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Migratory connectivity is a metric of the co-occurrence of migratory animals originating from different breeding sites, and like their spatio-temporal distributions, can vary substantially during the annual cycle. Together, both these properties affect the optimal times and sites of population censusing.We tracked taiga bean geese (Anser fabalis fabalis) during 2014–2021 to study their migratory connectivity and non-breeding movements, and determine optimal periods to assess the size of their main flyway population. We also compared available census data with tracking data, to examine how well two existing censuses covered the population.Daily Mantel’s correlation between breeding and non-breeding sites lay between 0 and 0.5 during most of the non-breeding season, implying birds from different breeding areas were not strongly separated other times in the annual cycle. However, the connectivity was higher among birds from the westernmost breeding areas compared to the birds breeding elsewhere. Daily Minimum Convex Polygons showed tracked birds were highly aggregated at census times, confirming their utility. The number of tracked birds absent at count sites during the censuses however exceeded numbers double-counted at several sites, indicating that censuses might have underestimated the true population size.Our results show that connectivity can vary in different times during the non-breeding period, and should be studied throughout the annual cycle. Our results also confirm previous studies, which have found that estimates using marked individuals usually produce higher population size estimates than total counts. This should be considered when using total counts to assess population sizes in the future.

Telomere length is increasingly used as a biomarker of long-term somatic state and future survival prospects. While most studies have overlooked this aspect, biological interpretations based on a given telomere length will benefit from considering the level of within-individual repeatability of telomere length through time. Therefore, we conducted a meta-analysis on 74 longitudinal studies in nonmammalian vertebrates, with the aim to establish the current pattern of within-individual repeatability in telomere length and to identify the methodological (e.g., qPCR/TRF) and biological factors (e.g., age class, phylogeny) that may affect it. While the median within-individual repeatability of telomere length was moderate to high (R = 0.55; 95% CI: 0.05-0.95; N = 82), marked heterogeneity between studies was evident. Measurement method affected the repeatability estimate strongly, with TRF-based studies exhibiting high repeatability (R = 0.80; 95% CI: 0.34-0.96; N = 25), while repeatability of qPCR-based studies was markedly lower and more variable (R = 0.46; 95% CI: 0.04-0.82; N = 57). While phylogeny explained some variance in repeatability, phylogenetic signal was not significant (λ = 0.32; 95% CI: 0.00-0.83). None of the biological factors investigated here significantly explained variation in the repeatability of telomere length, being potentially obscured by methodological differences. Our meta-analysis highlights the high variability in within-individual repeatability estimates between studies and the need to put more effort into separating technical and biological explanations. This is important to better understand to what extent biological factors can affect the repeatability of telomere length and thus the interpretation of telomere length data.

Changing climate can modify predator-prey interactions and induce declines or local extinctions of species due to reductions in food availability. Species hoarding perishable food for overwinter survival, like predators, are predicted to be particularly susceptible to increasing temperatures. We analysed the influence of autumn and winter weather, and abundance of main prey (voles), on the food-hoarding behaviour of a generalist predator, the Eurasian pygmy owl (Glaucidium passerinum), across 16 years in Finland. Fewer freeze-thaw events in early autumn delayed the initiation of food hoarding. Pygmy owls consumed more hoarded food with more frequent freeze-thaw events and deeper snow cover in autumn and in winter, and lower precipitation in winter. In autumn, the rotting of food hoards increased with precipitation. Hoards already present in early autumn were much more likely to rot than the ones initiated in late autumn. Rotten food hoards were used more in years of low food abundance than in years of high food abundance. Having rotten food hoards in autumn resulted in a lower future recapture probability of female owls. These results indicate that pygmy owls might be partly able to adapt to climate change by delaying food hoarding, but changes in the snow cover, precipitation and frequency of freeze-thaw events might impair their foraging and ultimately decrease local overwinter survival. Long-term trends and future predictions, therefore, suggest that impacts of climate change on wintering food-hoarding species could be substantial, because their 'freezers' may no longer work properly. Altered usability and poorer quality of hoarded food may further modify the foraging needs of food-hoarding predators and thus their overall predation pressure on prey species. This raises concerns about the impacts of climate change on boreal food webs, in which ecological interactions have evolved under cold winter conditions.

Specialist individuals within animal populations have shown to be more efficient foragers and/or to have higher reproductive success than generalist individuals, but interspecific reproductive consequences of the degree of diet specialisation in vertebrate predators have remained unstudied. Eurasian pygmy owls (hereafter POs) have less vole‐specialised diets than Tengmalm's owls (TOs), both of which mainly subsist on temporally fluctuating food resources (voles). To test whether the specialist TO is more limited by the main prey abundance than the generalist PO, we studied breeding densities and reproductive traits of co‐existing POs and TOs in central‐western Finland during 2002–2019. Breeding densities of POs increased with augmenting densities of voles in the previous autumn, whereas breeding densities of TOs increased with higher vole densities in both the previous autumn and the current spring. In years of vole scarcity, PO females started egg‐laying earlier than TOs, whereas in years of vole abundance TO females laid eggs substantially earlier than PO females. The yearly mean clutch size and number of fledglings produced of both POs and TOs increased with abundance of voles in the current spring. POs laid large clutches and produced large broods in years of both high and low vole abundance, whereas TOs were able to do so only in years of high vole abundance. POs were able to raise on average 73% of the eggs to fledglings whereas TOs only 44%. The generalist foraging strategy of POs including flexible switching from main prey to alternative prey (small birds) appeared to be more productive than the strictly vole‐specialized foraging strategy of TOs. In contrast to earlier studies at the individual‐level, specialist predators at the species level (in this case TOs) appear to be less effective than generalists (POs), but diet specialisation was particularly costly under conditions when scarcity of main foods limited offspring production.

Many organisms adjust their reproductive phenology in response to climate change, but phenological sensitivity to temperature may vary between species. For example, resident and migratory birds have vastly different annual cycles, which can cause differential temperature sensitivity at the breeding grounds, and may affect competitive dynamics. Currently, however, adjustment to climate change in resident and migratory birds have been studied separately or at relatively small geographical scales with varying time series durations and methodologies. Here, we studied differential effects of temperature on resident and migratory birds using the mean egg laying initiation dates from 10 European nest box schemes between 1991 and 2015 that had data on at least one resident tit species and at least one migratory flycatcher species. We found that both tits and flycatchers advanced laying in response to spring warming, but resident tit populations advanced more strongly in relation to temperature increases than migratory flycatchers. These different temperature responses have already led to a divergence in laying dates between tits and flycatchers of on average 0.94 days per decade over the current study period. Interestingly, this divergence was stronger at lower latitudes where the interval between tit and flycatcher phenology is smaller and winter conditions can be considered more favorable for resident birds. This could indicate that phenological adjustment to climate change by flycatchers is increasingly hampered by competition with resident species. Indeed, we found that tit laying date had an additional effect on flycatcher laying date after controlling for temperature, and this effect was strongest in areas with the shortest interval between both species groups. Combined, our results suggest that the differential effect of climate change on species groups with overlapping breeding ecology affects the phenological interval between them, potentially affecting interspecific interactions., T.E. acknowledges the funding by the Academy of Finland (project 265859). MM and RM were financially supported by the Estonian Research Council (institutional research funding IUT number 34‐8). I.K. received financial support from the Estonian Ministry of Education and Science (grant PUT1223). The data collection in Russia was financially supported by RSF grant number 14‐50‐00029. JP was supported by projects CGL2014‐55969‐P and CGL2015‐70639‐P of the Spanish Ministry of Economy and Competitiveness.

In a substantial number of species, females show some development of secondary sexual characters. These traits can function as signals of individual phenotypic or genetic qualities and status to conspecifics. Individuals may benefit potentially from expressing signals or badges of status if they are reliable and honest signals of individual quality. In many species, badge sizes have been shown to correlate with dominance rank, which may be mediated by testosterone (T) levels. Here, we explored geographic variation in the size and properties of the white wing patch of female pied flycatchers Ficedula hypoleuca and its relation to circulating T levels in three populations (two southern populations in central Spain and a northern population in Finland). Furthermore, we aimed at detecting if the size of the white wing patch and its ultraviolet (UV) reflectance indicate individual quality. We found that females in Spain had larger, brighter and more UV reflecting wing patches than those in Finland. Females with higher UV reflectance and larger primary white patches bred earlier. Younger females and females with larger primary white wing patches showed higher T levels. In contrast, higher values of UV reflectance in feathers from these patches were associated with low T levels. Despite genetic differentiation and differences in trait expression between populations, female pied flycatchers from different populations may converge and use the size of white wing patches to signal their T levels and thereby their social dominance., This study was financed by project CGL2013-48193-C3-3-P to JM from Spanish Ministerio de Economía y Competitividad (MINECO).

Mothers may affect the future success of their offspring by varying allocation to eggs and embryos. Allocation may be adaptive based on the environmental conditions perceived during early breeding. We investigated the effects of food supplementation and predation risk on yolk hormone transfer in the pied flycatcher Ficedula hypoleuca. In a food supplementation experiment, females were food-supplemented prior to and during egg-laying and androgen concentrations were measured throughout the laying order. Predation risk was investigated in three different studies combining both correlative data, where flycatchers bred in close proximity to two different predator species that prey upon adult flycatchers (either Tengmalm's owl Aegolius funereus or pygmy owl Glaucidium passerinum), and an experimental manipulation, where flycatchers were exposed to cues of a nest predator (least weasel Mustela nivalis). Females receiving food supplementation laid eggs with lower concentrations of androstenedione (A4) than females not receiving food supplements. Yolk testosterone (T) concentration showed the same pattern but the difference was not statically significant. Testosterone (but not A4) concentration increased within clutches, from the first to the last egg, independently of the food supplementation. Females breeding under high predation risk did not differ from control females in their yolk androgen levels (A4, T or progesterone). However, concentrations of A4 tended to be lower in the proximity of pygmy owls, which could indirectly increase offspring survival after fledging. Food supplementation during egg-laying seems to have a stronger impact on maternal transfer of androgens than predation risk. Food availability and predation risk could differentially affect the trade-offs of androgen allocation for the offspring when raised in good vs. dangerous environments.

Understanding what drives or prevents long-distance migrants to respond to environmental change requires basic knowledge about the wintering and breeding grounds, and the timing of movements between them. Both strong and weak migratory connectivity have been reported for Palearctic passerines wintering in Africa, but this remains unknown for most species. We investigated whether pied flycatchers Ficedula hypoleuca from different breeding populations also differ in wintering locations in west-Africa. Light-level geolocator data revealed that flycatchers from different breeding populations travelled to different wintering sites, despite similarity in routes during most of the autumn migration. We found support for strong migratory connectivity showing an unexpected pattern: individuals breeding in Fennoscandia (S-Finland and S-Norway) wintered further west compared to individuals breeding at more southern latitudes in the Netherlands and SW-United Kingdom. The same pattern was found in ring recovery data from sub-Saharan Africa of individuals with confirmed breeding origin. Furthermore, population-specific migratory connectivity was associated with geographical variation in breeding and migration phenology: birds from populations which breed and migrate earlier wintered further east than birds from late' populations. There was no indication that wintering locations were affected by geolocation deployment, as we found high repeatability and consistency in C-13 and N-15 stable isotope ratios of winter grown feathers of individuals with and without a geolocator. We discuss the potential ecological factors causing such an unexpected pattern of migratory connectivity. We hypothesise that population differences in wintering longitudes of pied flycatchers result from geographical variation in breeding phenology and the timing of fuelling for spring migration at the wintering grounds. Future research should aim at describing how temporal dynamics in food availability across the wintering range affects migration, wintering distribution and populations' capacity to respond to environmental changes.

The hormonal environment during early development, such as maternally derived androgens in bird eggs, has been shown to shape offspring in ways that may have important short- and long-term consequences for phenotype and behavior. However, studies on the effects of androgens on begging behavior and growth in birds have provided variable results and sex-specific effects are not well understood. We experimentally elevated yolk androgen levels in whole clutches in a passerine bird, the pied flycatcher Ficedula hypoleuca and studied effects on male and female begging behavior, ingestion and digestion. Yolk androgen elevation led to shorter latencies to beg in both male and female nestlings but to greater begging rates only in males. Both responses could be used as cues to solicit more food from parents. Yolk androgens had no effect on food intake, fecal wet mass or fecal water content. However, the proportion of organic matter in feces was lower in androgen-treated than control males, potentially suggesting that the androgen-treated individuals had more efficient digestion. We discuss the alternative pathways of effects of yolk androgens on begging, growth and digestion. If the effects of yolk androgens on phenotype and behavior are causally linked, there is less scope for the mother to adjust specific offspring traits independently from other traits. The sex-specific effects of yolk androgens should be studied more, also in within-clutch context, as the costs and benefits for each sex may further differ in relation to hatching order.

Forest age structure has become younger and the amount of old forest has decreased due to intensive forestry in Northern Europe. Consequently, large forest-dwelling hawks may lack nest sites as they need large trees to support their nests. Construction of artificial nests has been a globally adopted conservation measure to provide nest sites for several raptors, but the impact of this practice has seldom been evaluated. We compared the nesting success of northern goshawk Accipiter gentilis, common buzzard Buteo buteo, and honey buzzard Pernis apivorus in natural and artificial nests based on long-term data in Finland. The probability of successful nesting of goshawk and common buzzard was lower in artificial nests, whereas no difference was found in the nesting success of honey buzzard between the nest types. Brood size did not differ between natural and artificial nests in successful nests of any of the three species. Finally, distances to anthropogenic features had no effect on nesting success. These results indicate that hawks switching from natural to artificial nests are potentially lured to an ecological trap with higher risk of nesting failure. However, breeding at artificial nests, even with lower success than in natural nests, can still add to the population, if otherwise nonbreeding individuals settle to nest. It should be investigated which part of the population breeds in artificial nests and whether the site selection for artificial nests needs revision. Our results emphasize the importance of monitoring and evaluation of well-intentioned conservation efforts.

Given the unprecedented biodiversity crisis and scarcity of conservation resources, each single conservation effort must be allocated cost-effectively. Therefore, there is a growing need to evaluate the effectiveness of such efforts, particularly in the case of large and charismatic predators that have attracted a large share of species conservation funding. In Finland, conservation interventions aimed at protecting nest sites of the white-tailed eagle Haliaeetus albicilla were initiated based on general concerns of increasing disturbance posed by human encroachment and forestry practices. We evaluated the current effectiveness of this conservation program. We found that neither nest occupancy nor breeding success of white-tailed eagles were affected by the protection status of the land in which the nest was located (protected areas or unprotected land), or by the size of the area under protection. In addition, the type of forest management at the nest site or the presence and vicinity of anthropogenic infrastructures (roads and buildings) did not have any apparent effect on nesting eagles. Our results are encouraging, as they show that the white-tailed eagle breeds successfully in landscapes under intensive forestry, and thus the future of the species appears not to be threatened by forest management. Our results furthermore allow updating the allocation of resources for nature conservation in Finland, potentially by prioritizing more cost-effective conservation programs on other species and ecosystems.

The genetic theory of morphological evolution postulates that form evolves largely by changing the expression proteins that are functionally conserved. It follows that understanding the function of proteins during different phases of development as well as the mechanisms by which the functions are modified is a prerequisite for understanding evolutionary change. Male pied flycatchers exhibit marked phenotypic variation in their breeding plumage. This variation has repeatedly been shown to have adaptive significance, but the molecular basis of this variation is not known. Here, we characterize the proteome of developing pied flycatcher feathers from differently pigmented males and also introduce a new method for examining the effect sizes of expression differences in protein interaction networks. Approximately 300 proteins were identified in the developing feathers of males. Gene products associated with cellular transport, cell metabolism and protein synthesis formed a large part of the developing feather proteome. Sixty-five proteins associated with the development of the epidermis and/or pigmentation were detected in the data. The examination of expression level differences of protein–protein interaction networks revealed an immunological signalling– related network to exhibit significantly higher expression in black compared to brown males. Additionally, indications of differences in energy balance and oxidative stress related characteristics were detected. Together, these results provide new insight into the molecular mechanisms and evolutionary significance of plumage colour variation.

Summary 1. Early growth conditions, for example hormonal environment during embryonic development, may have long-lasting effects on behaviour and phenotype, with subsequent fitness consequences. Yolk androgens have been shown to affect various offspring traits in the short-term, but fitness consequences for either offspring or parents, a prerequisite for studying the adaptiveness of this maternal effect, are poorly known in the wild. 2. We experimentally elevated yolk androgen levels of whole clutches in a wild population of collared flycatchers (Ficedula albicollis) and investigated the long-term effects of yolk androgens on offspring local recruitment, parental return rate, and timing and success of breeding in both parents and offspring in the breeding season(s) following the manipulation. 3. Yolk androgen elevation lowered the local recruitment rate of male, but not female offspring, but had no effect on any breeding parameters of offspring of either sex. Furthermore, yolk androgen elevation of the clutch had no effect on the return rate or any breeding parameters of the parents. 4. Our results indicate that high yolk androgen levels may impose a potential direct fitness cost for male offspring, but no long-lasting additional indirect fitness costs for parents (direct costs of transferring androgens to eggs not considered). Such a sex-specific cost on offspring could constrain yolk androgen deposition and select for sex-specific deposition mechanisms. 5. As yolk androgen levels in this population are heritable, our results on long-term fitness effect of yolk androgens support the idea that hormone-mediated maternal effects may evolve under selection and thereby affect evolutionary processes.

Cost-effective conservation strategies are needed because resources for nature conservation are often limited. When areas of high biodiversity value rest on private land, voluntary participation of landowners to conservation should be sought. Voluntary programs are still rarely implemented and evaluated in their effectiveness in achieving conservation targets. From 2000 onwards, a conservation program was conducted to preserve raptor nests from forest clear-cutting in private lands of North Karelia (Finland). Landowners were proposed to voluntarily set aside a forest buffer around nests of Accipiter gentilis, Buteo buteo and Pernis apivorus on their land. Participation was based on self-motivation, with no incentive involved. Participation to the program was extremely high (97% of 327 approached landowners during 2002–2006 joined). This positive attitude reduced nest losses as a result of forestry from 54% in the 1990s (n = 327 known nests) to only 2% since the start of the program (n = 519 nests). The voluntary action proved effective as a before-after-control-impact analysis showed that low-buffered nests (which had clear-cutting within 100 m; n = 48) had similar occupancy as high-buffered nests (no clear-cutting within 100 m; n = 144). Occupancy, however, decreased with shorter distance to the clear-cut within the 100 m radius, and thus as wide a buffer as possible is recommended to retain occupancy of a nest site. We conclude that voluntary-based approaches can be useful for cost-effective conservation and they should be considered more often in conservation practice.

1. Climate warming has led to shifts in the seasonal timing of species. These shifts can differ across trophic levels, and as a result, predator phenology can get out of synchrony with prey phenology. This can have major consequences for predators such as population declines owing to low reproductive success. However, such trophic interactions are likely to differ between habitats, resulting in differential susceptibility of populations to increases in spring temperatures. A mismatch between breeding phenology and food abundance might be mitigated by dietary changes, but few studies have investigated this phenomenon. Here, we present data on nestling diets of nine different populations of pied flycatchers Ficedula hypoleuca, across their breeding range. This species has been shown to adjust its breeding phenology to local climate change, but sometimes insufficiently relative to the phenology of their presumed major prey: Lepidoptera larvae. In spring, such larvae have a pronounced peak in oak habitats, but to a much lesser extent in coniferous and other deciduous habitats. 2. We found strong seasonal declines in the proportions of caterpillars in the diet only for oak habitats, and not for the other forest types. The seasonal decline in oak habitats was most strongly observed in warmer years, indicating that potential mismatches were stronger in warmer years. However, in coniferous and other habitats, no such effect of spring temperature was found. 3. Chicks reached somewhat higher weights in broods provided with higher proportions of caterpillars, supporting the notion that caterpillars are an important food source and that the temporal match with the caterpillar peak may represent an important component of reproductive success. 4. We suggest that pied flycatchers breeding in oak habitats have greater need to adjust timing of breeding to rising spring temperatures, because of the strong seasonality in their food. Such between-habitat differences can have important consequences for population dynamics and should be taken into account in studies on phenotypic plasticity and adaptation to climate change.

Costs and benefits of dispersal can vary in space and time, depending on environmental factors and individual state. Plastic, condition-dependent dispersal strategies, in which individuals rely on external cues such as food abundance to adjust their dispersal distances, are therefore expected to evolve in temporally fluctuating environments. We examined factors affecting breeding dispersal distances in Eurasian kestrels Falco tinnunculus subsisting on multi-annually and cyclically fluctuating voles as their main food. We attempted to avoid traditional bias in dispersal studies by having large study areas and by taking detection probabilities into account. We observed 320 dispersal events of male and 215 events of female kestrels from our study areas in western Finland during 24 yr. After correcting for distance-specific detection probability, the estimates of mean dispersal distances increased two-fold being still clearly higher for females than males. Vole abundance in the spring of settlement was more important in determining average dispersal distances than vole abundance in the previous autumn. At the population level (cross-sectional model), both males and females dispersed longer distances when the spring abundance of their main food (voles) was low compared to when it was abundant, as predicted by the food depletion hypothesis. At the individual level (longitudinal model), only females responded to the food situation by dispersing more when food abundance was low in the spring of settlement. Females also dispersed longer when vole abundance in the previous autumn had been high. Individual males did not respond to vole abundance, which implies that the population level response in males might have been caused by long-distance dispersers, which breed in the study area only in good vole years. Our results show that the dispersal distances of kestrels at northern latitudes depend both on individual properties (gender, age, and possibly individual tendency to disperse) and environmental conditions (temporal variation in main food abundance).

Fluctuating selection pressure may maintain phenotypic variation because of different types of individuals being adapted to different environmental conditions. We show that the extensive variation in the coloration of male pied flycatchers (Ficedula hypoleuca) can be maintained through differences in the reproductive success of male phenotypes under different conditions. The effects of weather conditions on the relative success of different male phenotypes varied between different phases of breeding. The reproductive output of black males was the highest when it was cold during egg-laying but warm during the nestling period, whereas the fledgling production of brown males was highest when it was continuously warm. In addition, male forehead and wing patch sizes had context-dependent effects on timing of breeding and nestling mortality, respectively. These results indicate that environmental heterogeneity plays a role in maintaining phenotypic variation. As melanin-based coloration is heritable, climate change may alter phenotype frequencies depending on the patterns of warming.

Large-scale climate fluctuations, such as the North Atlantic Oscillation (NAO), have a marked effect on the timing of spring migration of birds. It has however been suggested that long-distance migrants wintering in Africa could respond less to NAO than short-distance migrants wintering in Europe, making them more vulnerable to climatic changes. We studied whether migratory boreal and arctic bird species returning from different wintering areas show differences in responses to the NAO in the timing of their spring migration. We used data on 75 species from two bird observatories in northern Europe (60°N). By extending the examination to the whole distribution of spring migration and to a taxonomically diverse set of birds, we aimed at finding general patterns of the effects of climate fluctuation on the timing of avian migration. Most species arrived earlier after winters with high NAO index. The degree of NAO-response diminished with the phase of migration: the early part of a species’ migratory population responded more strongly than the later part. Early phase waterfowl responded strongest to NAO, but in later phases their response faded to non-significant. This pattern may be related to winter severity and/or ice conditions in the Baltic. In the two other groups, gulls and waders and passerines, all phases of migration responded to NAO and fading with phase was non-significant. The difference between waterfowl and other groups may be related to differences between the phenological development of their respective macrohabitats. Wintering area affected the strength of NAO response in a complicated way. On average medium distance migrants responded most strongly, followed by short-distance migrants and partial migrants. Our results concerning the response of long-distance migrants were difficult to interpret: there is an overall weak yet statistically significant effect, but patterns with phase of migration need further study. Our results highlight the importance of examining the whole distribution of migration and warrant the use of data sets from several sampling sites when studying climatic effects on the timing of avian life-history events.

Migratory species are of special concern in the face of global climate change, since they may be affected by changes in the wintering area, along the migration route and at the breeding grounds. Here we show that migration and breeding times of a trans-Saharan migrant, the pied flycatcher Ficedula hypoleuca, closely follow local temperatures along the migration route and at the breeding grounds. Because of differences in long-term temperature trends of short within-spring periods, the migration period and the time interval between migration and breeding dates of this species have extended in SW Finland. Temperatures in northern parts of Central Europe have risen at the time when the first migrants arrive there, facilitating their migration northward. Temperatures later in the spring have not changed, and the last individuals arrive at the same time as before. The timing of breeding has not advanced because temperatures at the breeding site after arrival have not changed. These results show that the pied flycatchers can speed up their migration in response to rising temperatures along the migration route. Our results strongly indicate that the effects of climate change have to be studied at the appropriate time and geographical scales for each species and population concerned.

Summary 1 Offspring sex ratio may be an unpredictable component of life history that might select for behavioural plasticity in parental care. If the parents do not have such plasticity and the two sexes of offspring differ in size, individuals in a brood or litter biased towards the larger sex offspring may suffer from food shortage. Sibling sex ratio could also affect the young through asymmetric sex-related sibling competition. 2 We created single-sex broods of sexually size-dimorphic Eurasian kestrels (Falco tinnunculus L.) with mixed-sex control broods in order to test whether parents have behavioural plasticity to respond to the sex ratio of their brood and whether smaller male chicks suffer from reduced health status because their larger female siblings outrival them in sibling competition. The experiment was conducted during two years (2000 and 2001) that differed in the abundance of voles, the main natural prey of kestrels. 3 No obvious effects of the brood sex ratio manipulation on parental effort were detected, as there were no differences in prey delivery rate, biomass of prey brought to the nest, body mass or heterophile/lymphocyte ratio of the parents. 4 Female chicks in all-female broods had lower haematocrit levels than those in mixed-sex broods in a year of low vole abundance. As haematocrit is an index of nutritional condition and health state, this result indicates that in the year of low vole abundance the female chicks in all-female broods fledged in poorer condition than those in mixed-sex broods. 5 These results indicate that when the parents do not alter their level of parental effort in response to the sex ratio of their brood, the young in a brood biased towards the larger sex offspring can be reared successfully when food is abundant, while they may suffer under food shortage situations. 6 Kestrel chicks of both sexes had low haematocrit levels and T cell-mediated immune responses in the year of low vole abundance. This result indicates that poor health state at fledging may provide a causal link for low recruitment of kestrels hatched in years of low vole abundance.

Many birds begin to incubate before their clutch is full, which results in the chicks hatching at different times. I propose that hatching asynchrony could serve as an adaptive parental strategy to produce phenotypic variation in the offspring through asymmetric sibling competition. Producing diverse offspring that follow variable life history strategies might be a risk-spreading strategy in spatially and temporally variable environments.

Summary 1. Two main mechanisms have been proposed to account for the changes in breeding performance of long-lived animals with age: progressive appearance or disappearance of different quality individuals within a cohort and within-individual improvements with age. 2. We studied age-related breeding performance of vole-eating Tengmalm’s owls ( Aegolius funereus , L.), whose prey populations fluctuate in a cyclic manner in western Finland. In years of low vole abundance, most yearling owls were not able to breed. In years of initially intermediate but increasing vole abundance, clutch size increased with female age and partners of old males initiated egg-laying earlier than those of young males. In years of initially high but decreasing vole abundance, the differences in laying dates were still detectable, whereas age-related differences in clutch size of females tended to disappear. 3. Within-cohort analyses did not indicate quality differences between surviving and dying males of the same age in years of increasing vole abundance, whereas in years of decreasing vole abundance early breeding males were more likely to survive than late-breeding males. This novel finding indicates that quality differences between individuals may induce observed age-related differences in only some years, and especially that differential mortality may occur in a different year when the difference in breeding performance is detectable. 4. Within-individual analyses showed that individual males advanced their nest initiation with age, and there was a trend that females laid larger clutches with increasing age. In nests of long-lived males, a significant decline in the clutch size from mid-age (2‐ 4 years) to older age (6‐10 years) was found. 5. We conclude that environmental variation may mask age-related differences in breeding performance at the population level, since the differences appeared to emerge more clearly in poor and intermediate food conditions. Some evidence for the differential mortality-hypothesis was found for males, which are mainly responsible for subsisting their families during the entire breeding season. Substantial improvements in competence or increased reproductive effort from 1 to 3 years of age, and deteriorated skills due to senescence at older ages may account for within-individual changes in breeding performance of males.

When resources are limited, parents should decide the optimal number, size, and sex of progeny, and offspring should decide the optimal allocation of resources to different costly functions, such as growth and immunity. We manipulated clutch sizes of Eurasian kestrels by one egg to estimate possible cumulative effects of incubation and chick rearing costs on parental body condition, feeding effort, and offspring viability. No obvious effects of clutch size manipulations on feeding effort were found while feeding effort was adjusted to the original clutch size. Enlarged and control nests suffered from higher nestling mortality than reduced nests, and chicks of the enlarged group were in poorer body condition than chicks of the reduced group. Controlling for body mass, male chicks exhibited lower cell-mediated immunity assessed by a cutaneous hypersensitivity response than females, but only in treatments suffering from food restrictions, as indicated by chick starvation. These novel results reveal inter-sexual differences in physiological strategies in early life, suggesting sex-related differences in susceptibility to disease and consequently in survival prospects of offspring.