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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., A. Artemyev acknowledges funding by IB KRC RAS no. 0218-2019-0080 and T. Eeva acknowledges funding by the Academy of Finland (project 265859). This study was funded by research project CGL-2016-79568-C3-3-P (to J. C. Senar), from the Ministry of Economy and Competitivity, Spanish Research Council.

The integration and synthesis of the data in different areas of science is drastically slowed and hindered by a lack of standards and networking programmes. Long-term studies of individually marked animals are not an exception. These studies are especially important as instrumental for understanding evolutionary and eco-logical processes in the wild. Furthermore, their number and global distribution provides a unique opportunity to assess the generality of patterns and to address broad-scale global issues (e.g. climate change)., To solve data integration issues and enable a new scale of ecological and evolution-ary research based on long-term studies of birds, we have created the SPI-Birds Network and Database (www.spibirds.org)—a large-scale initiative that connects data from, and researchers working on, studies of wild populations of individually recognizable (usually ringed) birds. Within year and a half since the establishment, SPI-Birds has recruited over 120 members, and currently hosts data on almost 1.5 million individual birds collected in 80 populations over 2,000 cumulative years, and counting., SPI-Birds acts as a data hub and a catalogue of studied populations. It prevents data loss, secures easy data finding, use and integration and thus facilitates collab-oration and synthesis. We provide community-derived data and meta-data stand-ards and improve data integrity guided by the principles of Findable, Accessible, Interoperable and Reusable (FAIR), and aligned with the existing metadata lan-guages (e.g. ecological meta-data language)., The encouraging community involvement stems from SPI-Bird's decentralized ap-proach: research groups retain full control over data use and their way of data management, while SPI-Birds creates tailored pipelines to convert each unique data format into a standard format. We outline the lessons learned, so that other communities (e.g. those working on other taxa) can adapt our successful model. Creating community-specific hubs (such as ours, COMADRE for animal demogra-phy, etc.) will aid much-needed large-scale ecological data integration., The SPI-Birds have been supported by an NWO personal grant (grant number 016.Veni.181.054) to A.C., and a Research Council of Norway grant: 223257 (SFF-III) and 267511 (EVOCLIM).

Studies of age-specific reproductive performance are fundamental to our understanding of population dynamics and the evolution of life-history strategies. In species with bi-parental care, reproductive ageing trajectories of either parent may be influenced by their partner's age, but this has rarely been investigated. We investigated within-individual age-specific performance (laying date and number of eggs laid) in wild female blue tits Cyanistes caeruleus and evaluated how the age and longevity of their male partner indirectly influenced the females' reproductive performance. Females showed clear age-dependence in both laying date and number of eggs laid. We found that female reproductive performance improved in early life, before showing a decline. Longer-lived females had an earlier laying date throughout their lives than shorter-lived females, but there was no difference in number of eggs laid between longer- and shorter-lived females. Within breeding pairs, the female's (age-specific) reproductive performance was not dependent on the age and longevity of the male partner. We conclude that the age and quality of the male partner may be of little importance for traits that are under direct female control. This article is protected by copyright. All rights reserved.

Heritable personality variation is subject to fluctuating selection in many animal taxa; a major unresolved question is why this is the case. A parsimonious explanation must involve a general ecological process: a likely candidate is the omnipresent spatiotemporal variation in conspecific density. We tested whether spatiotemporal variation in density within and among nest box plots of great tits (Parus major) predicted variation in selection acting on exploratory behaviour (n = 48 episodes of selection). We found viability selection favouring faster explorers under lower densities but slower explorers under higher densities. Temporal variation in local density represented the primary factor explaining personality-related variation in viability selection. Importantly, birds did not anticipate changes in selection by means of adaptive density-dependent plasticity. This study thereby provides an unprecedented example of the key importance of the interplay between fluctuating selection and lack of adaptive behavioural plasticity in maintaining animal personality variation in the wild.

Timing of reproduction in birds is important for reproductive success and is known to depend on environmental cues such as day length and food availability. However, in equatorial regions, where day length is nearly constant, other factors such as rainfall and temperature are thought to determine timing of reproduction. Rainfall can vary at small spatial and temporal scales, providing a highly fluctuating and unpredictable environmental cue. In this study we investigated the extent to which spatio-temporal variation in environmental conditions can explain the timing of breeding of Red-capped Lark, Calandrella cinerea, a species that is capable of reproducing during every month of the year in our equatorial east African study locations. For 39 months in three climatically-distinct locations, we monitored nesting activities, sampled ground and flying invertebrates, and quantified rainfall, maximum (T-max) and minimum (T-min) temperatures. Among locations we found that lower rainfall and higher temperatures did not coincide with lower invertebrate biomasses and decreased nesting activities, as predicted. Within locations, we found that rainfall, Tmax, and Tmin varied unpredictably among months and years. The only consistent annually recurring observations in all locations were that January and February had low rainfall, high Tmax, and low Tmin. Ground and flying invertebrate biomasses varied unpredictably among months and years, but invertebrates were captured in all months in all locations. Red-capped Larks bred in all calendar months overall but not in every month in every year in every location. Using model selection, we found no clear support for any relationship between the environmental variables and breeding in any of the three locations. Contrary to popular understanding, this study suggests that rainfall and invertebrate biomass as proxy for food do not influence breeding in equatorial Larks. Instead, we propose that factors such as nest predation, female protein reserves, and competition are more important in environments where weather and food meet minimum requirements for breeding during most of the year.

Quantifying the fitness costs of reproduction is essential to understand the evolution of reproductive behavior. Recent work shows that increased reproductive investment reduced parental survival in more competitive environments. Here we experimentally test the hypothesis that reproductive investment has a negative effect on the ability of parents to compete for resources in later life. In a nest-box population of Great tits (Parus major), we manipulated family size by reducing or enlarging broods with 2 or 3 nestlings, relative to a control group, in 2 years (N = 237 broods). Parental feeding effort was positively related to experimental family size but leveled off for the enlarged broods. In the next breeding season we manipulated the nest box quality in early spring by reducing the depth of 80% of the nest boxes (deeper boxes are safer and preferred). We analyzed parents' probability of obtaining a deep breeding box in relation to their previous year's family size manipulation. We found for both years that increased reproductive investment negatively affected the probability of parents to claim a high-quality nest box in the subsequent breeding season. We thus confirm that family size has a negative effect on the future competitive ability of parents. Such carry-over effects are important because they show that selection on individual optimal clutch size will depend on 1) resource abundance and the level of competition in the next breeding season and 2) the reproductive investment of the competitors in the current breeding season because it affects their future competitive ability as well.

O objetivo deste estudo é descrever aspectos da biologia reprodutiva de Taraba major tais como período reprodutivo, ninhos, ovos, ninhegos, tamanho de ninhada, período de incubação e permanência dos ninhegos no ninho. Os dados foram coletados durante as estações reprodutivas de 2009 e 2010, na fazenda Retiro Novo, município de Poconé, Pantanal, Mato Grosso, Brasil. A procura dos ninhos ocorreu de forma ativa e através da observação dos adultos transportando material para o ninho ou alimento para o filhote. Os ninhos foram visitados com intervalos de três a quatro dias. Foram monitorados 51 ninhos ativos. O período reprodutivo da espécie estendeu-se de outubro a dezembro. O tempo médio de construção dos ninhos foi de 06 ± 1,6 dias (n = 08). Os ovos são de formato ovóide, esbranquiçados com manchas marrons espalhadas por toda sua extensão. A massa, comprimento e largura média dos ovos foram 6,1 ± 0,5 g, 26,8 ± 1,5 mm, 19,5 ± 1,7 mm, respectivamente (n = 74). O tamanho da ninhada variou de dois (n = 13) a três (n = 1) ovos. Ambos os sexos constroem o ninho e revezam na incubação. O tempo médio de incubação foi de 15,5 ± 1,7 dias (n = 08), e a permanência dos ninhegos nos ninhos de 13,2 ± 1,6 dias (n = 08). We describe some aspects of the reproductive biology of the Great Antshrike (Taraba major), such as: reproductive period, nests, eggs, nestlings, incubation period, and permanence of nestlings in the nest. The data were collected during the breeding seasons of 2009 and 2010 at Retiro Novo Farm, municipality of Poconé, Pantanal, Mato Grosso, Brazil. Nest searching was conducted through active searching and observation of adults carrying nest material or food for the nestlings. The nests were visited at intervals of three to four days. Fifty-one active nests were monitored. The reproductive period of the species lasted from October to December. The average time of nest construction was 06 ± 1.6 days (n = 08). Eggs were ovoid, white with brown spots scattered throughout their length. The mass, length, and average width of the eggs were 0.5 g, 6.1 ± 1 mm, and 26.8 ± 19.5 ± 1 mm, respectively (n = 74). Litter size ranged from two (n = 13) to three (n = 1) eggs. Both sexes built the nest and took turns during incubation. The average time of incubation was 15.5 ± 1.7 days (n = 08), and the nestling period ranged from 13.2 ± 1.6 days (n = 08).

An increase of competition among adults or nestlings usually negatively affects breeding output. Yet little is known about the differential effects that competition has on the offspring sexes. This could be important because it may influence parental reproductive decisions.In sexual size dimorphic species, two main contradictory mechanisms are proposed regarding sex-specific effects of competition on nestling performance assuming that parents do not feed their chicks differentially: (i) the larger sex requires more resources to grow and is more sensitive to a deterioration of the rearing conditions ('costly sex hypothesis'); (ii) the larger sex has a competitive advantage in intra-brood competition and performs better under adverse conditions ('competitive advantage hypothesis').In the present study, we manipulated the level of sex-specific sibling competition in a great tit population (Parus major) by altering simultaneously the brood size and the brood sex ratio on two levels: the nest (competition for food among nestlings) and the woodlot where the parents breed (competition for food among adults). We investigated whether altered competition during the nestling phase affected nestling growth traits and survival in the nest and whether the effects differed between males, the larger sex, and females.We found a strong negative and sex-specific effect of experimental brood size on all the nestling traits. In enlarged broods, sexual size dimorphism was smaller which may have resulted from biased mortality towards the less competitive individuals i.e. females of low condition. No effect of brood sex ratio on nestling growth traits was found.Negative brood size effects on nestling traits were stronger in natural high-density areas but we could not confirm this experimentally.Our results did not support the 'costly sex hypothesis' because males did not suffer from higher mortality under harsh conditions. The 'competitive advantage hypothesis' was also not fully supported because females did not suffer more in male-biased broods.We conclude that male nestlings are not likely to be more expensive to raise, yet they have a size-related competitive advantage in large broods, leading to higher mortality of their on average lighter female nest mates.

5 pages; International audience; The invasive leaf-mining moth, Cameraria ohridella, revealed to be a consistent eruptive species throughout Europe, at the expense of its host, the common horse chest-nut tree Aesculus hippocastanum. Its repeated outbreaks, year after year, are admittedly caused, in part, by the inadequacy of the ambient cortege of natural enemies as an effective mean of control of the dynamics of populations of this pest.Less attention has been given to other parameters also contributing to the moth’s impact in term of mines density, such as (i) the degree of selectivity of C. ohridella mothers among host-leaves prior to oviposition and (ii) the average clutch-size.Although these behaviour-related factors might reduce to a lesser extent the density of C. ohridella than the inefficient control of Cameraria by natural enemies, they nevertheless participate to the remarkable originality of C. ohridella, as compared to the bulk of other common but non-eruptive leaf-mining insects. Hereafter, we estimate the respective contributions of these two behaviour-related factors to the level of mining impact, using an appropriate indirect procedure. An unusually low level of mothers’ selectivity and a relatively high clutch-size are highlighted in C. ohridella, by comparison to other common mining moth species on trees. Both parameters are shown to contribute to a 50% increase of the parasite impact on its host.

Perrins (1970) galvanized thinking on the timing of birds' breeding seasons by pointing out that most individuals laid too late for the offspring to profit fully from the seasonal peak of food abundance, and suggested that the proximate cause was a shortage of food for the female when forming the eggs. This idea (the food constraint hypothesis) stimulated field experiments with supplementary food, and also catalyzed analysis of the seasonal trends of fitness for both parents and offspring. Most experiments resulted in minor advances in laying date hinting that other factors are also important, and the fitness comparisons underlined the view that laying date must be viewed as a trade-off between opposing seasonal trends affecting parents (later is better) and offspring (the earlier the better). Experimental advances of laying have revealed fitness costs (lowered survival) for the parent(s) counteracting the higher output of young. Laying date is thus best considered as an individually based compromise (individual optimisation hypothesis), with exact timing subject to local environmental control. Recent analyses of climate change (warming trend in spring) confirm responses in many but not all bird species, and where detailed data are available the advance documented in laying date seems often to fall short of the shift in the food peak. Migrants in particular may be unable to speed up their spring travels, in some species on account of time conflicts with moult. The larger bodied migrants breeding in the Arctic face severe time constraints as they must lay relatively early on the snowy tundra. Postulated to rely on endogenous stores to lay eggs and incubate them, finer resolution of their travel schedules by means of satellite telemetry combined with investigation of the isotopic signature of body tissues, eggs, and food sources show a mixed strategy to prevail: transport what you can ('capital') and supplement this on the breeding grounds ('income'). Stores acquired at stopover sites are of crucial importance for both survival and breeding and any'mismatch' caused by uncoupling of rates of climate change along the migratory pathway may have profound effects on the population level.

A crucial assumption underlying the breeders' equation is that selection acts directly on the trait of interest, and not on an unmeasured environmental factor which affects both fitness and the trait. Such an environmentally induced covariance between a trait and fitness has been repeatedly proposed as an explanation for the lack of response to selection on avian breeding time. We tested this hypothesis using a long-term dataset from a Dutch great tit (Parus major) population. Although there was strong selection for earlier breeding in this population, egg-laying dates have changed only marginally over the last decades. Using a so-called animal model, we quantified the additive genetic variance in breeding time and predicted breeding values for females. Subsequently, we compared selection at the phenotypic and genetic levels for two fitness components, fecundity and adult survival. We found no evidence for an environmentally caused covariance between breeding time and fitness or counteracting selection on the different fitness components. Consequently, breeding time should respond to selection but the expected response to selection was too small to be detected.

1. Dispersal of parents and offspring in relation to manipulated brood size were analysed in the great tit Parus major (L.) to study the potential confusion between dispersal and survival. The study area consisted of eight woodlots interspersed with nonbreeding habitat. The maximum distance between nestboxes was 10 km.2. The brood size of pairs with similar clutch size and laying date was manipulated in 3 years when chicks were 2 days old (1995, 1997 and 1998). Three nestlings were removed from one and added to another brood while a third was kept as a control. Offspring were measured, weighed and marked and breeding birds were captured and marked to allow dispersal estimates.3. For the offspring, dispersal was estimated as the distance between the natal nestbox and the nestbox of first breeding (natal dispersal distance) and, for the parents, as the distance between breeding boxes in two subsequent seasons (breeding dispersal distance).4. Natal dispersal distance was positively affected by brood size manipulation. This effect was more pronounced in males than in females. Breeding dispersal distance was not affected by manipulation.5. The practical consequence of this finding is that fitness estimates used to measure selection on brood size did depend on the spatial scale of the study area. For the Lauwersmeer population measured selection pressure changed from positive to stabilizing when I restricted the spatial scale of recovery. Other brood size manipulation experiments may suffer from similar biases in their fitness estimates.6. The biological consequence of this finding is that, if clutch size has a heritable component, local adaptation of clutch size will depend on the spatial heterogeneity of the habitat.

The ultimate reason why birds should advance their phenology in response to climate change is to match the shifting phenology of underlying levels of the food chain. In a seasonal environment, the timing of food abundance is one of the crucial factors to which birds should adapt their timing of reproduction. They can do this by shifting egg-laying date (LD), and also by changing other life-history characters that affect the period between laying of the eggs and hatching of the chicks. In a long-term study of the migratory Pied Flycatcher, we show that the peak of abundance of nestling food (caterpillars) has advanced during the last two decades, and that the birds advanced their LD. LD strongly correlates with the timing of the caterpillar peak, but in years with an early food peak the birds laid their eggs late relative to this food peak. In such years, the birds advance their hatching date by incubating earlier in the clutch and reducing the interval between laying the last egg to hatching of the first egg, thereby partly compensating for their relative late LD. Paradoxically, they also laid larger clutches in the years with an early food peak, and thereby took more time to lay (i.e. one egg per day). Clutch size therefore declined more strongly with LD in years with an early food peak. This stronger response is adaptive because the fitness of an egg declined more strongly with date in early than in late years. Clearly, avian life-history traits are correlated and Pied Flycatchers apparently optimize over the whole complex of the traits including LD, clutch size and the onset of incubation. Climate change will lead to changing selection pressures on this complex of traits and presumably the way they are correlated. [KEYWORDS: climate change ; clutch size ; Ficedula hypoleuca ; food availability ; laying date]

Question: How is variation in offspring size (between broods) related to brood size? Hypotheses: Variance in offspring size (between broods) should decrease with increasing brood size as predicted by Charnov and colleagues' (Charnov and Downhower, 1995; Charnov et al., 1995) small brood invariant. The range in resources put towards reproduction (for mothers producing a certain brood size) should be invariant over brood size (Downhower and Charnov, 1998). We also test assumptions underlying these predictions. Data studied: We use previously collected data on six parasitoid wasp species. Conclusions: As predicted, variance in offspring size among broods decreased with increasing brood size. However, this decrease did not follow closely the quantitative predictions of Charnov and colleagues (Charnov and Downhower, 1995; Charnov et al., 1995). We found some support for the prediction that the range in resources invested in reproduction is invariant over brood size. The assumption that mean offspring size is constant over brood size was violated in three of six species. The assumption that resources are shared equally between individuals within a brood generally held.

We investigated the effects of increased egg production on body condition as well as on measures of reproductive performance in great skuas, Stercorarius skua, over two subsequent years. We experimentally increased egg production from the normal two to six eggs. Six eggs might also be produced under natural circumstances after repeated clutch loss. After the production of the last egg we measured: (i) body mass, (ii) pectoral muscle, and (iii) haematocrit, total red blood cell count and mean corpuscular volume, as indicators of body condition. We took the same measurements of control females who had produced the normal clutch of two eggs. The measurements were repeated one year after the manipulation, and survival, laying dates, clutch sizes and hatching success were recorded for up to three consecutive years. After producing six eggs, females were lighter, had smaller pectoral muscles and lower haematological values than control females. Hatching success of eggs was significantly reduced. Even one year after the experiment there were still differences in body condition. Annual survival was not affected by the manipulations, although there was an indication that survival costs depended on whether chicks were raised after the increased egg production. While pair bonds and egg sizes were not affected in the post-experimental year.. females started breeding significantly later than in the previous year. Two years after the experiment laying dates had advanced again and were not different from those of control females. This pattern of maintaining survival and egg sizes, but delaying breeding in the post-experimental year was found for two independent groups of females which had both been subjected to increased egg production. These results present evidence that increased egg production can have long-term effects on female body condition and aspects of reproduction. However, although present, the costs of extra eggs appear to have been relatively small in the great skua in comparison to the two other bird species for which inter-annual effects have been reported.

The amount of time a bird allocates to incubation is likely to be limited by energetic constraints. If food is abundant, energetic constraints may be reduced and the time spent incubating (incubation attendance) may increase. Moreover, the onset of incubation in relation to clutch completion may be advanced, resulting in a higher degree of hatching asynchrony. We measured the effect of experimentally increased food availability on incubation attendance and an estimate of hatching asynchrony in the Australian reed warbler Acrocephalus australis. Supplementary food was provided every other day, from a few days before the start of egg laying until just prior to hatching. Incubation attendance was measured with temperature loggers at nests receiving supplementary food and control nests. Hatching asynchrony was inferred from mass and size differences between siblings shortly after hatching. We found that 1) food supplementation resulted in an increase in incubation attendance, when comparing both nests receiving supplementary food to control nests as well as feeding to non-feeding days in nests receiving supplementary food, and 2) food supplementation resulted in a greater hatching asynchrony, without affecting clutch size, average egg volume or the likelihood of eggs hatching. This suggests that food availability acts in a proximate way to modify the extent of incubation attendance and hatching asynchrony. We discuss the adaptive significance of increased incubation attendance and a shift in the degree of hatching asynchrony in relation to food availability.

We report the results of an expedition to a barnacle-goose (Branta leucopsis) breeding area in Kolokolkova Bay, west of the lower Pechora delta in northern Russia, undertaken in July 2002. In total, 6 breeding colonies were found within the study area, harbouring 1,324 nests. Mean clutch size was 2.77+/-0.10 but may have been underestimated because of nest predation. Nest predation was high and correlated with the density of breeding gulls, Larus. The 2002 season was relatively cold and peak hatch occurred late, on 14 July. More than 11,000 barnacle geese were found to moult in the area which, together with the large number of nests found, emphasises the importance of Kolokolkova Bay for barnacle geese. Adult barnacle geese (341) were captured, marked and measured during their annual wing moult. Birds with broods started to moult approximately 2 weeks later than non- and failed breeders. Weight loss during moult was 3 times as rapid as reported for barnacle geese breeding in the Baltic, and a large cost of reproduction seemed to exist in the form of reduced body weight at the onset of moult for birds leading broods. Work in the area will continue over the coming years to document and explain the differences in major life-history parameters, dynamics and environmental effects between arctic and temperate breeding barnacle-goose populations.

Sexual conflict occurs when the optimal solution regarding e.g. a life history trait differs between co-operating individuals of different sex. When deciding a conflict is not instantaneous, some form of negotiation can be expected to evolve. In great tits, Parus major, a sexual conflict exists over the number of clutches that are reared, because the fitness costs of a second clutch are greater for females. A conflict is also likely to exist over investment in the first brood - each parent benefiting from a greater investment by the partner. Male great tits sing when rearing the first brood, and if acoustic signals play a role in the negotiation of a sexual conflict, a positive association between male song rate and maternal investment is predicted. In agreement with this hypothesis, maternal effort (in kJ/day) relative to paternal effort was positively correlated with male song rate. Furthermore, females were more likely to start a second clutch when their male had a high song rate, and high song rate was associated with shorter inter-clutch intervals. Song rate was higher when brood size was experimentally reduced and, independent of brood size manipulation, males with high song rate produced higher quality fledglings. These results indicate that song rate reflects the males' state, suggesting it may function as a handicap signal. Although song rate seems too low (

We collected 328 freshly laid Great Tit (Parus major) eggs from 38 clutches in 1999 to determine the relationship of whole egg weight, wet yolk weight, wet albumen weight, dry shell weight, and the occurrence of laying gaps with mean ambient temperature in the three days preceding laying, while controlling for laying date and position in the laying sequence. We also reanalyzed existing data on whole egg weight in 1978 and 1979 in relation to temperature, controlling for the same other variables. Egg weight was correlated with temperature in 1978 and 1979, but not in 1999. None of the egg components was related to temperature preceding laying, apart from a nonsignificant tendency for albumen weights to be lower at higher temperature. Egg weight increased with laying date in 1978 and 1979 after controlling for other variables. In 1999, albumen and shell weight increased seasonally, but shell weight only did so to the extent expected on the basis of the seasonal increase in whole egg weight. Yolk weight did not increase significantly seasonally, but neither did yolk weight relative to whole egg weight decrease significantly. Whole egg weight and the weights of all components varied between clutches. Most weights also varied through the laying sequence. The probability of a laying gap occurring varied between clutches, and increased with position in the laying sequence and decreased with temperature and laying date. Eggs preceding laying gaps were lighter and contained less albumen but had higher shell weights than other eggs. Eggs following laying gaps also had heavier shells relative to the size of the egg than other eggs.

In many bird species, females undergo a marked decline in body condition during the first days of the nestling period. This decline may be because brooding young chicks reduces the time available for foraging. Alternatively, it might be viewed as an adaptive way to reduce flight costs when the food demand of the brood is highest. To test these hypotheses we modified the brooding commitment of House Sparrows Passer domesticus by manipulating brood size to see if changes in time spent brooding affects adult body condition. During the nestling period, females provided on average three times as much brooding as males. Reduced broods received 14% more brooding than large broods and time spent brooding declined with brood size and chick age according to an exponential decay function. Male body condition was unaffected by brood size and remained stable throughout the reproductive period. Body condition of females with enlarged broods decreased gradually during the nestling period, whereas that of females tending reduced broods dropped abruptly and significantly upon hatching. This resulted in females with reduced broods having lower body condition during the first half of the nestling period than those with enlarged broods. The sharp drop in body condition of females with reduced broods coincided with the period that brooding was most intensive. Indeed, female body condition at the end of the nestling period was negatively correlated with the proportion of time they spent brooding during the first half of the nestling period. Thus, the probable lower homeothermic capacities of reduced broods implies a higher brooding commitment for female House Sparrows that, in turn, may reduce their opportunity to forage and consequently also their body condition.

Life-history theory suggests that the variation in the seasonal timing of reproduction within populations may be explained on the basis of individual optimization. Optimal breeding times would vary between individuals as a result of trade-offs between fitness components. The existence of such trade-offs has seldom been tested empirically. We experimentally investigated the consequences of altered timing of current reproduction for future reproductive output in the European coot (Fulica atra). First clutches of different laying date were cross-fostered between nests, and parents thereby experienced a delay or an advance in the hatching date. The probability and success of a second brood. adult survival until and reproduction in the next season were then compared to the natural variation among control pairs. Among control pairs the probability of a second brood declined with the progress of season. Delayed pairs were less likely and advanced pairs were more likely to produce a second brood. These changes were quantitatively as predicted from the natural seasonal decline. The number of eggs in the second clutch was positively related to egg number in the first clutch and negatively related to laying date. Compared to the natural variation, delayed females had more and advanced females had fewer eggs in their second clutch. The size of the second brood declined with the progress of season, but there was no significant effect of delay or advance. Local adult survival was higher following a delay and reduced following an advance. The effect of the experiment on adult survival was independent of sex. Laying date and clutch size of females breeding in the next year were not affected by treatment. The study demonstrates the existence of a trade-off between increased probability of a second brood and decreased parental survival for early breeding. Timing-dependent effects of current reproduction on future reproductive output may thus play an important role in the evolution of the seasonal timing of reproduction. [KEYWORDS: adult survival, Fulica atra, future reproduction, life-history evolution, multiple breeding, timing of reproduction]

During egg-formation, energy and protein are deposited in the developing eggs but are, at the same time, needed by the laying female herself. This has been largely overlooked in the discussion on income and capital breeders (Drent & Daan 1980, Thomas 1988). We discuss data on exogenous versus endogenous energy and nutrients used during egg-formation for 12 well-studied species ranging from the Adelie Penguin Pygoscelis adelie (3400 g) to the Blue Tit Parus caeruleus (11 g) and calculate which part of the total energy and nutrient requirements (of clutch and laying female) originates from direct food intake and/or from body reserves. Because energy and nutrients are also needed by the laying female, some large species breeding in cold regions deposit sufficient reserve that they can fast completely during egg-formation (like the Adelie Penguin) and even throughout incubation (like the Elder Somateria mollissima). However, almost all smaller species must forage for most of their energy and nutrients during the egg-formation period. For the large species, energy and protein of the clutch represent 30% and 70%, respectively, of the total requirements of laying females, much more than in small species like passerines (4% and 40%). Therefore, the requirements for the clutch are much greater in larger than in smaller species, and egg-production is much more limited by protein than by energy. The effects of food supplementation on timing of laying (moderate advance), on number of eggs laid (not more, when corrected for date) and on egg size (not larger) of income and capital breeders/layers are discussed. It seems that neither the start of egg-laying nor the number or quality of eggs are directly related to the energetics of the laying female.

1. In precocial birds, where the young feed themselves, the costs and benefits of brood size are still poorly understood. An experimental manipulation of brood size was employed to examine the effects of brood size on both parents and young in a wild population of barnacle geese [Branta leucopsis (Bechstein)] during brood-rearing on Svalbard.2. Social dominance of the family unit, the amount of vigilance behaviour of the parents, the growth of the goslings in the family unit and an index of body condition for female parents during moult were all positively correlated with brood size.3. When brood size changed as a result of natural events (i.e. predation or adoption) or experimental manipulation, rates of dominance, parental vigilance, gosling growth and female parent condition changed in a similar direction to the observed relation between the variable and brood size in unchanged broods.4. After fledging, the fast-growing goslings in large broods survived better during autumn migration, while there was no apparent net cost in survival or next-year breeding for the parents.5. Via a direct effect of brood size on dominance of the family unit, large broods were beneficial for both parent and young in a situation where there was strong intraspecific competition for the available food resources.6. This study provides a clear demonstration of a causal relationship between brood size and various components of both gosling and adult fitness and is of direct relevance to the phenomenon of adoption and the evolution of brood size in this species.

During egg-formation, energy and protein are deposited in the developing eggs but are, at the same time, needed by the laying female herself. This has been largely overlooked in the discussion on income and capital breeders (Drent & Daan 1980, Thomas 1988). We discuss data on exogenous versus endogenous energy and nutrients used during egg-formation for 12 well-studied species ranging from the Adelie Penguin Pygoscelis adelie (3400 g) to the Blue Tit Parus caeruleus (11 g) and calculate which part of the total energy and nutrient requirements (of clutch and laying female) originates from direct food intake and/or from body reserves. Because energy and nutrients are also needed by the laying female, some large species breeding in cold regions deposit sufficient reserve that they can fast completely during egg-formation (like the Adelie Penguin) and even throughout incubation (like the Elder Somateria mollissima). However, almost all smaller species must forage for most of their energy and nutrients during the egg-formation period. For the large species, energy and protein of the clutch represent 30% and 70%, respectively, of the total requirements of laying females, much more than in small species like passerines (4% and 40%). Therefore, the requirements for the clutch are much greater in larger than in smaller species, and egg-production is much more limited by protein than by energy. The effects of food supplementation on timing of laying (moderate advance), on number of eggs laid (not more, when corrected for date) and on egg size (not larger) of income and capital breeders/layers are discussed. It seems that neither the start of egg-laying nor the number or quality of eggs are directly related to the energetics of the laying female.

A brood manipulation experiment on great tits Parus major was performed to study the effects of nestling age and brood size on parental care and offspring survival. Daily energy expenditure (DEE) of females feeding nestlings of 6 and 12 days of age was measured using the doubly-labeled water technique. Females adjusted their brooding behavior to the age of the young. The data are consistent with the idea that brooding behavior was determined primarily by the thermoregulatory requirements of the brood. Female DEE did not differ with nestling age; when differences in body mass were controlled for, it was lower during the brooding period than later. In enlarged broods, both parents showed significantly higher rates of food provisioning to the brood. Female DEE was affected by brood size manipulation, and it did not level off with brood size. There was no significant effect of nestling age on the relation between DEE and manipulation. Birds were able to raise a larger brood than the natural brood size, although larger broods suffered from increased nestling mortality rates during the peak demand period of the nestlings. Offspring condition at fledging was negatively affected by brood size manipulation, but recruitment rate per brood was positively related to brood size, suggesting that the optimal brood size exceeds the natural brood size in this population.

We present data on the natural variation in daily energy expenditure (DEE, measured by the doubly labelled water technique) of individual female Great Tits Parus major tending their brood near the northern border of the species' distribution. We expected that longer foraging days would result in a higher energy expenditure. Female DEE, visitation rate and the duration of the active period were measured at the nestling age of 12 days. There was a positive relationship between the duration of activity period and brood size. Female DEF was positively associated with the duration of the activity period and the number of young in the nest. Energy expenditure was not related to temperature or feeding rates. Despite the longer foraging day and lower ambient temperatures, female DEE did not differ from the values obtained in central Europe. This result suggests that there are differences in foraging cost and/or mode with latitude.

1. The role of food in avian reproduction is generally studied from the perspective of the protein and energy demand of birds, This study provides the first experimental evidence that calcium availability may limit reproduction in wild birds as well. 2. Data are presented showing that a large proportion of great tits Parus major on calcium-poor soils in the Netherlands produce eggs with thin and porous shells and desert the clutch before hatching. About 10% of the females do not lay at all. 3. Free-living great tits were supplied with an additional calcium source, i.e. snail shells and chicken eggshells. This treatment reduced the number of females without eggs, the frequency of clutch desertion, the proportion of nests with defective eggshells and the proportion of non-hatched eggs in clutches that produced young. The calcium supplements did not affect clutch size or laying date. 4. We suggest that possible adaptations to a limited calcium supply are not yet evident because the low calcium availability is a recent phenomenon caused by acid deposition and because a large part of the breeding population consists of immigrants from calcium-rich areas. 5. We provide evidence that calcium limitation in avian reproduction may be widespread on calcium-poor soils. 6. The results imply that the costs of egg formation in calcium-poor areas can be much higher than is currently estimated and that food conditions during egg-laying have a greater impact on avian reproduction than is presently believed. [KEYWORDS: eggshell; food availability; limiting nutrients; Parus major L reproduction Brown-headed cowbirds; tit parus-major; great tit; nutrient reserves; quelea-quelea; clutch-size; food; reproduction;birds; limitation]

We studied temporal variation in reproductive performance of Marsh Harriers Circus aeruginosus in two land reclamations in The Netherlands, i.e. South Flevoland and the Lauwersmeerpolder, embanked in 1968 and 1969 respectively. The number of breeding pairs in Flevoland rapidly increased to a maximum of 350 pairs (± 1 pair km-2) in 1977, followed by a sharp decline in the 1980s due to large-scale cultivation. The same trend was observed in the Lauwersmeer, although colonization was retarded and peak densities were reached later. In both study areas mean clutch size as well as the number of fledglings per nest decreased in the course of the twenty years of study. Two factors were responsible for this decline in reproductive output: (1) decrease of food abundance in the course of the years, and (2) an increase of nest predation, mainly by the Red Fox Vulpes vulpes. Superimposed on these long-term changes, annual fluctuations in density of the Common Vole Microtus arvalis had a considerable effect on the number of breeding pairs, as well as the fledgling production. Mean annual clutch size was associated positively, and laying date negatively, with the average annual temperature during the pre-laying phase. Our results indicate that the early stages of succession, during the first decade after reclamation, are characterized by high prey abundance (i.e. vole 'plagues') and low densities of ground predators, and offer favorable breeding conditions for Marsh Harriers. The simultaneous negative effects in recent years of less breeding habitat, decreased prey abundance and increased predation on reproductive output, exert pressure on these populations. The intense nest predation (50% of all nests), as recorded in the Lauwersmeer since 1990, may eventually cause local extinction in the absence of, relatively safe, inundated breeding habitat.

Reproductive, phenotypic and life-history traits in many animal and plant taxa show geographic variation, indicating spatial variation in selection regimes. Maternal deposition to avian eggs, such as hormones, antibodies and antioxidants, critically affect development of the offspring, with long-lasting effects on the phenotype and fitness. Little is however known about large-scale geographical patterns of variation in maternal deposition to eggs. We studied geographical variation in egg components of a passerine bird, the pied flycatcher (Ficedula hypoleuca), by collecting samples from 16 populations and measuring egg and yolk mass, albumen lysozyme activity, yolk immunoglobulins, yolk androgens and yolk total carotenoids. We found significant variation among populations in most egg components, but ca. 90% of the variation was among individuals within populations. Population however explained 40% of the variation in carotenoid levels. In contrast to our hypothesis, we found geographical trends only in carotenoids, but not in any of the other egg components. Our results thus suggest high within-population variation and leave little scope for local adaptation and genetic differentiation in deposition of different egg components. The role of these maternally-derived resources in evolutionary change should be further investigated.

Life-history theory predicts that there should be negative fitness consequences, in terms of future reproduction and survival, for parents with increased reproductive effort. We examined whether increased incubation demand affected innate immunity and body condition by performing a clutch-size manipulation experiment in black guillemots (Cepphus grylle). We found that plasma from males incubating experimentally enlarged clutches exhibited significantly reduced lysis titers compared with plasma from males incubating control clutches, while this was not observed in females. The increased incubation demand also impacted agglutination titers differently in males and females, although the effect of treatment was not significant in either sex. Among all birds, lysis titers increased and haptoglobin concentrations decreased from mid-to late incubation. Natural antibody-mediated agglutination titers and body condition were highly repeatable within the incubation bout and between years. This suggests that agglutination titers may serve as a reliable and resilient index of the immunological character of individuals in future studies. Overall, this study demonstrates that increased incubation demand impacts indices of innate immunity differently in males and females. The potential for different components of the immune system to be impacted sex-specifically should be considered in future studies linking immune function and life-history trade-offs.

Although the causes and consequences of extrapair paternity (EPP) have been studied extensively in birds, little is known about the regulation of extrapair copulation (EPC) behavior and how it may tie in with other aspects of avian reproduction. In birds, the presence of eggs stimulates incubation and, subsequently, the cessation of egg production. We propose that the same mechanism also regulates female motivation to engage in EPCs. To test this idea, we simulated the earlier onset of laying in blue tits (Cyanistes caeruleus), by adding model eggs to nests before natural laying commenced. Most females accepted these eggs, covering them with nest material in the natural way. As predicted, these broods hatched more asynchronously than control broods, revealing an earlier onset of incubation, and were less likely to contain extrapair offspring (EPO) suggesting that stimulation from eggs also inhibits motivation to seek EPCs. Egg stimulation is thought to cause cessation of laying a fixed number of days before clutch completion, after a certain hormonal threshold is exceeded. Similarly, a lower threshold may inhibit engagement in EPCs relative to clutch completion, explaining the proportional increase in EPO toward the extremes of clutch size, which we also observed, and would be consistent with a fertility or compatibility insurance function for EPCs. Our findings may represent the best experimental evidence for female-mediated effects on the timing of EPCs and suggest how EPP may be integrated within the regulatory mechanism of avian reproduction. Key words: extrapair fertilization, good genes, laying order, sexual conflict, sperm competition. [Behav Ecol]

We compared the use of body stores in breeding Barnacle Geese (Branta leucopsis) in traditional Arctic colonies in the Barents Sea with that in recently established temperate-zone breeding colonies in the Baltic Sea and North Sea by studying female body-mass loss and use of fat and protein stores during incubation. Average daily body-mass loss was almost identical in the 2 temperate-breeding populations (17.0 g and 16.5 g in Baltic Sea and North Sea, respectively), whereas Arctic-breeding females lost significantly less (10.6 g day(-1)). Temperate-breeding females initiated incubation with body mass 1.25 g higher than that of Arctic breeders, but at the end of incubation, body mass was similar among the 3 populations, averaging 1,458 g. Body-mass loss during incubation amounted to 23% (North Sea), 22% (Baltic Sea), and 15% (Barents Sea). Fat mass, as measured by isotope dilution in a subsample of females, was consistently higher in North Sea than in Barents Sea birds, but both Populations showed similar rates of fat-mass loss (94 9 day(-1) on average). By contrast, loss of fat-free mass (assumed to represent wet protein) amounted to 9.3 g day(-1) in North Sea birds but only 1.5 g day(-1) in Barents Sea birds. Energy content of I g utilized body mass was 21.1 kj (North Sea) and 34.9 kj (Barents Sea), which equates to 376 kj day(-1) and 415 kj day'drawn from stored energy, respectively. We suggest that differences in nest-attendance and post-incubation demands are responsible for the differential use of body stores in temperate- and Arctic-breeding Barnacle Geese. Received 1 November 2008, accepted 20 August 2009.

Breeding and moult cycles were investigated in pairs of European Kestrels Falco tinnunculus exposed to four different photoperiods (LD regimes of 17.5:6.5, 15:9, 13:11, and 8:16). Laying, incubation and feeding of the young occurred in all four photoperiods, with all birds moulting after the breeding season. The time until the first egg, the duration of the period during which first clutches were laid, and the time until birds began to moult, were all negatively correlated with daylength.Two groups of birds (LD 13:11 and 8:16) were held under constant conditions for 4.5 and 2 yr, respectively. After the first breeding/moult cycle, the LD 13:11 group did not begin a second cycle for the next 19 months, and only started laying again, and subsequently moulting, after exposure to an 8 h day for 6 weeks. The LD 8:16 group went through two breeding/moult cycles during the two years.It is concluded that the development and expression of photorefractoriness is related to daylength, and that photorefractory Kestrels need short days to become photosensitive again. The possibility that the development of photorefractoriness is involved in the seasonal decrease in clutch size, found in both free-living and captive breeding Kestrels, is discussed.

Temporal variation in survival, fecundity, and dispersal rates is associated with density-dependent and density-independent processes. Stable natural populations are expected to be regulated by density-dependent factors. However, detecting this by investigating natural variation in density is difficult because density-dependent and independent factors affecting population dynamics may covary. Therefore, experiments are needed to assess the density dependence of demographic rates. In this study, we investigate the effect of density on demographic rates of the Seychelles Warbler (Acrocephalus sechellensis). This species, endemic to a few islands in the Indian Ocean, went through a severe population bottleneck in the middle of the last century, with only approximately 30 individuals left on one small island, but has since recovered. Our monitoring shows that since reaching the island's carrying capacity, population density has remained stable. However, we detected neither density-dependent reproduction nor survival on the basis of natural density variation during this stable period. For conservation reasons, new populations have been established by transferring birds to nearby suitable islands. Using the change of numbers during the process of saturation as a natural experiment, we investigated whether we can detect regulation of numbers via density-dependent survival and reproduction within these new populations. We found that populations were mainly regulated by density-dependent reproduction, and not survival. Variation in density between islands can be explained by food abundance, measured as insect density. Islands with the highest insect densities also had the highest bird densities and the largest breeding groups. Consequently, we suggest that the density-dependent effect on reproduction is caused by competition for food.

Life-history theory assumes that organisms trade-off current against future reproduction to maximize fitness. Experimental explorations of the costs of reproduction have not yielded a clear understanding of the nature of these costs but rather point to a complex set of allocation possibilities among several physiological functions and behaviors. We investigated how experimentally increased flight costs affected the trade-off between parental investment and self-maintenance in tropical house wrens, which have relatively high annual survival and multiple breeding opportunities per year. We predicted that handicapped wrens would not increase their energy expenditure but instead decrease their effort to rear young in order to maintain their own body condition. Our results largely supported these predictions: handicapped parents decreased their nestling feeding frequency but did neither alter their field metabolic rate (FMR) nor compromise their body condition as measured by basal metabolic rate (BMR) and several measures of innate immune function. Reduced feeding rates did not affect nestling body mass growth but resulted in decreased structural growth (length of tarsus). The latter result can be explained if parents shifted the type of prey brought to offspring or altered the amount of food brought per trip. The experiment-wide positive correlations among FMR, BMR, and feeding frequency are in agreement with the hypothesis that hard work requires elevated levels of BMR. These correlations, in combination with the absence of a handicap treatment effect on FMR or BMR, do not lend support for predictions from studies in the laboratory that birds compensate hard work during the day by lowering their BMR at night. Considering a complex set of allocation possibilities among several physiological functions and behaviors, we conclude that tropical wrens take out the costs of a handicap largely on their offspring quality not on self-maintenance processes. Copyright 2008, Oxford University Press.

We investigated the effects of increased egg production on body condition as well as on measures of reproductive performance in great skuas, Stercorarius skua, over two subsequent years. We experimentally increased egg production from the normal two to six eggs. Six eggs might also be produced under natural circumstances after repeated clutch loss. After the production of the last egg we measured: (i) body mass, (ii) pectoral muscle, and (iii) haematocrit, total red blood cell count and mean corpuscular volume, as indicators of body condition. We took the same measurements of control females who had produced the normal clutch of two eggs. The measurements were repeated one year after the manipulation, and survival, laying dates, clutch sizes and hatching success were recorded for up to three consecutive years. After producing six eggs, females were lighter, had smaller pectoral muscles and lower haematological values than control females. Hatching success of eggs was significantly reduced. Even one year after the experiment there were still differences in body condition. Annual survival was not affected by the manipulations, although there was an indication that survival costs depended on whether chicks were raised after the increased egg production. While pair bonds and egg sizes were not affected in the post-experimental year.. females started breeding significantly later than in the previous year. Two years after the experiment laying dates had advanced again and were not different from those of control females. This pattern of maintaining survival and egg sizes, but delaying breeding in the post-experimental year was found for two independent groups of females which had both been subjected to increased egg production. These results present evidence that increased egg production can have long-term effects on female body condition and aspects of reproduction. However, although present, the costs of extra eggs appear to have been relatively small in the great skua in comparison to the two other bird species for which inter-annual effects have been reported.

We measured the selection pressure on brood size in a recently established population of great tits (Parus major L.) in the northern Netherlands by manipulating brood size in three years (1995: n ¼ 51, 1997: n ¼ 66, 1998: n ¼ 51), and we estimated fitness consequences in terms of local survival of both offspring and parents. Enlarged broods had highest fitness; the offspring fitness component was positively affected by manipulation and the parental fitness component was unaffected. Parental survival and the probability that parents produced a second clutch were not affected by the treatment. However, parents that had raised enlarged broods produced their second clutch later in the season. Clutch size, brood size, and laying date of birds recaptured in the next breeding season were largely independent of the treatment. We conclude that there is strong evidence for selection for larger brood size and reject the individual optimization hypothesis for this population because the number of young in the nest predicts fitness independently of the manipulation history.

10 pages; International audience; In a diverse array of avian and mammalian species, experimental manipulations of clutch size have tested the hypothesis that natural selection should adjust numbers of neonates produced so as to maximize the number of via-ble offspring at the end of the period of parental care. Reptiles have not been studied in this respect, probably because they rarely display parental care. However, females of all python species brood their eggs until hatching, but they do not care for their neonates. This feature provides a straightforward way to experimentally increase or reduce clutch size to see whether the mean clutch size observed in nature does indeed maximize hatching success and/or optimize offspring phenotypes. Eggs were removed or added to newly laid clutches of Ball Pythons (Python regius) in tropical Africa (nine control clutches, eight with 50% more eggs added, six with 42% of eggs removed). All clutches were brooded by females throughout the 2-month incubation period. Experimental manipulation of clutch-size did not sig-nificantly affect the phenotypes (morphology, locomotor ability) of hatchlings, but eggs in 'enlarged' clutches hatched later, and embryos were more likely to die before hatching. This mortality was due to desiccation of the eggs, with females being unable to cover 'enlarged' clutches sufficiently to retard water loss. Our results support the notion of an optimal clutch size, driven by limitations on parental ability to care for the offspring. However, the proximate mechanisms that generate this optimum value differ from those previously described in other kinds of animals.

We collected 328 freshly laid Great Tit (Parus major) eggs from 38 clutches in 1999 to determine the relationship of whole egg weight, wet yolk weight, wet albumen weight, dry shell weight, and the occurrence of laying gaps with mean ambient temperature in the three days preceding laying, while controlling for laying date and position in the laying sequence. We also reanalyzed existing data on whole egg weight in 1978 and 1979 in relation to temperature, controlling for the same other variables. Egg weight was correlated with temperature in 1978 and 1979, but not in 1999. None of the egg components was related to temperature preceding laying, apart from a nonsignificant tendency for albumen weights to be lower at higher temperature. Egg weight increased with laying date in 1978 and 1979 after controlling for other variables. In 1999, albumen and shell weight increased seasonally, but shell weight only did so to the extent expected on the basis of the seasonal increase in whole egg weight. Yolk weight did not increase significantly seasonally, but neither did yolk weight relative to whole egg weight decrease significantly. Whole egg weight and the weights of all components varied between clutches. Most weights also varied through the laying sequence. The probability of a laying gap occurring varied between clutches, and increased with position in the laying sequence and decreased with temperature and laying date. Eggs preceding laying gaps were lighter and contained less albumen but had higher shell weights than other eggs. Eggs following laying gaps also had heavier shells relative to the size of the egg than other eggs.

In many bird species, females undergo a marked decline in body condition during the first days of the nestling period. This decline may be because brooding young chicks reduces the time available for foraging. Alternatively, it might be viewed as an adaptive way to reduce flight costs when the food demand of the brood is highest. To test these hypotheses we modified the brooding commitment of House Sparrows Passer domesticus by manipulating brood size to see if changes in time spent brooding affects adult body condition. During the nestling period, females provided on average three times as much brooding as males. Reduced broods received 14% more brooding than large broods and time spent brooding declined with brood size and chick age according to an exponential decay function. Male body condition was unaffected by brood size and remained stable throughout the reproductive period. Body condition of females with enlarged broods decreased gradually during the nestling period, whereas that of females tending reduced broods dropped abruptly and significantly upon hatching. This resulted in females with reduced broods having lower body condition during the first half of the nestling period than those with enlarged broods. The sharp drop in body condition of females with reduced broods coincided with the period that brooding was most intensive. Indeed, female body condition at the end of the nestling period was negatively correlated with the proportion of time they spent brooding during the first half of the nestling period. Thus, the probable lower homeothermic capacities of reduced broods implies a higher brooding commitment for female House Sparrows that, in turn, may reduce their opportunity to forage and consequently also their body condition.

The variation in time and energy allocation of female great tits, Parus major, was studied in five different European populations across a latitudinal gradient. Daily energy expenditure (DEE) was measured in females tending 12-day-old broods. The number of daylight hours used by the parents to collect food for the brood increased with latitude, while DEE and feeding rate per brood tended to level off with latitude. Individual variation in DEE could be explained by variation in ambient temperature (–), the duration of activity period (+) and area, but not by brood size, female body mass, brood mass or feeding rate. When the effect of ambient temperature and the duration of the activity period on the day of energy expenditure measurements were controlled for, female DEE still tended to level off with latitude. Temperature and activity alone can thus not explain the observed pattern. The present study suggests that parents at southern latitudes may be under a time constraint and do not increase energy expenditure because they have no more daylight hours available for foraging, while birds at northern latitudes may be under an energy constraint because they do not make full use of the long daylight period available.

We studied temporal variation in reproductive performance of Marsh Harriers Circus aeruginosus in two land reclamations in The Netherlands, i.e. South Flevoland and the Lauwersmeerpolder, embanked in 1968 and 1969 respectively. The number of breeding pairs in Flevoland rapidly increased to a maximum of 350 pairs (± 1 pair km-2) in 1977, followed by a sharp decline in the 1980s due to large-scale cultivation. The same trend was observed in the Lauwersmeer, although colonization was retarded and peak densities were reached later. In both study areas mean clutch size as well as the number of fledglings per nest decreased in the course of the twenty years of study. Two factors were responsible for this decline in reproductive output: (1) decrease of food abundance in the course of the years, and (2) an increase of nest predation, mainly by the Red Fox Vulpes vulpes. Superimposed on these long-term changes, annual fluctuations in density of the Common Vole Microtus arvalis had a considerable effect on the number of breeding pairs, as well as the fledgling production. Mean annual clutch size was associated positively, and laying date negatively, with the average annual temperature during the pre-laying phase. Our results indicate that the early stages of succession, during the first decade after reclamation, are characterized by high prey abundance (i.e. vole 'plagues') and low densities of ground predators, and offer favorable breeding conditions for Marsh Harriers. The simultaneous negative effects in recent years of less breeding habitat, decreased prey abundance and increased predation on reproductive output, exert pressure on these populations. The intense nest predation (50% of all nests), as recorded in the Lauwersmeer since 1990, may eventually cause local extinction in the absence of, relatively safe, inundated breeding habitat.