait. Our findings reveal that restricted dispersal and spatial heterogeneity may lead to genetic differentiation among bird populations at a surprisingly small scale. In this respect, it is worth highlighting that such differentiation occurs for an organism with high dispersal capacity and within a continuous woodland. Moreover, we show that small-scale ecological differences, together with limited gene flow, can result in selection favouring different phenotypes even within the same continuum population.

After an overview of the discussion about the existence of intra- and interspecific competition that illustrates the contradictory opinions I conclude that long-term field experiments are needed for firm conclusions. I discuss in some detail the role of two factors that limit population size of secondary cavity nesting birds e.g. territorial behavior and adequate cavities. This is followed by an overview of experimental long-term field studies in Belgium showing that intra- and interspecific competition in a great tit-blue tit system exists. By using nestbox configurations with high densities of nestboxes that differ in the diameter of their entrance hole in replicate study plots it is possible to manipulate the breeding densities of great tit Parus major and blue tit Cyanistes caeruleus independently, thereby varying the intensity of intra- and interspecific competition between these two coexisting species. When blue tit densities are experimentally increased local recruitment of great tits increases, and adult great tit post-breeding dispersal to other study plots decreases, implying that great tits use blue tit density to evaluate habitat quality and that high blue tit density results in heterospecific attraction. The reverse is not true. An experimental increase in great tit density leading to an increase in interspecific competition in a plot where blue tit density was already high leads to a decrease in blue tit nestling mass (illustrating interspecific competition for food), but to a gradual increase in blue tit body size. Both are primarily caused by an increase in the body size of immigrants (caused by intraspecific competition for protected roosting holes) in contrast to the control plot, where neither is observed. I also summarize behavioral, ecological and possible evolutionary effects of sparrowhawks on blue tits after sparrowhawks settled in an isolated study plot halfway through the study: adult survival substantially decreased for both sexes, but more for females that laid large clutches, leading to selection for females that laid a smaller clutch. This led to a change in the reproduction/survival life-history trade-off. Adult winter weights and nestling weights decreased, and the heaviest fledglings were selected against. Furthermore the frequency of polygyny increased. The long-term experiments also document the role of the use of public information and that species that compete can be attracted to sites in which competitor density is high. [ABSTRACT FROM AUTHOR]

The spring dawn and dusk chorus of birds is a widespread phenomenon, yet its origin remains puzzling. We propose that a dawn and dusk chorus will inevitably arise if two criteria are met: (1) females leave their roost later in the morning and go to roost earlier in the evening than their mate, and (2) males sing more when separated from their mate. Previous studies on blue tits (Cyanistes caeruleus) support the first criterion. We here report that males sing at a higher rate whenever they are separated from their mate and that song rate increases with the duration of female absence. These findings can explain the existence of the dawn and dusk chorus in blue tits, and they can explain why the dawn chorus is more pronounced than the dusk chorus, as is typically observed. An exhaustive literature search provides support for both criteria of the 'absent mate' hypothesis in several passerine birds. We found no evidence contradicting the hypothesis. The new hypothesis is not inconsistent with many of the existing hypotheses about dawn singing, but may be a more general explanation for the occurrence of a dawn and dusk chorus. We describe how the 'absent mate' hypothesis leads to testable predictions about daily and seasonal variation in song output. [ABSTRACT FROM AUTHOR]

Extra-pair paternity occurs frequently in socially monogamous birds, but there is substantial variation in extra-pair siring success among males. Several studies have shown that siring success relates to the timing of morning activity, with the earliest active males being more successful, suggesting that early activity is important for acquiring extra-pair copulations. However, these studies are correlational, and it, therefore, remains unclear whether the relationship between timing and extra-pair siring success is causal. An alternative explanation is that successful extra-pair sires tend to be active earlier (e.g. because they are of high quality or in good condition), but that early activity in itself does not increase siring success. We experimentally advanced the emergence time of male blue tits by exposing them to light about half an hour before their natural emergence time. Although males that were exposed to the light treatment emerged from their roost substantially earlier than males that were exposed to a control treatment, light-treated males were not more likely to sire extra-pair offspring. Furthermore, whereas control males showed the expected relation between emergence time and siring success (although not statistically significant), there was no relation between emergence time and extra-pair siring success among light-treated males. Our results suggest that the timing of emergence from the roost is not an important factor underlying extra-pair siring success. [ABSTRACT FROM AUTHOR]

Copulations outside the pair bond are common among socially monogamous birds, but males differ in their extrapair siring success. A large body of research has focused on the ultimate causes and consequences of this variation, but the behavioural mechanisms underlying extrapair siring success remain poorly understood. Previous work showed that male songbirds that start singing earlier at dawn sire more extrapair offspring, suggesting that early morning activity influences the likelihood of obtaining extrapair copulations. However, this pattern may be confounded by male age, because adult males typically start singing earlier in the morning and have higher extrapair success than yearling males. We used data from an automated monitoring system to examine whether males' start and end of daily activity, that is, the time of exiting their roosting cavity in the morning and of entering their roost in the evening, relates to patterns of extrapair paternity in a population of blue tits, Cyanistes caeruleus. In line with previous work, we found that adult males emerged earlier from their roosting nestbox and had higher extrapair siring success than yearling males, whereas paternity loss was unaffected by age class and timing of activity. However, the effect of early emergence on paternity gain was not simply due to age, because earlier emerging males had higher extrapair success even after controlling for variation in age class. Males that ended their activity earlier in the evening were also more likely to sire extrapair offspring, independently of their age class. We discuss potential mechanisms that can explain how the timing of activity influences extrapair mating success and highlight the need for experimental manipulation of the timing of activity to test the causality of the reported relationship. [ABSTRACT FROM AUTHOR]

Many animals make visits outside of their territory during the breeding period, but these are typically infrequent and difficult to observe. As a consequence, comprehensive data on extra-territorial movements at the population-level are scarce and the function of this behavior remains poorly understood. Using an automated nest-box visit tracking system in a wild blue tit population over six breeding seasons, we recorded all extra-territorial nest-box visits (n = 22 137) related to 1195 individual breeding attempts (761 unique individuals). Sixty-two percent of breeders made at least one extra-territorial visit between the onset of nest building and the day of fledging of their offspring, and individuals visited another nest-box on average on 11% of the days during this period. Visit behavior differed clearly between the sexes, with males making over three times as many extra-territorial forays as females. There was a strong overall seasonal decline in visit behavior, but this was sex dependent, with females showing a strong reduction in the number of extra-territorial visits before the onset of egg laying and males showing a strong and sudden reduction on the day their offspring hatched. The likelihood of visiting a particular nest-box declined sharply with the distance to that box, and blue tits almost exclusively visited direct neighbors. Individuals were more likely to have extrapair offspring with an individual whose box they visited, but they were not more likely to disperse to a box they had visited. Thus, our results are inconsistent with the hypothesis that extra-territorial nest-box visits serve to inform dispersal decisions, but suggest that such visits are linked to extrapair mating opportunities. [ABSTRACT FROM AUTHOR]

Hatching asynchrony is a frequent phenomenon in altricial birds and can lead to brood reduction due to sibling competition. There are a number of adaptive hypotheses to explain its occurrence, relating hatching asynchrony to sibling competition and timing of breeding. Incubation prior to clutch completion (early incubation) is the main cause of hatching asynchrony. We used temperature loggers inside the nests of breeding Blue Tits Cyanistes caeruleus to provide a detailed account of female incubation over most of the egg-laying period. We relate this early incubation to the time interval between clutch completion and hatching as well as hatching asynchrony. Our study shows the frequent occurrence of early incubation during the beginning of the laying period, with all females showing more early incubation towards clutch completion. At first, early incubation mostly occurs at night, but as egg laying progresses, it also occurs during the day. However, overall there was more nocturnal than diurnal early incubation. These results were obtained using two different methods for quantifying incubation from temperature profiles, which we compared and cross-validated in this study. Moreover, the amount of early incubation related negatively to the time between clutch completion and first hatching and positively to the extent of hatching asynchrony. While we did not directly investigate the mechanisms driving variation in early incubation, the exceptionally cold March/April period followed by a warm May in our study year may explain the comparatively great amounts of early incubation we observed. We hypothesise that spring temperatures may influence the amount of early incubation, with warmer springs resulting in more early incubation and consequently shorter times from clutch completion until first hatching as well as increased hatching asynchrony. Such a mechanism of adjustment of incubation time and hatching asynchrony may also be important for the adaptation of birds to climate change. [ABSTRACT FROM AUTHOR]

Episodic-like memory in non-human animals represents the behavioral characteristics of human episodic memory—the ability to mentally travel backward in time to "re-live" past experiences. A focus on traditional model species of episodic-like memory may overlook taxa possessing this cognitive ability and consequently its evolution across species. Experiments conducted in the wild have the potential to broaden the scope of episodic-like memory research under the natural conditions in which they evolved. We combine two distinct yet complementary episodic-like memory tasks (the what-where-when memory and incidental encoding paradigms), each targeting a different aspect of human episodic memory, namely the content (what-where-when) and process (incidental encoding), to comprehensively test the memory abilities of wild, free-living, non-caching blue tits (Cyanistes caeruleus) and great tits (Parus major). Automated feeders with custom-built programs allowed for experimental manipulation of spatiotemporal experiences on an individual-level basis. In the what-where-when memory experiment, after learning individualized temporal feeder rules, the birds demonstrated their ability to recall the "what" (food type), "where" (feeder location), and "when" (time since their initial visit of the day) of previous foraging experiences. In the incidental encoding experiment, the birds showed that they were able to encode and recall incidental spatial information regarding previous foraging experiences ("where" test), and juveniles, but not adults, were also able to recall incidentally encoded visual information ("which" test). Consequently, this study presents multiple lines of converging evidence for episodic-like memory in a wild population of generalist foragers, suggesting that episodic-like memory may be more taxonomically widespread than previously assumed. • Wild blue tits and great tits were tested with two episodic-like memory tasks • Computerized feeders manipulated foraging experiences at an individual level • Birds recalled what happened, where, and when, as well as incidental information • Flexible memory systems may enable generalist foragers to exploit patchy resources Davies et al. show evidence for episodic-like memory in wild blue tits and great tits across two different tasks using automated, computerized feeders. These birds can recall what occurred, where, and how long ago. Additionally, they can recall incidentally encoded spatial information, and juveniles, but not adults, can recall incidentally encoded visual information. [ABSTRACT FROM AUTHOR]

The role of the availability of calcium in limiting the breeding success and intensity of eggshell specking in birds remains unclear. Experimentally, we provided blue tits (Cyanistes caeruleus) and great tits (Parus major) in woodlands with underlying calcium-rich limestone or calcium-poor gritstone with calcium during the egg stage or during the entire reproductive period and examined their breeding success and eggshell speckling. When provided with calcium during the egg stage, experimental blue tits laid larger clutch sizes with higher clutch weights than controls, whilst experimental great tits had higher fledging success than controls. When provided with calcium throughout reproduction, there was no effect of treatment on blue tit reproductive success, but experimental great tits had higher hatching success and nestling masses than controls. Additional calcium during the egg stage resulted in experimental great tits, but not blue tits, laying less speckled eggs than controls, and additional calcium throughout reproduction led to experimental blue tits and great tits laying less speckled eggs than controls. The provision of calcium led to both species generally having higher levels of reproductive success and less speckled eggs than controls, irrespective of habitat geology, meaning that calcium availability does determine avian breeding success and eggshell specking. [ABSTRACT FROM AUTHOR]

A considerable number of avian species tend to use bryophytes as nest construction material, but the selectivity of birds in their bryophyte use in nest construction in relation to the availability of particular mosses in individual territories remains largely unexplored. We studied the bryophyte composition of nests in two passerine species: the Blue Tit Cyanistes caeruleus and the Great Tit Parus major in two floristically and structurally contrasting study areas (a mature deciduous forest and an urban parkland). We also studied territories within a radius of 10 m from each occupied nest box, listing all bryophyte species and checking to what extent the moss composition of the nest reflected what was available in the surrounding of the nest box. Both tit species appeared to be selective. The tits used a total of 31 bryophyte species in their nests (27 found in Blue Tits and 19 in Great Tits), whereas 25 additional bryophyte species were found around (10 m radius) the studied nests, but not in the nests. The number of bryophyte species found in individual nests varied in the urban parkland between 1 and 6 and in the forest between 1 and 11. The average number of bryophytes in the nest ranged from 2.31 in Great Tit from the urban parkland to 3.52 in Blue Tit in the urban parkland. Only a few species were used in a higher proportion than 5% of the volume within a single nest and, considering only these most abundant bryophyte species, the mean number ranged from 1.28 in Great Tits in the forest to 1.57 in Blue Tits in the urban parkland. Our study also showed that, in general, the bryophyte species composition of the nests seems to be affected by the site (the urban park vs the forest) rather than the parid species (Blue Tit vs Great Tit).

Microbiome constitutes an important axis of individual variation that, together with genes and the environment, influences an individual's physiology and fitness. Microbiomes are dependent not only on an individual's body condition but also on external factors, such as diet or stress levels, and as such can be involved into feedbacks between the external ecological factors and internal physiology. In our study, we used a wild population of blue tits Cyanistes caeruleus to investigate the impact of external habitat composition on the microbiome of adult birds. We hypothesized that – through differences in plant composition, potentially affecting diet complexity – habitat type may impact the diversity and structure of the gut microbiome. Blue tits breeding in dense deciduous forests tended to have more diverse microbiomes and be significantly different in terms of microbiome composition from birds breeding in open, sparsely forested hay meadows. Distinct study plots also tended to differ in a number of parameters describing microbiome diversity. We observed no microbiome differentiation according to individual characteristics such as sex or age. The study emphasizes that external environment is one of the important modulators of microbiome diversity and calls for more such studies in wild animal populations. [ABSTRACT FROM AUTHOR]

Earlier phenology induced by climate change, such as the passerines' breeding time, is observed in many natural populations. Understanding the nature of such changes is key to predict the responses of wild populations to climate change. Genetic changes have been rarely investigated for laying date, though it has been shown to be heritable and under directional selection, suggesting that the trait could evolve. In a Corsican blue tit population, the birds' laying date has significantly advanced over 40 years, and we here determine whether this response is of plastic or evolutionary origin, by comparing the predictions of the breeder's and the Robertson‐Price (STS) equations, to the observed genetic changes. We compare the results obtained for two fitness proxies (fledgling and recruitment success), using models accounting for their zero inflation. Because the trait appears heritable and under directional selection, the breeder's equation predicts that genetic changes could drive a significant part of the phenological change observed. We, however, found that fitness proxies and laying date are not genetically correlated. The STS, therefore, predicts no evolution of the breeding time, predicting correctly the absence of trend in breeding values. Our results also emphasize that when investigating selection on a plastic trait under fluctuating selection, part of the fitness‐trait phenotypic covariance can be due to within individual covariance. In the case of repeated measurements, splitting within and between individual covariance can shift our perspective on the actual intensity of selection over multiple selection episodes, shedding light on the potential for the trait to evolve. [ABSTRACT FROM AUTHOR]

Territorial behaviour arises as a strategy of ensuring individuals' access to a variety of potentially limiting resources. While aggressiveness is a well-studied widespread trait across taxa, the mechanisms that allow for a range of aggressive phenotypes to coexist in the wild remains unclear. In this study, we analyse environmental, social and intrinsic variables that can modulate the expression of different strategies of male–male aggressiveness. Furthermore, through network analysis we explore the role of this trait in the establishment of territories during the breeding season as the intensity of different aggressiveness strategies may limit or grant access to resources. Simulating territorial intrusions during the early incubation period, we assessed the aggressiveness of breeding male blue tits (Cyanistes caeruleus). We defined three types of conspecific aggressiveness (nonconfrontational intimidating, nonconfrontational cautious and confrontational) and analysed the effect of habitat structure, territory quality, presence of other breeding species and male condition on the type and intensity of the aggressive display. The results obtained suggest that yearling males rely on intimidating behaviour more than older males, that perform more cautious displays. Furthermore, smaller and heavier males opted for confrontational strategies. The density and nature of neighbours, as well as the territory quality and the habitat structure, also conditioned the intensity and type of display. Surprisingly, the network analysis revealed that the intensity of male–male aggressive displays did not condition the establishment of breeding territories. Our results suggest that aggressiveness is a context-specific trait shaped by a complex array of environmental and intrinsic parameters. [ABSTRACT FROM AUTHOR]

Mycoplasma spp. are important pathogens in poultry and cause high economic losses for poultry industry worldwide. In other bird species (e.g. white storks, birds of prey, and several waterfowl species), Mycoplasma spp. are regularly found in healthy individuals, hence, considered apathogenic or part of the microbiota of the upper respiratory tract. However, as Mycoplasma spp. are absent in healthy individuals of some wild bird species, they might play a role as respiratory pathogen in these bird species, e.g. Mycoplasma gallisepticum in house finches. The knowledge on the occurrence of Mycoplasma spp. in wild birds is limited. To evaluate the relevance of Mycoplasma spp. in free-ranging nightingales and tits, 172 wild caught birds were screened for the presence of mycoplasmas. The birds were sampled via choanal swabs and examined via molecular methods (n = 172) and, when possible, via culture (n = 142). The Mycoplasma sp. was determined by sequencing the 16S rRNA gene and 16S-23S Intergenic Transcribed Spacer Region. All birds were tested negative for mycoplasmas via PCR and/or mycoplasmal culture. Hence, free-ranging nightingales and tits do not show any mycoplasma in their microbial flora of the respiratory tract. Therefore, these songbird species may suffer from clinical mycoplasmosis when being infected. We hypothesize that birds relying on their vocal ability for reproduction have excluded mycoplasmas from their respiratory flora compared to other bird species. [ABSTRACT FROM AUTHOR]

Abstract Understanding the ecology and evolution of parasites is contingent on identifying the selection pressures they face across their infection landscape. Such a task is made challenging by the fact that these pressures will likely vary across time and space, as a result of seasonal and geographical differences in host susceptibility or transmission opportunities. Avian haemosporidian blood parasites are capable of infecting multiple co‐occurring hosts within their ranges, yet whether their distribution across time and space varies similarly in their different host species remains unclear. Here, we applied a new PCR method to detect avian haemosporidia (genera Haemoproteus, Leucocytozoon, and Plasmodium) and to determine parasite prevalence in two closely related and co‐occurring host species, blue tits (Cyanistes caeruleus, N = 529) and great tits (Parus major, N = 443). Our samples were collected between autumn and spring, along an elevational gradient in the French Pyrenees and over a three‐year period. Most parasites were found to infect both host species, and while these generalist parasites displayed similar elevational patterns of prevalence in the two host species, this was not always the case for seasonal prevalence patterns. For example, Leucocytozoon group A parasites showed inverse seasonal prevalence when comparing between the two host species, being highest in winter and spring in blue tits but higher in autumn in great tits. While Plasmodium relictum prevalence was overall lower in spring relative to winter or autumn in both species, spring prevalence was also lower in blue tits than in great tits. Together, these results reveal how generalist parasites can exhibit host‐specific epidemiology, which is likely to complicate predictions of host–parasite co‐evolution.

Conserving species relies upon acquiring an understanding of their use of habitat, yet our understanding of the use of habitat by co-existing species of different guilds at microgeographic scales remains poor. In particular, the use of habitat by woodland species is of conservation concern because of widespread declines in woodland biodiversity. Woodland bird declines have been ascribed, in part, to high deer densities because their browsing reduces the availability of nesting sites and food. We quantify the microgeographic use of habitat by Roe Deer (Capreolus capreolus) and of Great Tits (Parus major), Blue Tits (Cyanistes caeruleus) and Pied Flycatchers (Ficedula hypoleuca) in a heterogenous woodland landscape. We examined the use of habitat at microgeographic scales by the deer and the three bird species in relation to whether the local habitat was flat or wet or had a path, fence or wall within a 25-m radius of 206 randomly selected locations. We first examined if the occupancy rates of nestboxes in those locations were correlated with the number of Roe Deer lays and second, examined if the use of habitat by the Roe Deer and the bird species were associated with each of the habitat features that we quantified. We begin by showing that the use of habitat by Roe Deer is incongruent with the use of habitat by Great Tits, Blue Tits and Pied Flycatchers during the breeding season. Also, whilst all three bird species showed no, or weak, habitat preferences, the Roe Deer preferred daytime lay sites that were in flat areas of wet woodland close to paths, whilst there were no significant effects of the presence of fences and walls. These findings show that the Roe Deer and the three bird species differ in the use of habitat within a heterogenous woodland landscape, meaning that their use of habitat did not overlap at microgeographic scales. Meanwhile, the deer showed preferences for flat areas of wet woodland, whilst none of the bird species exhibited such preferences, and we discuss the implications of our findings for the management of woodlands.

Acoustic and visual signals are well known to play important roles in social communication in birds. Growing evidence suggests that many bird species, including species of songbirds, additionally have a well-developed sense of smell. However, we are still at the beginning of understanding the potential importance of chemical communication in the social lives of birds, for example in mate choice. The secretion of the preen gland may be an important contributor to the chemical phenotype of birds. Here, we report on a first characterisation of the chemical composition of the preen gland secretion of the Blue Tit (Cyanistes caeruleus), a common songbird which is an often used model species in animal behaviour and ecology, in particular also in studies of sexual selection and (extra-pair) mate choice. We found sex differences in the composition of the preen gland secretion in breeding Blue Tits. Females further tended to have a larger number of putative compounds in their secretions compared to males. We briefly discuss the possible implications of these findings and speculate that the chemical composition of the preen gland secretion may be a sexually selected trait in Blue Tits. Our preliminary findings warrant follow-up research into the patterns of within- and among individual variation in the chemical composition of the preen gland secretion as well as the identification of the main chemical compounds involved. [ABSTRACT FROM AUTHOR]

A considerable number of avian species tend to use bryophytes as nest construction material, but the selectivity of birds in their bryophyte use in nest construction in relation to the availability of particular mosses in individual territories remains largely unexplored. We studied the bryophyte composition of nests in two passerine species: the Blue Tit Cyanistes caeruleus and the Great Tit Parus major in two floristically and structurally contrasting study areas (a mature deciduous forest and an urban parkland). We also studied territories within a radius of 10 m from each occupied nest box, listing all bryophyte species and checking to what extent the moss composition of the nest reflected what was available in the surrounding of the nest box. Both tit species appeared to be selective. The tits used a total of 31 bryophyte species in their nests (27 found in Blue Tits and 19 in Great Tits), whereas 25 additional bryophyte species were found around (10 m radius) the studied nests, but not in the nests. The number of bryophyte species found in individual nests varied in the urban parkland between 1 and 6 and in the forest between 1 and 11. The average number of bryophytes in the nest ranged from 2.31 in Great Tit from the urban parkland to 3.52 in Blue Tit in the urban parkland. Only a few species were used in a higher proportion than 5% of the volume within a single nest and, considering only these most abundant bryophyte species, the mean number ranged from 1.28 in Great Tits in the forest to 1.57 in Blue Tits in the urban parkland. Our study also showed that, in general, the bryophyte species composition of the nests seems to be affected by the site (the urban park vs the forest) rather than the parid species (Blue Tit vs Great Tit). [ABSTRACT FROM AUTHOR]

Leaving the nest is a key transition in the life of altricial birds, whereby fledging decisions should depend on multiple factors, including the risk of predation. High postfledging predation risk may favour fledging at a more advanced stage of development, if more developed fledglings are better at escaping predation, or together with others. While comparative studies have highlighted the role of predation risk for between-species variation in the timing of fledging, drivers of within-species variation in fledging behaviour remain largely unknown. We presented owl models near blue tit, Cyanistes caeruleus, nests during the first half of the day throughout the fledging period to simulate an increased risk of postfledging predation. Using an automated monitoring system, we then recorded the precise fledging times of 595 nestlings from 105 nests (52 predator-treated, 53 control nests). Contrary to our predictions, the predator presentations did not affect the age at which nestlings fledged, the time of day of fledging or other aspects of fledging behaviour. The tendency to fledge together with siblings was affected, but the effect was in the opposite direction to that expected, with nestlings exposed to the predator treatment being more likely to fledge alone. Parents visited predator-treated nests less often, but this effect diminished over the course of the morning. We suggest several explanations for why the behavioural responses to the predation risk manipulation were generally limited. • In altricial birds, the duration of the nestling period varies substantially. • Time of fledging may depend on relative risk of pre- and postfledging predation. • We presented owl models near blue tit nests to simulate increased predation risk. • The treatment did not affect the age or time of day at which nestlings fledged. • The treatment did affect other aspects of fledging and parental behaviour. [ABSTRACT FROM AUTHOR]

Las cajas-nido constituyen una herramienta muy útil para el desarrollo de diversos estudios en aves, pues permiten el diseño experimental y la comparación de muestras sometidas a distintos tratamientos o localizaciones. Igualmente, son útiles para la conservación de aves en hábitats con muy poco arbolado maduro, como ocurre en muchos parques urbanos. Con el fin de impulsar la biodiversidad en el ámbito urbano, la Sociedad de Ciencias Aranzadi inició en 2018 el proyecto Hirilife, entre cuyas acciones estaba la colocación de cajas nido para aves paseriformes insectívoras en arbolado en calles, paseos y parques urbanos de la ciudad de San Sebastián. Este artículo se presenta con el fin de (1) determinar la tasa de ocupación de estas cajas-nido; y (2) explorar los factores que condicionan esta ocupación, con el fin de establecer, a futuro, una guía de buenas prácticas orientada a optimizar su colocación. A lo largo de la primavera de 2020 se revisaron 202 cajas en total, de las 220 que se habían colocado. Se ocuparon un total de 91 (45%). La tasa de ocupación, no obstante, varió significativamente entre zonas. Las cajas en las que se pudo determinar la especie fueron ocupadas por dos especies de páridos: el herrerillo común Cyanistes caeruleus y el carbonero común Parus major. Se observa una dominancia del carbonero común (31 versus 10 cajas; el resto se corresponde con las que no se pudo determinar la especie). Según los modelos las cajas son ocupadas con más probabilidad cuando se sitúan en entornos con una gran cantidad de hábitat natural. Abstract: Nest-boxes constitute a very useful tool in ornithological studies, since they allow the development of experimental designs and permit comparisons to be made between samples subjected to different treatments and locations. Moreover, they are also useful for bird conservation in habitats with low amounts of mature trees, as occurs in many urban parks. The ‘Hirilife’ project, developed by the Aranzadi Sciences Society, was launched in 2018 with the aim of promoting biodiversity in urban areas. This project included actions oriented to install and monitor nest-boxes for insectivorous passerine birds in the streets, promenades and parks of the city of San Sebastian. As a result, in this article we (1) describe the nest-box occupation rate, and (2) examine those factors that determine this rate, with the aim of drawing up a good practice guide to optimize the installation of new nestboxes in the future. During the spring of 2020, 202 nest-boxes were surveyed. Overall, 91 were occupied (45%). The occupation rate, however, varied substantially between zones. The nest-boxes were occupied by two species: the blue tit Cyanistes caeruleus and the great tit Parus major. The latter species was dominant (31 versus 10 nest-boxes). According to the model of nest-box, these were more likely to be occupied when they were situated in areas close to/surrounded by a high amount of natural habitat.

Aposematic organisms warn predators of their unprofitability using a combination of defenses, including visual warning signals, startling sounds, noxious odors, or aversive tastes. Using multiple lines of defense can help prey avoid predators by stimulating multiple senses and/or by acting at different stages of predation. We tested the efficacy of three lines of defense (color, smell, taste) during the predation sequence of aposematic wood tiger moths (Arctia plantaginis) using blue tit (Cyanistes caeruleus) predators. Moths with two hindwing phenotypes (genotypes: WW/Wy = white, yy = yellow) were manipulated to have defense fluid with aversive smell (methoxypyrazines), body tissues with aversive taste (pyrrolizidine alkaloids) or both. In early predation stages, moth color and smell had additive effects on bird approach latency and dropping the prey, with the strongest effect for moths of the white morph with defense fluids. Pyrrolizidine alkaloid sequestration was detrimental in early attack stages, suggesting a trade-off between pyrrolizidine alkaloid sequestration and investment in other defenses. In addition, pyrrolizidine alkaloid taste alone did not deter bird predators. Birds could only effectively discriminate toxic moths from non-toxic moths when neck fluids containing methoxypyrazines were present, at which point they abandoned attack at the consumption stage. As a result, moths of the white morph with an aversive methoxypyrazine smell and moths in the treatment with both chemical defenses had the greatest chance of survival. We suggest that methoxypyrazines act as context setting signals for warning colors and as attention alerting or “go-slow” signals for distasteful toxins, thereby mediating the relationship between warning signal and toxicity. Furthermore, we found that moths that were heterozygous for hindwing coloration had more effective defense fluids compared to other genotypes in terms of delaying approach and reducing the latency to drop the moth, suggesting a genetic link between coloration and defense that could help to explain the color polymorphism. Conclusively, these results indicate that color, smell, and taste constitute a multimodal warning signal that impedes predator attack and improves prey survival. This work highlights the importance of understanding the separate roles of color, smell and taste through the predation sequence and also within-species variation in chemical defenses.

In many socially monogamous bird species, females produce offspring sired by males other than their social partner. A large body of research has aimed to elucidate the evolutionary causes and consequences of such extra‐pair paternity, but relatively little is known about the underlying behaviour. The number of sperm on the egg's perivitelline layers (PVL) is related to recent copulation activity and may thus give some insight into the female's mating behaviour. We used a simple technique that allowed us to remove embryonic cells from the blastoderm for DNA extraction whilst keeping the PVL intact for sperm counts. Using 243 eggs from 99 blue tits Cyanistes caeruleus, we show that PVL sperm numbers were repeatable within clutches (r = 0.40 [95% CI: 0.25–0.53]). However, neither overall sperm numbers nor changes in sperm numbers across the laying sequence differed between clutches that contained extra‐pair sired eggs and those that did not. Our results, therefore, provide no evidence that females with and without extra‐pair young differ in their copulation activity. [ABSTRACT FROM AUTHOR]

The materials that birds use to build their nests have a profound effect on nest quality and consequently on the builder's reproductive success. Given that the common method to quantify nest materials by dismantling nests takes time and limits study species, we developed a non‐destructive and much quicker method for quantifying nest materials using nest photographs. Using our photographic method, the proportions of the main materials in 45 Blue Tit Cyanistes caeruleus and 20 Dartford Warbler Sylvia undata nests, including grass, heather and moss, matched those found by dismantling the nests, but the proportions of rarer animal‐derived materials differed between the two methods. Provided that there is an initial calibration with the dismantling method, the photographic method offers two key advantages: a reduction in the time it takes to quantify the major components of nests, and application to previously inaccessible data, including museum collections. Together, these advantages encourage further study of nesting materials and enable a better understanding of avian nest diversification. [ABSTRACT FROM AUTHOR]

Social cues can improve estimates of foraging site predation risk beyond that of individual assessment alone, as they provide current, accurate information, and the importance of these cues may be greater in higher risk environments. We assessed the foraging behaviour of mixed‐species flocks of tits in both high‐risk, open environments and lower risk, closed environments in winter by measuring foraging intensity on standardized feeders. Responses to acoustic playback of bird calls differed in open and closed sites, suggesting that reliance on social cues is dependent on the habitat. Audible social cues from known species, and to a lesser extent unfamiliar calls at a similar pitch, were most important in evaluating safety in open, riskier sites, with more seeds eaten when playback was present at open‐site feeders compared with silent controls, but no difference was observed between treatments in closed environments. [ABSTRACT FROM AUTHOR]

From April to June 2020, during a period of increased mortality of tits (Cyanistes caeruleus, Parus major, Periparus ater) in Germany, 84 tits were pathomorphologically and microbiologically investigated in governmental veterinary investigation offices in North Rhine-Westphalia using a broad diagnostic panel. Although Suttonella ornithocola was the most frequently detected bacterium (36/84), other pathogens, in particular Chlamydia psittaci (12/65), were also identified, sometimes as concurrent infection (3/36). In conclusion, S. ornithocola-infection seems to play an important role within the epidemic tit mortality in North Rhine-Westphalia, Germany in 2020. However, these cases may be combined with sporadic and/or endemic causes of mortality (e.g., other pathogens, trauma, or predation) for these species. [ABSTRACT FROM AUTHOR]

Most birds are socially monogamous with both parents providing offspring care, but sometimes individuals are observed to provision at a nest that is not their own. One possible explanation for this behaviour is that it is a fitness maximising strategy by males who have copulated with the female and hence potentially sired extra‐pair offspring in the focal nest. Over a period of 8 years and among a total of 854 nests, we observed 12 blue tits Cyanistes caeruleus that provisioned at a nest that was not their own. In all cases, the provisioning individual was a male who had no nestlings of its own, either because its breeding attempt had failed (n = 5), because it was a non‐breeder (n = 6) or because the eggs in its own nest had not hatched yet (n = 1). Only two out of the 12 males had sired offspring in the nest where they helped, but most others had had interactions with the female during her presumed fertile period or were close neighbours, and hence may have performed extra‐pair copulations that did not result in fertilisations. In 10 out of the 12 cases, the resident male was not present anymore, either because it had disappeared (n = 7) or because it was socially polygynous and provided care at its other, primary nest (n = 3). Our findings suggest that females can benefit from extra‐pair copulations by obtaining help in raising their brood when they do not receive help from their social mate. Extra‐pair copulations could thus function as an 'insurance' strategy with a relatively small cost. [ABSTRACT FROM AUTHOR]

Vision is a vital attribute to foraging, navigation, mate selection and social signalling in animals, which often have a very different colour perception in comparison to humans. For understanding how animal colour perception works, vision models provide the smallest colour difference that animals of a given species are assumed to detect. To determine the just-noticeable-difference, or JND, vision models use Weber fractions that set discrimination thresholds of a stimulus compared to its background. However, although vision models are widely used, they rely on assumptions of Weber fractions since the exact fractions are unknown for most species. Here, we test; i) which Weber fractions in long-, middle- and shortwave (i.e. L, M, S) colour channels best describe the blue tit (Cyanistes caeruleus) colour discrimination, ii) how changes in hue of saturated colours and iii) chromatic background noise impair search behaviour in blue tits. We show that the behaviourally verified Weber fractions on achromatic backgrounds were L: 0.05, M: 0.03 and S: 0.03, indicating a high colour sensitivity. In contrast, on saturated chromatic backgrounds, the correct Weber fractions were considerably higher for L: 0.20, M: 0.17 and S: 0.15, indicating a less detailed colour perception. Chromatic complexity of backgrounds affected the longwave channel, while middle- and shortwave channels were mostly unaffected. We caution that using a vision model whereby colour discrimination is determined in achromatic viewing conditions, as they often are, can lead to misleading interpretations of biological interactions in natural - colourful - environments. [ABSTRACT FROM AUTHOR]

Parasites exert a strong selection pressure on their hosts as manifested in behavioural antiparasite traits to reduce negative impacts on fitness. The numerous nest‐dwelling ecto‐parasites residing in avian nests make altricial birds excellent model‐systems for investigating the relationship between parasites and their hosts. Here, we experimentally increased natural levels of hen fleas Ceratophyllus gallinae in blue tit Cyanistes caeruleus nests during incubation, and tested their effects on parental incubation behaviours and reproductive performance. Our experimental addition of fleas resulted in an increase in feeding effort of males to incubating females. Frequency of male feedings was also positively related to clutch size. These results suggest that males increase their reproductive effort in flea manipulated and large broods. This will, at least partly, compensate female costs in nests with high ecto‐parasite density and many nestlings. Furthermore, nestling mass at day six in experimental nests decreased with brood size, which was not the case in nests with a natural level of fleas. In line with male incubation feeding, parents may try to compensate for the costs inflicted by the fleas but can only partly compensate when brood size is large.

Climate warming affects trophic interactions in ecosystems, which has been documented for relationships between plants, herbivores and predators, including their relative densities, varying between taxons/guilds. In the case of leaf-eating caterpillars feeding in tree canopies the use of caterpillar frass fall as an indicator of density and production has proven to be advisable. Here we analysed annual variation in the peak mass of caterpillar frass fall and the timing of the occurrence of the peak mass in a deciduous forest site as compared with an urban park site during 16 consecutive years (2003–2018) in central Poland. Variation in the mean date of the frass fall peak mass among years was extensive, up to 20 days, with the sequence of the occurrence in the study habitats differing between years. There was a significant advance in the date of the peak frass fall in the course of the study years. There was also great variation in the peak frass fall mass between years and study sites. The tree canopy caterpillars were consistently less numerous in the urban park than in the deciduous forest. Cyclical outbreaks of caterpillars, whose signs were visible in the data set, possibly tended to decline. A decline in the annual peak mass of frass fall over the study period was found. No evidence for synchronization of the timing of blue tit Cyanistes caeruleus breeding with the timing of peak caterpillar abundance was found. The annual average peak mass of caterpillar frass fall proved to be a significant predictor of the annual mean number of fledgling blue tits per brood. Both the consistent pattern of spatio-temporal variation in the timing of peak availability and the abundance of tree-canopy caterpillars and the effects of caterpillar abundance on an insectivorous bird confirm that frass fall is a useful indicator of tri-trophic links to birds that prey upon caterpillars.

In altricial birds, leaving the nest is a key life history transition associated with a high risk of mortality. Studies of numerous species have shown that young typically fledge early in the day, and it is often asserted that early fledging is important for survival; however, evidence for this hypothesis is limited. We used an automated monitoring system to obtain precise fledging times of 1582 young blue tits, Cyanistes caeruleus , from 230 nests. As expected, nestlings primarily fledged early in the day (84% fledged before midday). However, we found no evidence that early fledging was associated with higher postfledging survival (i.e. recorded the following autumn or later). We propose two alternative explanations for the morning peak in fledging. Hypothesis 1 is that some offspring reach a developmental threshold for fledging overnight and leave the nest early the next day. This is supported by our observation that offspring that fledged early in the day tended to be more developed than those that fledged later in the day, that is, they were older and had a high body mass (measured at 14 days of age) for their fledging age. Hypothesis 2 is that the timing of fledging is related to parental provisioning behaviour. Our results do not support this hypothesis. Parents reduced their nest visit rate over the course of the day, but offspring did not fledge earlier when their parents decreased their visit rate more strongly with time of day. In conclusion, our results do not support the notion that the time of fledging affects survival but suggest a link with nestling development. • In altricial birds, nestlings typically fledge early in the day. • This is often assumed to improve survival, but with little supporting evidence. • We monitored the precise fledging times of 1582 blue tits from 230 nests. • Fledging time was not associated with the probability of local survival. • Instead, fledging time was related to the developmental stage of nestlings. [ABSTRACT FROM AUTHOR]

Classic explanations for polygyny consider habitat, genetic makeup and paternal care, but little attention has been paid to social inertia. We studied facultative social polygyny in a population of Blue Tits Cyanistes caeruleus with a low rate of polygyny (3% of males across 12 years). Occurrence of polygyny was best predicted by social turnover after the disappearance of one or more individuals; habitat quality and individual phenotypic traits were unimportant. Females settled as secondary females with a male and in an area they had previously associated with, indicating a role of familiarity in the formation of polygyny. Females mated to polygynous males received less help feeding nestlings. Reduced paternal care is potentially costly, because these females survived less well than females mated to monogamous males. Reduction in care occurred at both primary and secondary nests although polygynous males rarely divided their care between nests. On any given day during the nestling period, polygynous males were more likely than monogamous males to not feed nestlings at all and – as a consequence – polygynous males provided less care in total than monogamous males. Polygynous males rarely intermingled visits at both nests, suggesting a cost of switching between feeding locations. Both primary and secondary nests had reduced fledging success, even after controlling for variation in habitat quality and paternal care, due to a higher rate of nest failure. Paternity loss was higher at secondary nests, presumably because the secondary female copulated with a previous mate that disappeared. Due to increased paternity loss and reduced fledging success polygynous males did not sire more fledglings than monogamous males in a given season. Thus, the benefit of social polygyny for males seems limited. Social polygyny in our population probably arises as a by-product of widowed females settling as secondary mates with a familiar male or in a familiar area, thereby making the best of a bad situation. [ABSTRACT FROM AUTHOR]

Parasites exert a strong selection pressure on their hosts as manifested in behavioural antiparasite traits to reduce negative impacts on fitness. The numerous nest‐dwelling ecto‐parasites residing in avian nests make altricial birds excellent model‐systems for investigating the relationship between parasites and their hosts. Here, we experimentally increased natural levels of hen fleas Ceratophyllus gallinae in blue tit Cyanistes caeruleus nests during incubation, and tested their effects on parental incubation behaviours and reproductive performance. Our experimental addition of fleas resulted in an increase in feeding effort of males to incubating females. Frequency of male feedings was also positively related to clutch size. These results suggest that males increase their reproductive effort in flea manipulated and large broods. This will, at least partly, compensate female costs in nests with high ecto‐parasite density and many nestlings. Furthermore, nestling mass at day six in experimental nests decreased with brood size, which was not the case in nests with a natural level of fleas. In line with male incubation feeding, parents may try to compensate for the costs inflicted by the fleas but can only partly compensate when brood size is large. [ABSTRACT FROM AUTHOR]

Climate change is associated with advancing phenology of seasonal traits in many taxa, but shifts by higher trophic levels are generally reduced compared with those of lower trophic levels. For example, the eclosion date of caterpillars and the lay date of insectivorous passerine birds have both advanced recently, but the former has done so more than the latter. While the ensuring phenological mismatch between predator and prey is well-documented, our understanding of the origins of this mismatch is more limited. Here we shed light on the interplay between ambient temperature, breeding phenology and reproductive success in a single population of blue tits (Cyanistes caeruleus) nesting over a 1,000 m (∼5°C) elevational gradient in the French Pyrenees. During the 6 years of this study, we found that average breeding phenology varied by 2–9 days among years, but was on average 11 days earlier at low versus high elevation. Despite the delay, breeding at high elevation was associated with lower and more variable temperatures during breeding. Early breeders within a given year generally had larger clutch sizes than late breeders, which led to more offspring fledged as typically found in other studies. However, in three of the 6 years, the probability of producing fledglings was actually lower among early layers. Additionally, birds breeding at high elevations who experience conditions typical of early breeders in other populations had reduced hatching success and were significantly less likely to fledge any young compared with those breeding at lower elevation. Reduced success at high elevation was not obviously driven by higher nest predation, which was exceptionally low, or reduced food availability because high elevation birds laid clutches of comparable size and fledged the same number of offspring of comparable mass as those breeding at low elevation. Our study reveals the capacity for substantial variation in breeding phenology within a population, but that the success of early breeders varies across years and temperature gradients. We suggest that the evolution of phenological advancements by small endotherms might be curtailed by increased probability of experiencing, and failure under, challenging meteorological conditions in late winter or very early spring.

The plastic regulation of internal energy reserves is acknowledged as the main adaptive response to winter conditions of resident small birds in northern latitudes, a strategy that may be altered whenever human‐supplemented food is available. We investigated the effects of supplementary feeding on the energy management strategy of two wild passerine species, the Willow Tit Poecile montanus and Blue Tit Cyanistes caeruleus, wintering in boreal conditions by measuring body mass and the energy cost of living, i.e. basal metabolic rate. Individuals of both species were heavier, larger and exhibited a higher energy cost of living when captured at the feeders than were individuals captured away from feeders. Fed Willow Tits expended more energy in maintenance, although this difference disappeared once mass was accounted for. Conversely, Blue Tits at feeders had higher mass‐adjusted energy cost of living, but only at low ambient temperatures. The results indicate that winter feeding has species‐specific effects on overall energy management strategy and modifies the response to environmental conditions of wintering passerines. [ABSTRACT FROM AUTHOR]

We studied the relationship between temperature and the coexistence of great tit Parus major and blue tit Cyanistes caeruleus, breeding in 75 study plots across Europe and North Africa. We expected an advance in laying date and a reduction in clutch size during warmer springs as a general response to climate warming and a delay in laying date and a reduction in clutch size during warmer winters due to density‐dependent effects. As expected, as spring temperature increases laying date advances and as winter temperature increases clutch size is reduced in both species. Density of great tit affected the relationship between winter temperature and laying date in great and blue tit. Specifically, as density of great tit increased and temperature in winter increased both species started to reproduce later. Density of blue tit affected the relationship between spring temperature and blue and great tit laying date. Thus, both species start to reproduce earlier with increasing spring temperature as density of blue tit increases, which was not an expected outcome, since we expected that increasing spring temperature should advance laying date, while increasing density should delay it cancelling each other out. Climate warming and its interaction with density affects clutch size of great tits but not of blue tits. As predicted, great tit clutch size is reduced more with density of blue tits as temperature in winter increases. The relationship between spring temperature and density on clutch size of great tits depends on whether the increase is in density of great tit or blue tit. Therefore, an increase in temperature negatively affected the coexistence of blue and great tits differently in both species. Thus, blue tit clutch size was unaffected by the interaction effect of density with temperature, while great tit clutch size was affected in multiple ways by these interactions terms. [ABSTRACT FROM AUTHOR]

Evolutionary selection pressures, and species-specific ecology and behavior, promote a great variability in the size and composition of nests. However, it would be expected that phylogenetically close species, with similar ecological needs, breeding at the same time in the same place, would also build similar nests. In contrast with this, previous studies have found differences in nest mass and composition among closely related sympatric species. These differences have been attributed to small differences in body size (smaller species building larger and/or more insulated nests), or to the different ways in which species perceive the environment (e.g. perceived predation risk). In this study, for the first time, we searched for differences between nest mass, composition, and importance of the different functional parts of the nest between Blue (Cyanistes caeruleus), Great (Parus major) and Coal tits (Periparus ater) breeding under the same conditions. We found that smaller species built larger nests and/or include more thermoregulatory materials, probably having greater insulating capacity, which agrees with previous hypotheses. In particular, Blue Tits made greater use of bark, feathers and vegetable fiber, while Great Tits used wild boar hair in greater proportions. In addition, for the first time, we described in detail the nest composition of Coal Tits, which contained large amounts of fluff compared to the other two species. All these results are in line with previous hypothesis linking nest size and composition to the size of the birds, and the existence of species-specific characteristics in the selection of materials for nest building. [ABSTRACT FROM AUTHOR]

Aim: Nest building is widespread among animals. Nests may provide receptacles for eggs, developing offspring and the parents, and protect them from adverse environmental conditions. Nests may also indicate the quality of the territory and its owner and can be considered as an extended phenotype of its builder(s). Nests may, thus, function as a sexual and social signal. Here, we examined ecological and abiotic factors—temperature, nest predation and interspecific information utilization—shaping geographical variation in a specific nest structure—hair and feather cover of eggs—and its function as an extended phenotype before incubation in great (Parus major) and blue tits (Cyanistes caeruleus) across Europe. We also tested whether egg covering is associated with reproductive success of great tits. Location: Fourteen different study sites and 28 populations across Europe. Taxon: Parus major, Cyanistes caeruleus. Methods: We recorded clutch coverage estimates and collected egg covering nest material from the tit nests. We also measured nest specific breeding parameters and phenotypic measurements on adults. We tested whether mean spring temperatures, nest predation rates and flycatcher (Ficedula spp) densities in the study areas explain the large‐scale geographical variation of clutch coverage and reproductive success of tits. Results: The degree of egg coverage of great tits increased with lower mean spring temperature, higher nest predation rate and higher flycatcher density. We did not find egg covering of blue tits to be associated with any of the ecological or abiotic factors. Moreover, egg covering of great tits was not associated with reproductive success in our cross‐sectional data, yet a rigorous assessment of fitness effects would require long‐term data. Main conclusions: Our findings suggest that, in great tits, egg covering may simultaneously provide thermal insulation against cold temperatures during egg‐laying in spring and also represent a counter‐adaptation to reduce information parasitism by flycatchers and nest predation. Hence, geographical variation in interspecific interactions, and consequently in co‐evolutionary processes, may affect the evolution of nest characteristics besides environmental conditions. [ABSTRACT FROM AUTHOR]

Accurately estimating genetic variance components is important for studying evolution in the wild. Empirical work on domesticated and wild outbred populations suggests that dominance genetic variance represents a substantial part of genetic variance, and theoretical work predicts that ignoring dominance can inflate estimates of additive genetic variance. Whether this issue is pervasive in natural systems is unknown, because we lack estimates of dominance variance in wild populations obtained in situ. Here, we estimate dominance and additive genetic variance, maternal variance, and other sources of nongenetic variance in eight traits measured in over 9000 wild nestlings linked through a genetically resolved pedigree. We find that dominance variance, when estimable, does not statistically differ from zero and represents a modest amount (2‐36%) of genetic variance. Simulations show that (1) inferences of all variance components for an average trait are unbiased; (2) the power to detect dominance variance is low; (3) ignoring dominance can mildly inflate additive genetic variance and heritability estimates but such inflation becomes substantial when maternal effects are also ignored. These findings hence suggest that dominance is a small source of phenotypic variance in the wild and highlight the importance of proper model construction for accurately estimating evolutionary potential. [ABSTRACT FROM AUTHOR]