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Hatching asynchrony is understood as a female tactic that shapes family dynamics, but its adjustment mechanisms and adaptiveness remain unclear. Using 32 years of individual data on a Neotropical seabird, the blue-footed booby, Sula nebouxii , we examined variation in hatching asynchrony in relation to sea surface temperature, a proxy for food abundance, and hatching asynchrony's adaptiveness. Under warm sea surface temperature, signalling low food abundance, females extended laying interval, resulting in increased hatching asynchrony. Longer hatching asynchrony was associated with more probable and earlier brood reduction. When brood reduction occurred, longer hatching asynchrony improved females' prospect of breeding the next year but not their timing of laying or production of fledglings and recruits. By extending laying interval, female boobies increase hatching asynchrony to cope with poor food conditions by prompting early brood reduction, thereby reducing reproductive costs and enhancing the probability of breeding the next year. Understanding the temperature sensitivity of plastic reproductive traits is crucial for predicting organisms' responses and resilience to global warming. • Hatching asynchrony (HA) positively correlated with sea surface temperature (SST). • Females rapidly and plastically extended laying intervals in response to warm SST. • Longer hatch intervals increased the probability and earliness of brood reduction. • When brood reduction occurred, HA increased females' next year breeding probability. • HA did not affect females' number of fledglings or recruits produced the next year. [ABSTRACT FROM AUTHOR]

The adaptive value of routinely laying more eggs than can be successfully fledged has intrigued evolutionary biologists for decades. Extra eggs could, for instance, be adaptive as insurance against hatching failures. Moreover, because recent literature demonstrates that sibling cannibalism is frequent in the Eurasian hoopoe (Upupa epops), producing extra offspring that may be cannibalized by older siblings might also be adaptive in birds. Here, directed to explore this possibility in hoopoes, we performed a food supplementation experiment during the laying period and a clutch size manipulation during the hatching stage. We found that females with the food supplement laid on average one more egg than control females and that the addition of a close-to-hatch egg at the end of the hatching period increased the intensity of sibling cannibalism and enhanced fledging success in hoopoe nests. Because none of the extra nestlings from the experimental extra eggs survived until fledging, these results strongly suggest that hoopoes obtain fitness advantages by using temporarily abundant resources to produce additional nestlings that will be cannibalized. These results therefore suppose the first experimental demonstration of the nutritive adaptive function of laying extra eggs in vertebrates with parental care. [ABSTRACT FROM AUTHOR]

In many raptor species, hatching asynchrony results in a size hierarchy among nestlings, which may facilitate brood reduction by means of sibling aggression-induced mortality, especially in times of low food resources. However, few researchers have investigated the relationships among hatching asynchrony, sibling aggression, and nestling feeding rates in raptors. We studied the influence of hatch rank, brood size, and brood age on aggression and feeding rates among Red-shouldered Hawk (Buteo lineatus) nestlings in suburban southwestern Ohio. Despite relatively modest hatching asynchrony, nestling hatch rank (along with brood size and brood age) was an important predictor of the amount of aggression exhibited by individual nestlings. The oldest nestlings exhibited more aggressive behavior than younger nestlings, although they did not consume significantly more food. Aggression rates were higher in broods of three than broods of two nestlings, and aggression decreased from week 2 to week 3, post-hatching. However, hatch rank was not an important determinant of nestling food consumption, and food consumption rates were unrelated to sibling aggression rates. Identifiable prey delivered to nestling Red-shouldered Hawks was diverse and consisted of 28.3% birds, 25.7% mammals, 23.4% amphibians, 16.4% reptiles, and 6.2% invertebrates (by biomass). The relatively high percentage of birds in the diet may reflect food availability in the suburban habitat surrounding the nests. Sibling aggression did not result in direct siblicide in our study, but four nestlings apparently starved. Overall, the oldest Red-shouldered Hawk nestlings in our study established their dominance early but this was not associated with a detectable advantage in terms of food consumption. En muchas especies de aves rapaces, la asincronía en la eclosión da como resultado una jerarquía de tamaño entre los polluelos, lo que puede facilitar la reducción de la nidada por medio de la mortalidad inducida por la agresión entre hermanos, especialmente en épocas de bajos recursos alimenticios. Sin embargo, pocos investigadores han estudiado las relaciones entre la asincronía en la eclosión, la agresión entre hermanos y las tasas de alimentación de los polluelos en las aves rapaces. Estudiamos la influencia del ranking de eclosión, el tamaño de la nidada y la edad de la nidada en la agresión y las tasas de alimentación entre polluelos de Buteo lineatus en los suburbios del suroeste de Ohio. A pesar de una asincronía en la eclosión relativamente moderada, el ranking de eclosión de los polluelos (junto con el tamaño y la edad de la nidada) fue un predictor importante de la cantidad de agresión exhibida por los polluelos individuales. Los polluelos mayores exhibieron un comportamiento más agresivo que los polluelos más jóvenes, aunque no recibieron una cantidad significativamente mayor de alimento. Las tasas de agresión fueron más altas en las nidadas de tres que en las nidadas de dos polluelos, y la agresión disminuyó de la semana dos a la semana tres después de la eclosión. Sin embargo, el ranking de eclosión no fue un determinante importante del consumo de alimentos de los polluelos, y las tasas de consumo de alimentos no estuvieron relacionadas con las tasas de agresión entre hermanos. Las presas identificables entregadas a los polluelos de B. lineatus fueron diversas y consistieron en 28.3% de aves, 25.7% de mamíferos, 23.4% de anfibios, 16.4% de reptiles y 6.2% de invertebrados (por biomasa). El porcentaje relativamente alto de aves en la dieta puede reflejar la disponibilidad de alimentos en el hábitat suburbano que rodea los nidos. La agresión entre hermanos no resultó en un fratricidio directo en nuestro estudio, pero aparentemente cuatro polluelos murieron de hambre. En general, los polluelos más viejos de B. lineatus en nuestro estudio establecieron su dominancia de forma temprana, pero esto no se asoció con una ventaja detectable en términos de consumo de alimentos. [Traducción del equipo editorial] [ABSTRACT FROM AUTHOR]

Birds face strong selective pressures to complete individual nesting attempts as quickly as possible to minimize exposure of nests to predators and weather, maximize renesting potential, and maximize hatching success. As a result, the duration of developmental periods and of overall nest periods are often relatively constant within species. However, birds may sometimes be subject to acute energetic constraints that may preclude them from initiating incubation at the optimal time. We report an extraordinary case of delayed incubation by a female Burrowing Owl (Athene cunicularia) in Oregon, USA, which contrasted sharply with a large sample of nests monitored by motion-activated video cameras in western North America from 2015 to 2018. The focal female delayed incubation by 11 d following clutch completion and subsequently experienced near total hatching failure of her clutch. This observation corroborates previous experimental studies regarding the limits to egg viability and trade-offs between the timing of incubation onset and hatching success. These findings illustrate the acute trade-offs faced by nesting birds, and the cost of delayed incubation that we observed in this owl could help explain the cause of asynchronous hatching. Las aves enfrentan fuertes presiones selectivas para completar sus intentos de anidamiento individuales tan rápido como sea posible, para minimizar la exposición del nido a depredadores y al tiempo, maximizar potencial de reanidamiento y maximizar éxito de eclosión. Como resultado, la duración de los periodos de desarrollo y los periodos de anidamiento generales son a menudo relativamente constantes dentro de las especies. Sin embargo, las aves pueden a veces estar sometidas a limitaciones energéticas agudas que pueden impedirles iniciar la incubación en un tiempo óptimo. Reportamos un caso extraordinario de retraso de incubación de una hembra de búho Athene cunicularia en Oregon, Estados Unidos, que contrastó fuertemente con una muestra grande de nidos monitoreados por video cámaras activadas por movimiento en el oeste de Norteamérica de 2015 a 2018. La hembra focal postergó la incubación 11 días después de la puesta y a continuación experimentó una pérdida casi total de la puesta. Esta observación corrobora estudios experimentales previos acerca del límite de viabilidad de los huevos y la compensación entre el tiempo de incubación y el éxito de eclosión. Estos hallazgos ilustran las fuertes compensaciones que enfrentan las aves ponedoras y el costo del retraso de la incubación que observamos en este búho podría ayudar a explicar la causa de su eclosión asincrónica. Palabras clave: asincronía de eclosión, fallo en eclosión, limitaciones energéticas, tamaño de puesta, viabilidad de huevos. [ABSTRACT FROM AUTHOR]

Resource limitations, either due to environmental conditions or constraints on parental provisioning effort, can drive intense competition among offspring. In communal groups, resource availability may increase if parents receive assistance from other group members; however, if those caregivers also produce young, offspring demand may increase at the same time. It is possible, therefore, that the costs of intrabrood competition in large broods may outweigh the benefits of provisioning from additional caregivers. We tested the relationships between group size, brood size, and provisioning rates in the greater ani (Crotophaga major), a communally nesting cuckoo in which multiple breeding pairs and nonreproductive helpers cooperatively raise a shared brood. Crucially, brood and group size can vary independently in this species, allowing us to test changes in each variable separately. Using video footage of 2255 prey deliveries across 10 nests, we found that an increase in the number of adult caregivers within a group did not sufficiently offset a corresponding increase in the number of dependent young within a brood: prey availability per average nestling decreased with brood size, regardless of group size. In larger broods, last-hatched nestlings received significantly less prey than their broodmates, in part due to greater hatching asynchrony that exacerbated competitive asymmetries and facilitated inequality in food allocation. Our results indicate that last-hatched ani nestlings suffer a "double cost" in large broods: they must compete with more nestmates, and suffer disproportionately from asynchronous hatching. These costs may contribute to increased parent–offspring conflict and may constrain group size in communal breeders. [ABSTRACT FROM AUTHOR]

The repayment hypothesis for offspring sex allocation predicts that breeders in cooperatively breeding groups should overproduce the helping sex, particularly when they lack helpers. In birds that produce sexually size-dimorphic nestlings, sex allocation is further predicted to vary across hatching order to maximize brood survival. This could occur by biasing late-hatched nestlings toward either the energetically inexpensive sex (the intra-brood sharing-out hypothesis) or the more competitive sex (the intra-brood competitive equilibrium hypothesis). Here, we test these hypotheses using data from 553 nestlings in 109 broods of greater anis (Crotophaga major), a cooperatively breeding bird that breeds in groups composed of multiple reproductive pairs and non-breeding helpers. Helpers are predominantly males and increase the reproductive output of breeders; late-hatched nestlings are more vulnerable to starvation. Therefore, the repayment hypothesis predicts that groups without helpers should produce male-biased broods, whereas the intra-brood sharing-out and competitive equilibrium hypotheses predict that either females (the energetically inexpensive sex) or males (the more competitive sex), respectively, should be overproduced at the end of the laying sequence. Contrary to these predictions, population-wide sex ratios did not differ significantly from 50:50, and we found no evidence for facultative sex ratio adjustment within broods by helper presence or by hatch order. These results support a growing consensus that facultative sex allocation is less widespread in birds than once thought, even in cooperatively breeding species with sex-biased helping behavior. Significance statement: Evolutionary biologists have long been interested in offspring sex ratios. Despite a rich body of literature, few clear trends have emerged among the many hypotheses proposed to explain offspring sex biases in birds, and relatively few studies have examined multiple hypotheses or long-term datasets. This study leverages an 11-year dataset on the cooperatively breeding greater ani to address three adaptive hypotheses for offspring sex allocation. However, consistent with recent meta-analyses, we find no support for adaptive sex allocation in line with the predictions of any of the three hypotheses. Facultative adjustment of sex ratios in birds may be more constrained than once thought, even in species in which it is predicted to be adaptive. [ABSTRACT FROM AUTHOR]

Avian reproductive strategies vary widely, and many studies of life-history variation have focused on the incubation and hatching stages of nesting. Birds make proximate decisions regarding reproductive investment during the laying stage, and these decisions likely constrain and tradeoff with other traits and subsequent behaviors. However, we know relatively little about egg-laying stage behaviors given the difficulty of locating and monitoring nest sites from the onset of laying. We used non-invasive continuous video recording to quantify variation in the egg-laying behaviors of burrowing owls (Athene cunicularia) along a 1400-km latitudinal gradient in western North America. Burrowing owls laid eggs disproportionately in the morning hours, and that tendency was strongest among first eggs in a clutch. However, selection appeared to act more strongly on laying intervals (the time between laying of consecutive eggs) than on the diel time of laying, and laying intervals varied widely among and within clutches. Laying intervals declined seasonally and with increasing clutch size but increased with increasing burrow temperature and as a function of laying stage nest attentiveness, which may be a strategy to preserve egg viability. Laying interval was positively correlated with the duration of hatching intervals, suggesting that laying interval duration is one mechanism (along with timing of incubation onset) that generates variation in hatching asynchrony. Our results lend support to two general hypotheses to explain laying schedules; selection favors laying eggs in the morning, but other selective pressures may override that pattern. These conclusions indicate that allocation decisions during laying are an important part of avian life-history strategies which are subject to energetic constraints and tradeoffs with other traits. [ABSTRACT FROM AUTHOR]

Simple Summary: In some species of birds, asynchronous hatching occurs when incubation begins before completion of clutch, resulting in age differences between nestlings. According to the brood reduction hypothesis, which explains the reason for asynchronous hatching, selective survival is permitted only for older nestlings when availability of food is unpredictable. Due to the feature of food distribution, passerines are not likely to practice selective feeding of older nestlings. The essential purpose of this study is to determine whether the brood reduction hypothesis can explain asynchronous hatching in passerines. Infrared cameras were installed inside nest boxes to determine whether great tit (Parus major) parents, which show asynchronous hatching, practices selective feeding of older nestlings. The results of this study showed that great tit parents did not practice selective feeding of older nestlings, highlighting that using the brood reduction hypothesis to explain asynchronous hatching of great tits is not reasonable. Further research is needed in order to explain the adaptiveness of asynchronous hatching. The brood reduction hypothesis, which explains asynchronous hatching in birds, as an adaptation that enables selective survival of older nestlings when availability of food is unpredictable. This study was conducted in order to determine whether the brood reduction hypothesis can explain asynchronous hatching in passerines. Infrared cameras were installed inside nest boxes where great tits (Parus major) were attempting to reproduce in order to determine whether the parents practiced selective feeding of older nestlings. According to the results of the study, no significant difference was observed between the hatching order and the average number of feedings per nestling. In addition, when examining the distribution of food according to hatching order over time, every 30 min, beginning at 9 a.m., selective distribution of food to older nestlings was not observed. In conclusion, use of the brood reduction hypothesis, which supports selective provision of beneficial feeding of older and larger nestlings, to explain the asynchronous hatching of passerines is problematic, thus conduct of future studies focusing on other hypotheses in order to explain the asynchronous hatching of this passerine bird will be necessary. [ABSTRACT FROM AUTHOR]

I examined sex allocation of offspring in northern flickers, Colaptes auratus , a woodpecker with biparental care and sexual size dimorphism. The analysis of data involving 8321 nestlings that were sexed over a span of 16 years revealed no significant bias towards the smaller sex (females) at the population level (all fledglings pooled), counter to the Fisherian hypothesis of equal investment at the end of parental care. At the level of the brood, high-quality parents (older and in better body condition) did not produce more of the expensive sex (males) counter to the Trivers–Willard hypothesis. Brood sex ratios were somewhat repeatable among females but did not vary significantly with year, clutch size or hatching date probably because the main food of flickers, ground-dwelling ants, is relatively invariable. Mother flickers allocated more of the cheaper sex to last-hatched offspring in asynchronous broods, consistent with the 'intrabrood sharing-out hypothesis' but not the 'competitive advantage hypothesis'. The female-bias in last-laid eggs was present in complete broods, which had no nestling mortality, suggesting that primary sex ratio manipulation was involved. However, a small shift in the direction of female-bias occurred as a result of a greater mortality of the more costly (male) nestlings. Female flickers may therefore attempt to minimize maladaptive brood reduction by allocating more of the cheaper sex to positions later in the laying sequence. • Population level offspring sex ratio of northern flickers did not differ from parity. • High-quality parents did not produce more males (the expensive sex). • Flickers allocated more females to last-hatched offspring within asynchronous broods. • Female bias in last-laid eggs was present in broods without nestling mortality. • Sex allocation was consistent with the 'intrabrood sharing-out hypothesis'. [ABSTRACT FROM AUTHOR]

Brood reduction is a frequent outcome in asynchronously hatching birds. In these species, first-hatched nestlings get a disproportionately larger share of the food delivered by parents, often leading the youngest nestling to starve to death soon after hatching. However, we still do not know the proximate mechanisms of such brood reduction. Starvation of the smallest nestling may be the outcome of scramble competition among nestmates (due to brood size hierarchies) or, alternatively, be caused by parents that ignore the persistent begging from their smallest nestling. To determine whether parents or scramble competition among siblings induces brood reduction, we used a cross-fostering approach to experimentally create asynchronous Eurasian blackbird, Turdus merula , broods. Then, we assessed food allocation by parents in two different situations: when sibling competition was allowed and when competition was prevented by physically separating nestlings with wooden barriers placed within the nest. We found that smaller nestlings experimentally introduced into blackbird nests received less food than their larger nestmates regardless of whether scramble competition among nestlings was allowed or not. Males preferentially fed larger nestlings regardless of the competitive context, while females showed no feeding differences. Our results suggest that nestlings may exert limited control over parental food allocation. • The smallest nestling usually starves in asynchronously hatched broods. • This can be due to nestling control, parental control or both. • Smaller nestlings received less food regardless of the competitive situation. • Parents actively ignore the smallest nestling. [ABSTRACT FROM AUTHOR]

Many songbirds begin active incubation after laying their penultimate egg, resulting in synchronous hatching of the clutch except for a last-hatched individual ("runt") that hatches with a size deficit and competitive disadvantage to siblings when begging for food. However, climate change may elevate temperatures and cause environmental incubation as eggs are laid, resulting in asynchronous hatching and larger size hierarchies among siblings. Although previous work demonstrated that asynchronous hatching reduces nestling growth and survival relative to synchrony, the physiological mechanisms underlying these effects are unclear. To test the effects of asynchronous hatching on runt growth, survival, physiology, and compensatory growth-related tradeoffs, we manipulated incubation temperature in nest boxes of European starlings (Sturnus vulgaris) to increase asynchronous hatching and collected nestling morphological measurements and blood samples to assess physiology and development. Independent of heating treatment, runts from asynchronously hatched nests had lower survival than runts from more synchronous nests. Surviving runts from asynchronous nests were smaller and had reduced stress-induced corticosterone concentrations and reduced circulating glucose compared with runts from synchronous nests. Despite persistent size and energy deficits, runts from asynchronous nests did not have significant deficits in immunity or telomere length when compared with runts from synchronous nests, suggesting no trade-off between investment in immune development or telomere maintenance with growth. Overall, these results suggest that increased asynchrony due to climate change could reduce clutch survival for altricial songbirds, especially for the smallest chicks in a clutch, and that the negative effects of asynchrony may be driven by persistent energetic deficits., (© 2024 Wiley Periodicals LLC.)

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]

Hatching asynchrony in birds is considered an adaptation to facilitate brood reduction because under conditions of food scarcity, the smallest nestling usually dies soon after hatching, thereby minimizing parental effort. However, in species with extreme hatching asynchrony, the last hatchlings paradoxically experience a very low probability of survival and death can take so long that it can hardly be considered an adaptation. Here, we propose and experimentally tested a new adaptive hypothesis explaining the brood reduction paradox, namely the “Male Manipulation Hypothesis”. Our hypothesis suggests that by inducing asynchronous hatching, females increase the feeding requirements of the brood, which will induce males to increase provisioning effort. In addition, females may extend the period of male manipulation by feeding the smallest nestling just enough to sustain life. Our study showed that male common blackbirds (Turdus merula) increased their effort (i.e., number of food items per hour) in experimental asynchronous broods compared to synchronous broods, while females reduced their contribution, as predicted by the hypothesis. [ABSTRACT FROM AUTHOR]

Sibling cannibalism is relatively common in nature, but its evolution in birds and certain other vertebrates with extended parental care had been discarded. Here, however, we demonstrate its regular occurrence in two European populations of the Eurasian hoopoe (Upupa epops) and explore possible adaptive and non-adaptive explanations. Results showed that sibling cannibalism was more frequently detected in Spain (51.7%) than in Austria (5.9%). In these two populations, the hoopoes laid similar clutch sizes, resulting in similar fledging production, but hatching failures were more frequent in the northern population. Consequently, having more nestlings condemned to die in the southern population may explain the higher incidence of sibling cannibalism. In accordance with this interpretation, hatching span and failure, but not breeding date, explained the probability of sibling cannibalism in the Spanish hoopoes, while all three variables predicted brood reduction intensity. Furthermore, experimental food supply reduced the probability of sibling cannibalism, but not the intensity of brood reduction. Finally, females allocated fewer resources to the smallest nestlings when they were going to starve, but not necessarily when they were going to be used as food for their siblings. These results suggest that hoopoes produce extra eggs that, in the case of reduced hatching failure and food scarcity, produce nestlings that are used to feed older siblings. These findings provide the first evidence that sibling cannibalism occurs regularly in a bird species, thus expanding our evolutionary understanding of clutch size, hatching asynchrony, parent-offspring conflict, infanticide, and sibling cannibalism in the animal kingdom. [ABSTRACT FROM AUTHOR]

Egg laying is one of the most important phases in a female bird's breeding cycle. Its cost is high because eggs contain all the resources needed for the development of an embryo. Variation in size and quality of eggs can have important long‐term consequences for offspring survival. Hatching asynchrony is known to influence sibling competition in many bird species. Last‐hatched chicks will have a competitive disadvantage throughout the pre‐fledgling period because they are smaller. The aim of this study was to analyze the effect of hatching asynchrony and egg size variation on the growth and fledging success of Magellanic penguin Spheniscus magellanicus chicks after disentangling the effects of parental condition. We simultaneously manipulated egg size dimorphism, hatching asynchrony and parental condition by performing a cross‐fostering experiment, creating broods with controlled egg size dimorphism and hatching asynchrony in a colony of Magellanic penguins located in Isla Quiroga, Santa Cruz, Argentina. We found that hatching asynchrony had a negative effect on last‐hatched chicks, but this disadvantage was mitigated by egg size dimorphism in their favor. Moreover, females in good condition invest more in second than in first chicks, which, added to a greater investment by foster fathers, leading to offspring fledging in good condition. On the contrary, for the first‐hatched chicks, we found that body condition of the biological father was an important factor for their growth. We conclude that raising more than one chick seems to be a decision based on parental condition throughout the breeding season. [ABSTRACT FROM AUTHOR]

Incubation length and hatching asynchrony are integral elements of the evolved reproductive strategies of birds. We examined intra- and interpopulation variation in both traits for Eastern Kingbird (Tyrannus tyrannus) populations from New York (NY), Kansas (KS), and Oregon (OR) and found that both incubation length and hatching asynchrony were not repeatable among females, after controlling for a repeatable trait, clutch size. Instead, incubation length and clutch size were influenced by ambient temperature and precipitation. Incubation length exhibited the same median (15 days) and range (13–17 days) at all sites. Model selection results indicated that incubation periods for the smallest and largest clutches were longer in NY than KS when rain was frequent throughout incubation, in replacement nests, and likely when ambient temperatures were low during egg-laying. Full hatching usually required 2 days (but up to 3), with synchronous hatching associated with small clutch sizes, short incubation periods, frequent rain during the egg-laying period, and low ambient temperatures during the first half of incubation. Nestling starvation was uncommon (5–9% of nestlings monitored) and not associated with greater hatching asynchrony. These results indicate that while clutch size, a repeatable female trait, contributed to variation in incubation length and hatching asynchrony in Eastern Kingbirds, weather was a greater source of variation, especially for incubation length. [ABSTRACT FROM AUTHOR]

Ambient temperature is assumed to be the major cue used by passerines to synchronize their laying and hatching dates to the expected peak of prey availability. While laying eggs, females are still able to fine‐tune their hatching date following increasing or decreasing patterns of ambient temperature, mostly via changes in incubation onset. The onset of incubation behaviour in relation to the laying sequence could have later consequences for the duration of the incubation period and the extent of hatching asynchrony. Clutch size is also known to affect incubation patterns and might therefore condition potential responses to changing temperatures. In this study we assessed the association of ambient temperature and clutch size with the onset of four different incubation behaviours: partial and full nocturnal incubation, and partial and full diurnal incubation. We also evaluated how the onset and duration of each incubation behaviour might predict the duration of diurnal full incubation and the extent of hatching asynchrony. To achieve our aims, we monitored incubation behaviour using temperature data loggers during the egg‐laying period in three Mediterranean Great Tit Parus major populations in three consecutive years. Our results showed that increasing temperatures were related to an advance of diurnal partial incubation, but not its duration or the onset of full incubation behaviour. We did not find any effect of ambient temperatures on nocturnal incubation. However, females lengthened nocturnal partial incubation and delayed the onset of nocturnal full and diurnal partial incubation when laying larger clutches. Longer diurnal incubation before clutch completion was associated with greater hatching asynchrony. Moreover, longer diurnal partial incubation shortened the duration of the full incubation period. In conclusion, increasing ambient temperatures during the egg‐laying period advanced diurnal partial incubation, indirectly shortening the full incubation period and increasing hatching asynchrony. [ABSTRACT FROM AUTHOR]

Variation in life‐history strategies is central to our understanding of population dynamics and how organisms adapt to their environments. Yet we lack consensus regarding the ecological processes that drive variation in traits related to reproduction and survival. For example, we still do not understand the cause of two widespread inter‐ and intraspecific patterns: (1) the ubiquitous positive association between avian clutch size and latitude; and (2) variation in the extent of asynchronous hatching of eggs within a single clutch. Well‐known hypotheses to explain each pattern have largely focused on biotic processes related to food availability and predation risk. However, local adaptation to maintain egg viability could explain both patterns with a single abiotic mechanism. The egg viability hypothesis was initially proposed to explain the cause of asynchronous hatching and suggests that asynchronous hatching results from early incubation onset in response to unfavorable nest microclimatic conditions, which otherwise reduce egg viability. However, allocation of resources to early incubation, prior to clutch completion, may energetically constrain clutch size and help explain the positive association between clutch size and latitude. We measured intraspecific variation in five functionally linked life‐history traits of burrowing owls at five study sites spanning a 1,400‐km latitudinal transect in western North America: clutch size, the timing of incubation onset, the degree of hatching asynchrony, the probability of hatching failure, and nestling survival. We found that most traits varied clinally with latitude, but all the traits were more strongly associated with individual nest microclimates than with latitude, and all varied with nest microclimate in the directions predicted by the egg viability hypothesis. Furthermore, incubation began earlier, hatching asynchrony increased, and clutch size declined across the breeding season. These results suggest that nest microclimate drives an important life‐history trade‐off and that thermal gradients are often sufficient to account for observed biogeographic and seasonal patterns in life‐history strategies. Furthermore, our results reveal a potentially important indirect mechanism by which reproductive success and recruitment could be affected by climate change. [ABSTRACT FROM AUTHOR]

Evolutionary theory suggests that individuals can benefit from deferring the fitness cost of developing under poor conditions to later in life. Although empirical evidence for delayed fitness costs of poor developmental conditions is abundant, individuals that die prematurely have not often been incorporated when estimating fitness, such that age‐specific fitness costs, and therefore the relative importance of delayed fitness costs are actually unknown.We developed a Bayesian statistical framework to estimate age‐specific reproductive values in relation to developmental conditions. We applied it to data obtained from a long‐term longitudinal study of common terns Sterna hirundo, using sibling rank to describe variation in developmental conditions. Common terns have a maximum of three chicks, and later hatching chicks acquire less food, grow more slowly and have a lower fledging probability than their earlier hatched siblings.We estimated fitness costs in adulthood to constitute c. 45% and 70% of the total fitness costs of hatching third and second, respectively, compared to hatching first. This was due to third‐ranked hatchlings experiencing especially high pre‐fledging mortality, while second‐ranked hatchlings had lower reproductive success in adulthood. Both groups had slightly lower adult survival. There was, however, no evidence for sibling rank‐specific rates of senescence.We additionally found years with low fledgling production to be associated with particularly strong pre‐fledging selection on sibling rank, and with increased adult survival to the next breeding season. This suggests that adults reduce parental allocation to reproduction in poor years, which disproportionately impacts low‐ranked offspring.Interpreting these results, we suggest that selection at the level of the individual offspring for delaying fitness costs is counteracted by selection for parental reduction in brood size when resources are limiting. [ABSTRACT FROM AUTHOR]

Like many raptors, Red-shouldered Hawks (Buteo lineatus) exhibit asynchronous hatching, which is thought to be a parental strategy for rearing the maximum number of offspring under conditions of unpredictable prey availability. Current knowledge of incubation patterns and the associated behavioral mechanisms by which asynchronous hatching occurs in raptors is limited, and few detailed quantitative studies are available for any species of Accipitriformes. We investigated the effects of egg-laying intervals and parental behavioral incubation patterns during egg-laying on hatching asynchrony in the Red-shouldered Hawk. We used 24-hr/d digital color/infrared video cameras at 15 Red-shouldered Hawk nests to record egg-laying and hatching, and to quantify the proportion of time parents spent incubating, standing, or absent from the nest. The mean egg-laying interval was 2.9 ± 0.1 d or 69.8 ± 2.2 hr (range = 65.8–74.2 hr), and the mean duration of incubation (from the onset of full incubation to hatch) for last-laid eggs was 33 d (32–34 d, n = 15). Red-shouldered Hawks exhibited partial incubation behavior, with the delay in the onset of full incubation varying by clutch size and among individuals; full incubation generally began with or just after the laying of the penultimate egg of the clutch. The mean interval between the hatching of the first and second egg was 0.56 ± 0.49 d; between the second and third eggs, 1.24 ± 0.71 d (P= 0.0215); and between the third and fourth eggs, 2.42 ± 0.38 d (P < 0.0004). The total amount of time parents at each nest spent incubating during the egg-laying period explained most of the variation in the amount of time between the hatching of their first and last egg (the hatching span; r2 = 0.83, P < 0.0001). Hatching patterns were the result of both the egg-laying intervals and the pattern of partial incubation, which may represent an adaptive mechanism to reduce the hatching interval while maintaining the hatching asynchrony. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Raptor Research is the property of Raptor Research Foundation and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Journal of Raptor Research is the property of Raptor Research Foundation and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Hatching asynchrony in birds occurs when incubation commences prior to clutch completion and results in broods containing different aged and sized nestlings. The brood reduction hypothesis predicts that hatching asynchrony is a strategy that facilitates the partial success of brood, as feeding to old and large nestlings which results in adaptive brood reduction under periods of food shortage. Using stochastic simulation, we tested and specified the outcome of the brood reduction hypothesis with various provisioning variables of birds to investigate under what conditions the brood reduction effectively explains the evolution of hatching asynchrony. Asynchronous hatching can be productive as provisioning by parents is preferentially feeding to old nestlings when food is not sufficiently available, while it did not create substantial differences in the number of nestlings fledged when food was equally allocated to all nestlings. Secondly, in the case where food items are large and fewer deliveries are needed, asynchronous hatching becomes more favoured compared to smaller, more frequently delivered, food items. Also, when parents provision less food than was needed on bad days, hatching asynchrony became favoured even when the bad days occur infrequently. Different life traits of each bird can change the expectation of the brood reduction hypothesis. [ABSTRACT FROM AUTHOR]

In all animals, susceptibility to parasites can differ among individuals. Young, nest-bound birds are exposed to a diversity of nest-dwelling ectoparasites that typically feed on their blood. Within broods, hatching asynchrony creates size hierarchies that result in morphological and physiological variation among nest mates, and susceptibility to parasites also may vary predictably with this size hierarchy. Our objective was to use a broad-spectrum, anti-parasite drug, ivermectin (IVM), to treat individual nestling Tree Swallows (Tachycineta bicolor) and assess how nestling susceptibility to parasites varied both within and among broods. Broods were either assigned to an IVM group, where half of the nestlings in a brood received IVM injections and half received control injections of pure sesame oil, or to a control group, where all nestlings received oil injections. We found that the IVM treatment reduced parasite loads for broods as a whole, thereby benefiting all nestlings in IVM broods and suggesting our treatment resulted in herd immunity. Specifically, nestlings from IVM broods had higher hemoglobin concentrations, regardless of whether they received injections with IVM or oil, and greater fledging success, than nestlings from control broods. On the contrary, IVM treatment did not strongly affect nestling morphology, with only marginal effects on the growth rate of ninth primary feathers, and the effects of the treatment on 2 other morphological traits depending on temporal factors. Variation in size within broods, however, influenced the chance of an individual fledging, which increased with relative size within a brood, but only under lower parasite loads (i.e. IVM broods). By experimentally manipulating nestling susceptibility to parasites, we have demonstrated variation in nestling response to an anti-parasite treatment both within and among broods, and future studies should investigate the underlying mechanism for why certain nestlings along the brood size hierarchy are more susceptible to parasites. [ABSTRACT FROM AUTHOR]

When eggs hatch asynchronously, offspring arising from last-hatched eggs often exhibit a competitive disadvantage compared with their older, larger nestmates. Strong sibling competition might result in a pattern of resource allocation favoring larger nestlings, but active food allocation towards smaller offspring may compensate for the negative effects of asynchronous hatching. We examined patterns of resource allocation by green-rumped parrotlet parents to small and large broods under control and food-supplemented conditions. There was no difference between parents and among brood sizes in visit rate or number of feeds delivered, although females spent marginally more time in the nest than males. Both male and female parents preferentially fed offspring that had a higher begging effort than the remainder of the brood. Mean begging levels did not differ between small and large broods, but smaller offspring begged more than their older nestmates in large broods. Male parents fed small offspring less often in both brood sizes. Female parents fed offspring evenly in small broods, while in large broods they fed smaller offspring more frequently, with the exception of the very last hatched individual. These data suggest male parrotlets exhibit a feeding preference for larger offspring—possibly arising from the outcome of sibling competition—but that females practice active food allocation, particularly in larger brood sizes. These differential patterns of resource allocation between the sexes are consistent with other studies of parrots and may reflect some level of female compensation for the limitations imposed on smaller offspring by hatching asynchrony.

Maternal and environmental effects can have profound effects on offspring performance by generating variation in offspring phenotypes, independent of genetic effects. Within avian broods, differential maternal investment of resources across the laying sequence is thought to be an adaptive strategy to modulate competitive hierarchies induced by hatching asynchrony. In this study, we evaluated the relative importance of maternally derived within-clutch variation and the asymmetric post-hatching environment for growth and survival of common tern (Sterna hirundo) siblings. We experimentally manipulated hatching order, resulting in chicks from last-laid eggs hatching first and vice versa. Although both initial age and size asymmetries were larger within experimental than control broods, the early survival of last-hatched chicks was similar between groups. Initial positive effects of egg size disappeared as siblings approached fledging. Ultimately at fledging, both within-brood growth and cumulative survival patterns were similar between experimental and control broods, suggesting that the effects of systematic variation of egg constituents (e.g., maternally derived yolk hormones) and egg size are too subtle and largely overwhelmed by the effects of hatching asynchrony. Therefore, we conclude that variation in offspring phenotypes is pre-dominantly determined by the social environment experienced post-hatching. Maternal effects may further fine-tune phenotypic variation in response to varying environmental conditions, but this needs to be tested through empirical studies in which multiple maternal effects are measured simultaneously under different environmental conditions. [ABSTRACT FROM AUTHOR]

Selective elimination of excess offspring with poor fitness prospects may occur prenatally (selective abortion) or postnatally (brood reduction). Postnatal reduction is the dominant strategy, presumably because surplus progeny serves as a hedge against environmental and developmental uncertainty. In birds, its main proximate mechanism is asynchronous hatching, generating within-brood competitive asymmetry. Here, clutch-size reduction via last-egg abandonment was investigated in the asynchronous [ABSTRACT FROM AUTHOR]

Abstract The early developmental trajectory is affected by genetic and environmental factors that co‐depend and interact often in a complex way. In order to distinguish their respective roles, we used canaries (Serinus canaria) of different genetic backgrounds (inbred and outbred birds). An artificial size hierarchy was created to provoke within‐nest competition, manipulating postnatal conditions. To this end, inbred birds were weight‐matched with outbred birds into duos, and each nest contained one duo of size‐advantaged, and one duo of size‐disadvantaged inbred and outbred nestlings. Prenatal (maternal) effects were taken into account also, enabling us to study the separate as well as the interactive effects of inbreeding, pre‐ and postnatal conditions on nestling development. We find that postnatal conditions were the most important determinant of early growth, with size‐advantaged nestlings growing faster and obtaining larger size/body mass at fledging in comparison with size‐disadvantaged nestlings. Prenatal conditions were important too, with birds that hatched from eggs that were laid late in the laying order obtaining a larger size at fledging than those hatched from early laid eggs. Inbreeding inhibited growth, but surprisingly this did not depend on (dis)advantageous pre‐ or postnatal conditions. Our findings imply that inbred individuals lose when they are in direct competition with same‐sized outbred individuals regardless of the rearing conditions, and we thus propose that reduced competitiveness is one of the driving forces of inbreeding depression.

The allocation of prey within broods of altricial nestlings may be the result of parents targeting certain offspring preferentially and/or the result of intrabrood competition related to features of nestlings such as body size. We investigated two mechanisms that could influence allocation of food within broods of Mountain Bluebirds (Sialia currucoides) by filming the size and type of prey that parents fed nestlings. According to the gape size constraint hypothesis, the inability of small nestlings to physically swallow large prey items might place junior brood members at a higher risk of starvation than older siblings if parents do not bring enough small items. Consistent with this idea, the frequency of 'testing', placing an item in the gape of a nestling but then withdrawing it, was positively correlated with prey size. The junior nestling of a brood was fed less often when parents brought larger prey items, and nestlings were tested more in the early nestling stage (0-4 days old) when their gapes were smaller than at older stages. Parents of many cavity-nesting species may feed the brood from the nest entrance and the resulting scramble competition among brood members to access the entrance hole also potentially increases the mortality of the least competitive (smallest) nestlings. Although the frequency of feeding from the entrance hole was prevalent during the late nestling stage (≥ 12 days old), feeding from the entrance was not associated with mortality rate in bluebird broods and the mortality of junior nestlings was not linked to the testing of prey. The results confirm that a mechanism of gape size constraint does direct large prey away from junior nestlings, but apparently small-sized prey items were abundant enough during our study so that this did not result in increased mortality of nestlings. [ABSTRACT FROM AUTHOR]

Inbreeding negatively affects various life-history traits, with inbred individuals typically having lower fitness than outbred individuals (= inbreeding depression). Inbreeding depression is often emphasized under environmental stress, but the underlying mechanisms and potential long-lasting consequences of such inbreeding–environment interactions remain poorly understood. Here, we hypothesize that inbreeding–environment interactions that occur early in life have long-term physiological effects, in particular on the adult oxidative balance. We applied a unique experimental design to manipulate early life conditions of inbred and outbred songbirds (Serinus canaria) that allowed us to separate prenatal and postnatal components of early life conditions and their respective importance in inbreeding–environment interactions. We measured a wide variety of markers of oxidative status in adulthood, resulting in a comprehensive account for oxidative balance. Using a Bayesian approach with Markov chain Monte Carlo, we found clear sex-specific effects and we also found only in females small yet significant long-term effects of inbreeding–environment interactions on adult oxidative balance. Postnatal components of early life conditions were most persuasively reflected on adult oxidative balance, with inbred females that experienced disadvantageous postnatal conditions upregulating enzymatic antioxidants in adulthood. Our study provides some evidence that adult oxidative balance can reflect inbreeding–environment interactions early in life, but given the rather small effects that were limited to females, we conclude that oxidative stress might have a limited role as mechanism underlying inbreeding–environment interactions. [ABSTRACT FROM AUTHOR]

Interactions among siblings are finely balanced between rivalry and cooperation, but the factors that tip the balance towards cooperation are incompletely understood. Previous observations of insect species suggest that (i) sibling cooperation is more likely when siblings hatch at the same time, and (ii) this is more common when parents provide little to no care. In this paper, we tested these ideas experimentally with the burying beetle, Nicrophorus vespilloides. Burying beetles convert the body of a small dead vertebrate into an edible nest for their larvae, and provision and guard their young after hatching. In our first experiment, we simulated synchronous or asynchronous hatching by adding larvae at different intervals to the carrion-breeding resource. We found that 'synchronously' hatched broods survived better than 'asynchronously' hatched broods, probably because 'synchronous hatching' generated larger teams of larvae, that together worked more effectively to penetrate the carrion nest and feed upon it. In our second experiment, we measured the synchronicity of hatching in experimental populations that had evolved for 22 generations without any post-hatching care, and control populations that had evolved in parallel with post-hatching care. We found that larvae were more likely to hatch earlier, and at the same time as their broodmates, in the experimental populations that evolved without post-hatching care. We suggest that synchronous hatching enables offspring to help each other when parents are not present to provide care. However, we also suggest that greater levels of cooperation among siblings cannot compensate fully for the loss of parental care. [ABSTRACT FROM AUTHOR]

Female birds can potentially utilise various mechanisms, such as intraclutch egg size variation (ICESV), to modulate offspring quality. If this represents an 'adaptive' strategy allowing females to adjust reproductive effort after laying of the first egg, as information on resources becomes more predictable later in the laying sequence, then we would predict that ICESV would vary with ecological context, e.g. in 'good' versus 'bad' years. Here, in European Starlings ( Sturnus vulgaris), we tested specific predictions that (1) females would lay relatively small last laid eggs in 'poor' years (with early laying and below-average breeding productivity); and (2) that ICESV would be lower in good years (late laying, above-average productivity) so that late-hatched chicks would not be disadvantaged. At the individual level we predicted (3) that repeatability of ICESV would be low if females adjusted this in relation to the environment in different years; and (4) that within-individual variation in ICESV should correlate with the number, and timing, of loss of chicks. Mean ICESV averaged −1.9% (measured as last egg mass relative to mean egg mass), typical of other hole-nesting passerines. However, our results did not support any of our hypotheses: (1) although there was annual variation in ICESV, in all years egg mass decreased through the laying sequence; (2) annual variation in ICESV was not consistent with variation in phenology or breeding productivity among years; (3) marked individual variation in ICESV (last laid egg varied from −20 to +10% relative to mean egg mass within clutches) was repeatable, i.e. individual females did not adjust ICESV in subsequent breeding attempts; and (4) variation in the magnitude of ICESV, alone and combined with hatching asynchrony, did not predict early brood loss. Our study suggests that ICESV is a relatively inflexible component of an individual female's phenotype that might contribute to variation in individual quality. [ABSTRACT FROM AUTHOR]

Hatching asynchrony in birds, which occurs when incubation begins before egg laying is complete, has been a topic of study for many decades. The 'nest failure hypothesis' posits that the distribution of the risk of nest predation across the nesting cycle (from egg laying to the fledging of young) determines the optimal degree of hatching asynchrony. If such risk is higher earlier in the nesting cycle, then much asynchrony (e.g., incubation on the first egg laid) is generally favored. Alternatively, if the risk of nest predation is concentrated later in the cycle, then greater synchrony (a late start to incubation) is favored. Adult mortality during incubation favors greater hatching synchrony. Existing models suggest that optimal hatching asynchrony should depend on the ratio of daily mortality risks prehatching and posthatching, or early and late in the nesting cycle. We show that these classic theoretical expectations depend critically on the assumption of a single nesting attempt per breeding season. Allowing for multiple nesting attempts per season leads to a large range of possible outcomes, often with large deviations from the expectations of single-attempt models. We show further that daily nest survival probabilities across the entire nesting cycle combine to influence optimal hatching asynchrony, not just a subset of values, or a ratio of values, as suggested by earlier theory. In addition, assumptions about the transitions in daily survival values between nesting stages (abrupt vs. gradual changes) are also important determinants of hatching asynchrony. Overall, however, a consideration of multiple nesting attempts does not alter the general expectation that high risk early in the nesting cycle favors hatching asynchrony, and that greater synchrony is favored by high risk later in the cycle or adult mortality during incubation. [ABSTRACT FROM AUTHOR]

Thyroid hormones (THs) - triiodothyronine (T3) and thyroxine (T4) - are essential for embryonic development in vertebrates. All vertebrate embryos are exposed to THs from maternal origin. As maternal TH levels are known to be essential to embryonic development, the natural variation of maternal THs probably represents a pathway of maternal effects that can modify offspring phenotype. However, potential fitness consequences of variation of maternal TH exposure within the normal physiological range and without confounding effects of the mother have never been experimentally investigated. We experimentally manipulated the levels of yolk T3 and T4 within the physiological range in a species in which the embryo develops outside the mother's body, the Rock Pigeon (Columba livia) eggs. Making use of the natural difference of yolk testosterone between the two eggs of pigeon clutches, we were also able to investigate the potential interaction between THs and testosterone. Elevated yolk TH levels enhanced embryonic development and hatching success, and reduced body mass but not tarsus length between day 14 and fledging. The yolk hormones increased plasma T4 concentrations in females but reduced it in males, in line with the effect on metabolic rate at hatching. Plasma concentrations of T3 and testosterone were not significantly affected. The effects of treatment did not differ between eggs with high or low testosterone levels. Our data indicate that natural variation in maternal yolk TH levels affects offspring phenotype and embryonic survival, potentially influencing maternal and chick fitness. [ABSTRACT FROM AUTHOR]

The early developmental trajectory is affected by genetic and environmental factors that co-depend and interact often in a complex way. In order to distinguish their respective roles, we used canaries ( Serinus canaria) of different genetic backgrounds (inbred and outbred birds). An artificial size hierarchy was created to provoke within-nest competition, manipulating postnatal conditions. To this end, inbred birds were weight-matched with outbred birds into duos, and each nest contained one duo of size-advantaged, and one duo of size-disadvantaged inbred and outbred nestlings. Prenatal (maternal) effects were taken into account also, enabling us to study the separate as well as the interactive effects of inbreeding, pre- and postnatal conditions on nestling development. We find that postnatal conditions were the most important determinant of early growth, with size-advantaged nestlings growing faster and obtaining larger size/body mass at fledging in comparison with size-disadvantaged nestlings. Prenatal conditions were important too, with birds that hatched from eggs that were laid late in the laying order obtaining a larger size at fledging than those hatched from early laid eggs. Inbreeding inhibited growth, but surprisingly this did not depend on (dis)advantageous pre- or postnatal conditions. Our findings imply that inbred individuals lose when they are in direct competition with same-sized outbred individuals regardless of the rearing conditions, and we thus propose that reduced competitiveness is one of the driving forces of inbreeding depression. [ABSTRACT FROM AUTHOR]

Size hierarchies are often seen when nestlings hatch asynchronously over a period of days. Shorter hatch periods are common across passerines, however, and while these may also give rise to asymmetries, their effects are rarely considered. Regardless of hatch period, the long-term consequences for later hatched nestlings that survive to fledge is unknown for wild birds. Here we explored the timing of hatch order in a free-living population of hihi nestlings, Notiomystis cincta , and followed any effects in and out of the nest. We found that while hatching time from first- to last-hatched nestlings was often less than 24 h, last-hatched individuals grew more slowly and were lighter and smaller at fledging than older siblings. Last-hatched nestlings were also less likely to fledge. These effects were greater in larger broods. Adult body size is correlated with fledging size in hihi; however, we found no evidence that hatch order affected longevity postfledging, or lifetime reproductive success. We then explored whether carotenoid availability might buffer these stressful rearing conditions (through food supplementation of parents) but found no evidence that increased access to carotenoids for mothers and/or growing nestlings influenced incubation schedules, or the effects of hatching late. Together these results suggest that while even a very short hatch period can influence adult phenotype, hatching asynchrony is not maladaptive for hihi: when last-hatched nestlings survive to fledge they can contribute as much to their mothers' fitness as first-hatched siblings. [ABSTRACT FROM AUTHOR]

Hatching asynchrony is common among bird species. It results from starting incubation before clutch completion and is often accompanied by brood reduction, an adaptive strategy of the parents to selectively starve the younger chicks in times of food scarcity. The different developmental stages of nestlings are expected to affect their competitive abilities, their hunger state and thereby parental food allocation patterns. Here we used the combination of a correlative and an experimental field study to investigate sex-specific feeding patterns in the hoopoe, Upupa epops , in relation to a chick's rank and hunger state. Male parents showed a strong bias in their feeding pattern towards larger chicks, while females' feeding pattern was significantly more even. Only females were responsive to experimentally altered chicks' hunger state, especially so with small chicks at the lower end of the size hierarchy. This differential food allocation pattern mainly resulted from females mostly entering the nestbox to feed the chicks, while males more often delivered prey from the nestbox entrance hole without entering the cavity. Hence, when entering the nestbox, females can exert some parental control over food allocation, whereas competition among chicks to access the entrance hole mainly rules food allocation when the parents feed from the nestbox entrance hole. Similar sex-specific feeding patterns might be widespread in cavity breeders and it remains to be investigated to what extent variation in these male and female feeding patterns can affect nestling mortality and ultimately breeding success. [ABSTRACT FROM AUTHOR]

Avian mothers can potentially alter the phenotypes of their offspring by varying the concentration of steroid hormones in their eggs. We explored variation in androstenedione (A4), testosterone (T), 5α-dihydrotestosterone (DHT), 17β-estradiol (E2), and corticosterone (CORT) in the yolks of 12 free-living great tit Parus major clutches. We analyzed variation and covariation in greater detail than previous studies, using models for variation with laying sequence that take into account variable clutch size and comparing correlations between pairs of hormones at the within- and between-clutch levels. We also investigated relationships between hormone levels and various environmental, life history, and parental traits. For three of the five steroids, we found no significant correlates, but based on individual statistical tests (a) DHT varied between clutches with male age (1 year old vs older); (b) DHT and CORT were negatively correlated within clutches with the average temperature on the day (DHT and CORT) or 3 days (DHT only) preceding laying; and (c) DHT in the last egg of the clutch relative to the clutch mean was positively correlated with the interval between clutch completion and the onset of incubation ( incubation delay). Relationships with ambient temperature and incubation delay have not previously been reported for any yolk hormone in birds. Intriguingly, the three relationships for DHT are consistent with more DHT being transferred to eggs in situations that could be more energetically challenging for the female. More research is needed to determine the generality of the patterns we found and to understand their functional significance. Significance statement: The yolks of birds' eggs contain steroid hormones produced by the mother which can affect the development and behavior of the resultant chicks. We analyzed five steroid hormones in the yolks of wild great tits and show for the first time that yolk hormone levels are related to ambient temperature in the day(s) just before laying and, in the last-laid egg, with the day it is laid relative to the onset of incubation, and that the concentrations of pairs of yolk hormones can vary with each other in a different way between and within clutches. These results contribute insights into the ways in which yolk hormones may adaptively modify the chicks or may reflect physiological processes occurring in the mother. [ABSTRACT FROM AUTHOR]

Begging behaviour is a crucial component of parent–offspring communication. It is used by offspring to trigger the transfer of parental resources while at the same time allowing parents to access cryptic information about their offspring's need. But in a brood of more than one, offspring may gain indirect fitness benefits from responding to the need of its (related) siblings, not to withdraw all resources, especially if these are needier and would possibly contest more vigorously. Thus each offspring is thought to adjust its begging behaviour to its own intrinsic need as well as to its social environment, which is also shaped by the parents potentially having control over the distribution of resources. Here, we experimentally satiated the heaviest nestling within canary broods, Serinus canaria , in order to analyse: (1) whether nestlings honestly communicate their intrinsic hunger level; (2) whether nestlings adjust their begging behaviour to the need of their siblings; and (3) how parents respond to these begging strategies. Only female nestlings responded honestly, begging significantly less when satiated. Male nestlings, by contrast, did not alter their begging according to the level of satiation. Nestlings only weakly responded to the need of their siblings, and again only female nestlings did so. Thus female nestlings appear to be more sensitive to both intrinsic need and changes in their social environment, potentially owing to their lower competitiveness. Parents preferentially fed needier nestlings irrespective of the observed sex differences in begging strategies. They appear to control food distribution according to (cryptic) signals of need, which is important to take into account when studying the adaptive significance of any begging behaviour and strategy. [ABSTRACT FROM AUTHOR]

For many animals, adult size is an important determinant of fitness. Thus, after a period of food restriction, offspring often grow quickly to approach an optimal size. Offspring can approach an optimal size by increasing mass faster than the peak growth of offspring that do not delay development (compensatory growth) or by extending the period of rapid growth to reach an optimal size (catch-up growth). Unfortunately, the most common statistical techniques make it difficult to differentiate alternative growth patterns among developing offspring. Here, I show how random effect estimates can be used to uncover important variation in growth in a short-lived passerine, the House Sparrow Passer domesticus. Specifically, I show that much of the variation in offspring growth can be explained by differences in the timing of peak growth and in final adult size, both within a single population and within treatments of an experimental manipulation. These results suggest that much of the variation in offspring growth may be explained by factors other than growth rate. I also show that offspring that delay development either maintain slow but steady growth across development and reach a small adult size, or extend the period of rapid growth to reach an optimal size, indicative of catch-up growth. This pattern of extending the period of rapid growth may allow offspring to minimize the cellular damage caused by compensatory growth but still maximize size-related fitness benefits (e.g. increased survival and fecundity) prior to fledging. [ABSTRACT FROM AUTHOR]

Differences in the deposition of limited maternal resources to eggs can reflect the optimal allocation to manipulate offspring phenotype, or constraints caused by maternal condition. We examined multiple maternal substances transferred to eggs in the magpie Pica pica to test the hypothesis that certain substances represent constraint and some optimal allocation. We did this by supplementary feeding magpies prior to egg-laying and then comparing the effect of food on maternal substances in conjunction with laying order relative to a control group. Certain substances such as carotenoids, immunoglobulins and avidin responded positively to food supplementation, whereas others, such as testosterone and total protein content of eggs, did not. Immunoglobulin allocation increased with laying order in fed but not in unfed groups. Our results suggest that patterns of optimal allocation and constraint in maternally-provisioned substances co-occur within clutches. Furthermore, constraint and optimal allocation may co-occur within the same substance i.e. immunoglobulins, highlighting the complexities of maternal resource allocation. [ABSTRACT FROM AUTHOR]

Hatching asynchrony (HA hereafter) leads to patent age and size hierarchies within broods of altricial birds, disadvantaging runts through a reduced condition/survival. The function of HA is controversial, although a general hypothesis states that HA would be an adaptive maternal mechanism for maximizing reproductive output under particular ecological conditions. Accordingly, when ecological conditions are not favourable, asynchronous broods would outperform synchronous broods because the formers would allow for an adaptive adjustment of brood size. A poorly untested prediction emerging from this hypothesis is that the adaptive value of HA should change with environmental conditions in relatively long time windows within a population. Using data from 8 years of a population of the asynchronous European roller Coracias garrulus, we studied variation in HA and fecundity selection on HA. Hatching span (ranging from 1 to 8 days) was longer in larger broods and later in the breeding season. Interestingly, we found that asynchronous broods were more fecund than synchronous ones in colder and drier years in April, which is the previous month to reproduction, and the opposite was true in years with warmer but rainier Aprils. Given that warmer and rainier Aprils relate to advanced and increased arthropod availability in the Mediterranean region, these results would suggest that HA in rollers might function as an adaptive trait that provides parents of asynchronous broods with some benefits in years with low productivity, in agreement with the Lack's brood reduction hypothesis, and illustrate the relevance of studying the adaptive value of HA over gradients of environmental conditions. [ABSTRACT FROM AUTHOR]