Daily variation in markers of nutritional condition in wintering Black‐capped Chickadees Poecile atricapillus

The plasma metabolites triglycerides (TRIG) and β‐hydroxybutyrate (BUTY) are used as indices of nutritional condition in migrating birds during refuelling and can provide a measure of relative fattening rates in individual birds. Because non‐migratory birds wintering at northern latitudes also fatten on a daily basis to support their overnight fast, blood metabolites could provide a useful tool to measure individual performance in energy acquisition. However, daily patterns of metabolite change may differ between species and could be affected by thermoregulatory requirements. We studied daily variation in TRIG and BUTY over a complete winter in Black‐capped Chickadees to determine the pattern of daily and seasonal change in these markers. We also assessed how short‐term variation (up to 7 days) in weather parameters that influence heat exchange may affect TRIG and BUTY levels. In contrast to a linear gain of body mass, TRIG increased non‐linearly during the day, with a rapid increase in the morning that levelled off in the afternoon, whereas BUTY did not change significantly. Metabolites varied with sampling time and the seasonal change in day length, suggesting higher fat catabolism and fattening rates in mid‐winter. TRIG and BUTY also differed between capture sites, possibly due to differences in shelter quality. Weather variation did not affect TRIG levels and had a significant but marginal effect on BUTY, explaining at best 3% of the variation. Our results suggest that these markers can be used as indicators of energy turnover in resident wintering passerines.     

Pre-incubation feeding activities and energy budgets of Snow Geese: can food on the breeding grounds influence fecundity?

The potential contribution of early spring feeding conditions in the Arctic to clutch size variation was examined in a population of Lesser Snow Geese Anser caerulescens caerulescens. Behavioural observations were combined with energetic analyses of food material to derive an estimate of the energy budgets of pre-laying and laying females. Food intake of females between arrival on the breeding grounds and incubation was considerable; estimated energy gains in this period were in the same magnitude as the cost of one or several eggs. The pre-laying period spent on the breeding grounds can thus be energetically beneficial rather than costly. Accumulation of resources for reproduction in Snow Geese is a continual process including the breeding grounds, and nutrient limitation after arrival in the Arctic cannot sufficiently explain the environmental component of clutch size variation. The timing of migration and follicle development is such that clutch size decisions are sometimes made during the late stages of migration and some-times after arrival. In the latter case food conditions on the breeding grounds may greatly influence clutch size; in the former case they may still influence readjustments of clutch size after the initial decision. The universal negative correlation between clutch size and laying date in Snow Geese can be explained by negative fitness consequences of late hatching, which outweigh the benefits of delayed laying and further nutrient accumulation. Food shortage on the breeding grounds may sometimes be a secondary factor contributing to seasonal clutch size decline.     

Sex-Related Effects of Reproduction on Biomarkers of Oxidative Damage in Free-living Barn Swallows (Hirundo rustica)

According to life-history theory, the allocation of limiting resources to one trait has negative consequences for other traits requiring the same resource, resulting in trade-offs among life-history traits, such as reproduction and survival. In vertebrates, oxidative stress is increasingly being considered among the physiological mechanisms forming the currency of life-history trade-offs. In this study of the barn swallow (Hirundo rustica), we focus on the oxidative costs of reproduction, especially egg laying, by investigating the effects of breeding stage (pre- vs. post-laying) and progression of the season on three biomarkers of oxidative damage (OD) to plasma proteins, namely the concentration of malondialdehyde (MDA)-protein adducts and of protein thiol groups (PSH), and the protein carbonyl (PCO) content. Moreover, we investigated whether males and females differed in plasma OD levels, because the inherent sex differences in reproductive roles and physiology may originate sex-specific patterns of OD during breeding. We found that MDA-protein adduct levels were higher in the pre-laying than in the post-laying phase, that males had lower levels of MDA-modified proteins than females, and that the decline of MDA-protein adduct concentration between the pre- and the post-laying phase was more marked for females than males. In addition, MDA-protein adduct levels declined with sampling date, but only during the pre-laying phase. On the other hand, plasma PCO levels increased from the pre- to the post-laying phase in both sexes, and females had higher levels of PCO than males. PSH concentration was unaffected by breeding stage, sex or sampling date. On the whole, our findings indicate that biomarkers of protein oxidation closely track the short-term variation in breeding stage of both male and female barn swallows. Moreover, the higher protein OD levels observed among females compared to males suggest that egg laying entails oxidative costs, which might negatively affect female residual reproductive value.     

Body reserves and nutritional needs during laying preparation in barn owls

To investigate the composition of the body reserves made during pre-laying by breeding European barn owls (Tyto alba), we have analysed the body composition of captive breeding and non-breeding females sacrificed during the laying period. The data obtained were compared to the daily requirement for egg formation obtained by an egg composition analysis and the timing of egg synthesis. This study demonstrates that body mass gain observed in breeding females (+38.3 g after eggs in formation and gonadal tractus were removed) was not the consequence of an accumulation of body fuels like lipids but of mainly water and lean material. The lipidic reserves were found to be less important in breeding than in non-breeding females and their localisation modified; lipids were absent from medullar bones in breeding females which liberated room for other storage. The subcutaneous tissue, which was homogeneous in non-breeding females, was located principally under the brood patch in breeding females. Nutrients and energy required during egg formation could be obtained without modification of daily food intake. These results show that a laying event can be initiated in 14 days and that the onset of reproduction is not triggered by body condition in barn owls. The water accumulation observed is suggested to be the mere consequence of an increase of protein metabolism (egg and moult). These results intimate that the body mass increase observed in diurnal and nocturnal raptors during laying preparation, interpreted as an energy safety strategy, ought to be reconsidered. Accepted: 25 February 2000     

Body size and determinants of laying date variation in the Snow Petrel Pagodroma nivea

We studied several determinants of laying date variation and the relationship between laying date and reproductive success in the Snow Petrel Pagodroma nivea . The effects of female body size and condition, year, individual laying period, colony size, mate fidelity, previous reproductive success, and duration of the pre-laying exodus on laying date, were investigated during a 3-year study. The average laying date was 4 December. The laying period was compressed into 10–16 days and was very constant from year to year, both for the population as a whole and for individual females. The laying period of individual females varied from year to year on average by less than one day. Body size explained 24% of the variation in laying dates, with large females laying their egg later than small ones. Laying in large colonies occurred c. 2 days later than in small colonies, mainly because a higher proportion of large females bred in large colonies. There was no effect of mate fidelity, age, body condition and previous reproductive success on laying date, but the duration of the pre-laying exodus was strongly correlated with laying date. Smaller females had shorter pre-laying exodus (c. 17.7 days) than larger ones (c. 20.4 days). During the three years of the study reproductive success either increased, decreased or did not vary with laying date. Although body size is not maintained by selection on laying date alone, the genetic body size component of this species suggests that balancing selection on body size may act through laying date.     

The adjustment of reproductive threshold to prey abundance in a capital breeder

1. Many organisms rely upon stored energy reserves to support reproduction and do not initiate breeding until their reserves exceed some 'reproductive threshold'. However, the determinants of such thresholds are poorly understood; for example, we do not know if they are fixed (invariant) or respond dynamically to fluctuations in resource availability.
2. An 8-year field study on water pythons ( Liasis fuscus Peters 1873) in tropical Australia shows that individual female pythons adjust their reproductive thresholds in response to annual variation in prey abundance.
3. In every year of the study, female pythons that reproduced were in better condition (mass relative to body length) than non-reproductive females. However, in years with low abundance of rats, female pythons reproduced despite being in relatively poor condition. Indeed, the mean condition of reproductive pythons in one particularly 'bad' year was as low as the mean condition of non-reproductive females in a 'good' year.
4. Clutch sizes were slightly reduced in a 'bad' year, but the main effect of the lowered threshold was greater emaciation of the females after laying.
5. Recapture records of marked snakes show that the annual variation in thresholds is due to flexibility of individual females, not to differential representation of cohorts with different reproductive thresholds.
6. The dynamic adjustment of threshold levels fits well with predictions from life-history models and is likely to be a widespread phenomenon.     

Individual heterogeneity in fitness in a long‐lived herbivore

Heterogeneity in the intrinsic quality and nutritional condition of individuals affects reproductive success and consequently fitness. Black brant (Branta bernicla nigricans) are long‐lived, migratory, specialist herbivores. Long migratory pathways and short summer breeding seasons constrain the time and energy available for reproduction, thus magnifying life‐history trade‐offs. These constraints, combined with long lifespans and trade‐offs between current and future reproductive value, provide a model system to examine the role of individual heterogeneity in driving life‐history strategies and individual heterogeneity in fitness. We used hierarchical Bayesian models to examine reproductive trade‐offs, modeling the relationships between within‐year measures of reproductive energy allocation and among‐year demographic rates of individual females breeding on the Yukon‐Kuskokwim Delta, Alaska, using capture–recapture and reproductive data from 1988 to 2014. We generally found that annual survival tended to be buffered against variation in reproductive investment, while breeding probability varied considerably over the range of clutch size‐laying date combinations. We provide evidence for relationships between breeding probability and clutch size, breeding probability and nest initiation date, and an interaction between clutch size and initiation date. Average lifetime clutch size also had a weak positive relationship with apparent survival probability. Our results support the use of demographic buffering strategies for black brant. These results also indirectly suggest associations among environmental conditions during growth, fitness, and energy allocation, highlighting the effects of early growth conditions on individual heterogeneity, and subsequently, lifetime reproductive investment.     

Short-term versus long-term effects of food intake on reproductive output in a viviparous snake, Vipera aspis

Feeding rates influence reproductive output in many kinds of animals, but we need to understand the timescale of this influence before we can compare reproductive energy allocation to energy intake. A central issue is the extent to which reproduction is fuelled by long-term energy stores ("capital" breeding) versus recently-acquired resources ("income" breeding). Our data on free-living aspic vipers show that there is no simple answer to this question: reproductive frequency is determined by long-term energy stores, offspring size is influenced by maternal food intake immediately before ovulation, and litter size is influenced by both long-term stores and short-term energy acquisition. Thus, offspring size in free-living vipers reflects the mother's energy balance over the preceding year (via a trade-off between litter size and offspring size) as well as her energy balance in the current breeding season. Hence, different components of a given reproductive output (litter) are not only functionally linked, but also respond to different temporal scales of prey availability. A female's body size has little effect on her reproductive output. Attempts to quantify reproductive energy allocation must take into account the fact that different reproductive traits (such as offspring size versus number) may respond to energy availability over different timespans. Thus, although the aspic viper is a typical "capital breeder" in terms of its reliance on stored reserves for maternal "decisions" concerning reproductive frequency, it is to some degree a facultative "income breeder" with respect to the determination of offspring size and litter size.     

Immunoglobulin plasma concentration in relation to egg laying and mate ornamentation of female barn swallows (Hirundo rustica)

In vertebrates, offspring have a relatively inefficient immune system soon after birth. Female birds transmit immunoglobulins to the egg, which can confer protection against parasites to their offspring after hatching, but allocation of immune factors can depend on the reproductive value of the offspring as affected, for example, by the quality of their father. We analyse the variation in immunoglobulin levels of female barn swallows (Hirundo rustica) during the breeding cycle in relation to the expression of a secondary sexual character of their mates. Circulating immunoglobulins peaked on approximately the day before that of laying of their first egg, but postlaying concentration was similar to the concentration well before laying. Immunoglobulin levels per unit volume of plasma were lower for females breeding late compared with those breeding early. Haematocrit of females reached an absolute minimum on the day of laying of the third egg. In males, concentration of immunoglobulins relative to other plasma proteins did not change in relation to the breeding stage. Smaller relative concentrations of immunoglobulins and haematocrit were observed in males breeding late in the season. Immunoglobulin concentration of females was positively correlated with the level of ornamentation of their mates. These results suggest that females alter their immune profile to transmit humoral factors providing immune defence against pathogens to their offspring after hatching. This enhancement of immune protection is larger when the offsprings are of relatively large reproductive value as when sired by high quality males. Alternatively, females with relatively large immunoglobulin concentration preferentially mate with the most ornamented males.     

The ontogeny of postmaturation resource allocation in turtles

Resource-allocation decisions vary with life-history strategy, and growing evidence suggests that long-lived endothermic vertebrates direct resources toward growth and self-maintenance when young, increasing allocation toward reproductive effort over time. Few studies have tracked the ontogeny of resource allocation (energy, steroid hormones, etc.) in long-lived ectothermic vertebrates, limiting our understanding of the generality of life-history strategies among vertebrates. We investigated how reproductively mature female painted turtles (Chrysemys picta) from two distinct age classes allocated resources over a 4-yr period and whether resource-allocation patterns varied with nesting experience. We examined age-related variation in body size, egg mass, reproductive frequency, and yolk steroids and report that younger females were smaller and allocated fewer resources to reproduction than did older females. Testosterone levels were higher in eggs from younger females, whereas eggs from second (seasonal) clutches contained higher concentrations of progesterone and estradiol. These allocation patterns resulted in older, larger females laying larger eggs and producing second clutches more frequently than their younger counterparts. We conclude that resource-allocation patterns do vary with age in a long-lived ectotherm.     

Tough decisions: Reproductive timing and output vary with individuals' physiology,behavior and past success in a social opportunistic breeder

This article is part of a Special Issue "SBN 2014".Photoperiod and the hormonal response it triggers are key determinants of reproductive timing in birds. However, other cues and physiological traits may permit flexibility in the timing of breeding and perhaps facilitate adaptation to global change. Opportunistic breeders are excellent models to study the adaptive significance of this flexibility, especially at the individual level. Here, we sought to quantify whether particular male physiological and behavioral traits were linked to reproductive timing and output in wild-derived zebra finches. We repeatedly assessed male stress-induced corticosterone levels (CORT), basal metabolic rate (BMR), and activity before releasing them into outdoor aviaries and quantifying each pair's breeding timing, investment, and output over a seven-month period. Despite unlimited access to food and water, the colony breeding activity occurred in waves, probably due to interpair social stimulations. Pairs adjusted their inter-clutch interval and clutch size to social and temperature cues, respectively, but only after successful breeding attempts, suggesting a facultative response to external cues. When these effects were controlled for statistically or experimentally, breeding intervals were repeatable within individuals across reproductive attempts. In addition, males' first laying date and total offspring production varied with complex interactions between pre-breeding CORT, BMR and activity levels. These results suggest that no one trait is under selection but that, instead, correlational selection acts on hormone levels, metabolism, and behavior. Together our results suggest that studying inter-individual variation in breeding strategy and their multiple physiological and behavioral underpinnings may greatly improve our understanding of the mechanisms underlying the evolution of breeding decisions.     

Plasma mammalian leptin analogue predicts reproductive phenology,but not reproductive output in a capital‐income breeding seaduck

To invest in energetically demanding life history stages, individuals require a substantial amount of resources. Physiological traits, particularly those related to energetics, can be useful for examining variation in life history decisions and trade‐offs because they result from individual responses to environmental variation. Leptin is a protein hormone found in mammals that is proportional to the amount of endogenous fat stores within an individual. Recently, researchers have confirmed that a mammalian leptin analogue (MLA), based on the mammalian sequence of leptin, is present with associated receptors and proteins in avian species, with an inhibitory effect on foraging and body mass gain at high circulating levels. While MLA has been both quantified and manipulated in avian species, little is currently known regarding whether plasma MLA in wild‐living species and individuals is associated with key reproductive decisions. We quantified plasma MLA in wild, Arctic‐nesting female common eiders (Somateria mollissima) at arrival on the breeding grounds and followed them to determine subsequent breeding propensity, and reproductive phenology, investment, and success. Common eiders are capital‐income breeding birds that require the accumulation of substantial fat stores to initiate laying and successfully complete incubation. We found that females with lower plasma MLA initiated breeding earlier and in a shorter period of time. However, we found no links between plasma MLA levels and breeding propensity, clutch size, or reproductive success. Although little is still known about plasma MLA, based on these results and its role in influencing foraging behaviors and condition gain, plasma MLA appears to be closely linked to reproductive timing and is therefore likely to underlie trade‐offs surrounding life history decisions.     

Do purely capital layers exist among flying birds? Evidence of exogenous contribution to arctic-nesting common eider eggs

The strategy of relying extensively on stored resources for reproduction has been termed capital breeding and is in contrast to income breeding, where needs of reproduction are satisfied by exogenous (dietary) resources. Most species likely fall somewhere between these two extremes, and the position of an organism along this gradient can influence several key life-history traits. Common eiders (Somateria mollissima) are the only flying birds that are still typically considered pure capital breeders, suggesting that they depend exclusively on endogenous reserves to form their eggs and incubate. We investigated the annual and seasonal variation in contributions of endogenous and exogenous resources to egg formation in eiders breeding at the East Bay colony in the Canadian Arctic. We collected prey items along with females and their eggs during various stages of breeding and used two complementary analytical approaches: body reserve dynamics and stable isotope [δ(13)C, δ(15)N] mixing models. Indices of protein reserves remained stable from pre-laying to post-laying stages, while lipid reserves declined significantly during laying. Similarly, stable isotope analyses indicated that (1) exogenous nutrients derived from marine invertebrates strongly contributed to the formation of lipid-free egg constituents, and (2) yolk lipids were constituted mostly from endogenous lipids. We also found evidence of seasonal variation in the use of body reserves, with early breeders using proportionally more exogenous proteins to form each egg than late breeders. Based on these results, we reject the hypothesis that eiders are pure capital layers. In these flying birds, the fitness costs of a strict capital breeding strategy, such as temporary loss of flight capability and limitation of clutch and egg size, may outweigh benefits such as a reduction in egg predation rate.     

Relationship between reproductive resource allocation and resource capacity in the matriphagous spider, <Emphasis Type="Italic">Chiracanthium japonicum </Emphasis>(Araneae: Clubionidae)

Females of the Japanese foliage spider, Chiracanthium japonicum, are eaten by their offspring at the end of the maternal care period. To examine the patterns of allocation of maternal investment to their offspring associated with female resource capacity, the amounts of female body reserves accumulated before oviposition, reproductive components at the egg-production phase and those at the matriphagy phase were measured using an artificial breeding nest. Regardless of size, female bodies were completely consumed by the offspring, and larger females, i.e. those having larger reserves, produced a larger number of offspring, but not larger offspring. Furthermore, the proportion of reserves allocated to egg production was not affected by the total amount of the reserves, which indicated that the females systematically divided the resources for individual offspring between egg yolk and food for the growth and survival of the offspring. These results suggest that C. japonicum females adjust egg production to their own resource capacity so that they can achieve an investment per individual offspring which is not dependent on resource capacity. Electronic Publication     

Sex-specific associations between reproductive output and hematozoan parasites of American kestrels

Parasites have the potential to decrease reproductive output of hosts by competing for nutrients or forcing hosts to invest in immune function. Conversely, reproductive output may affect parasite loads if hosts allocate resources to reproduction such that allocation to immune function is compromised. Both hypotheses implicitly have a temporal component, so we sampled parasites both before and after egg laying to examine the relationship between reproductive output (indexed using a combined measure of clutch size, egg volume, and initiation date) and blood parasite loads of American kestrels (Falco sparverius). Parasite loads measured prior to egg laying had no adverse effects on subsequent reproductive output. Females that previously had large reproductive outputs subsequently had lower parasite intensities than those whose outputs were smaller, suggesting that females were capable of allocating energy to both forming clutches and reducing parasite loads. Because male kestrels provide most of their mate's energetic needs before, during, and after egg laying, mate choice by females may have consequences for their parasite loads. Females choosing high-quality mates may not only have increased reproductive output, but may also obtain sufficient resources from their mates to enable them to reduce their parasite burdens. Males whose mates had large reproductive outputs were more likely to subsequently be parasitized and have more intense infections. For individual males sampled both before and after egg laying, those whose mates had larger reproductive outputs were also more likely to become parasitized, or remain parasitized, between sampling periods. Increased parasite loads of males may be one mechanism by which the costs of reproduction are paid.     

Unpredictable perturbation reduces breeding propensity regardless of pre‐laying reproductive readiness in a partial capital breeder

Theoretically, individuals of migratory species should optimize reproductive investment based on a combination of timing of and body condition at arrival on the breeding grounds. A minimum threshold body mass is required to initiate reproduction, and the timing of reaching this threshold is critical because of the trade‐off between delaying breeding to gain in condition against the declining value of offspring with later reproductive timing. Long‐lived species have the flexibility within their life history to skip reproduction in a given year if they are unable to achieve this theoretical mass threshold. Although the decision to breed or not is an important parameter influencing population dynamics, the mechanisms underlying this decision are poorly understood. Here, we mimicked an unpredictable environmental perturbation that induced a reduction in body mass of Arctic pre‐breeding (before the laying period) female common eiders Somateria mollissima; a long‐lived migratory seaduck, while controlling for individual variation in the pre‐laying physiological reproductive readiness via vitellogenin (VTG) – a yolk‐targeted lipoprotein. Our aim was to causally determine the interaction between body condition and pre‐laying reproductive readiness (VTG) on breeding propensity by experimentally reducing body mass in treatment females. We first demonstrated that arrival body condition was a key driver of breeding propensity. Secondly, we found that treatment and VTG levels interacted to influence breeding propensity, indicating that our experimental manipulation, mimicking an unpredictable food shortage, reduced breeding propensity, regardless of the degree of pre‐laying physiological reproductive readiness (i.e. timing of ovarian follicles recruitment). Our experiment demonstrates that momentary environmental perturbations during the pre‐breeding period can strongly affect the decision to breed, a key parameter driving population dynamics.     

Impact of body reserves on energy expenditure, water flux, and mating success in breeding male northern elephant seals

In capital breeders, individual differences in body size and condition can impact mating effort and success. In addition to the collateral advantages of large body size in competition, large nutrient reserves may offer advantages in endurance rivalry and enable the high rates of energy expenditure associated with mating success. We examined the impacts of body reserves and dominance rank on energy expenditure, water flux, mating success, and breeding tenure in the adult male northern elephant seal, a polygynous, capital breeder. Adult males expended energy at a rate of 159 ± 49 MJ d (-1), which is equivalent to 3.1 times the standard metabolic rate predicted by Kleiber's equation. Despite high rates of energy expenditure and a long fasting duration, males spared lean tissue effectively, deriving a mean of 7% of their metabolism from protein catabolism. Body composition had a strong impact on the ability to spare lean tissue during breeding. When controlling for body size, energy expenditure, depletion of blubber reserves, and water efflux were significantly greater in alpha males than in subordinate males. Large body size was associated with increased reproductive effort, tenure on shore, dominance rank, and reproductive success. Terrestrial locomotion and topography appeared to strongly influence energy expenditure. Comparisons with conspecific females suggest greater total seasonal reproductive effort in male northern elephant seals when controlling for the effects of body mass. In polygynous capital breeding systems, male effort may be strongly influenced by physiological state and exceed that of females.     

Feast or famine: evidence for mixed capital–income breeding strategies in Weddell seals

Evolved patterns of resource expenditure for reproduction have resulted in a life history continuum across species. A strictly capital-breeding strategy relies extensively on stored energy for reproduction, whereas income breeding uses energy acquired throughout the reproductive period. However, facultative income breeding has been shown in some classically capital-breeding animals, and was originally thought to provide a nutritional refuge for smaller females incapable of securing sufficient reserves during pre-partum foraging. We examined milk composition and milk output for the Weddell seal to determine to what degree lactation was aided by food intake, and what factors contributed to its manifestation. Milk composition was independent of maternal post-partum mass and condition, but did change over lactation. Changes were most likely in response to energetic and nutritional demands of the pup at different stages of development. During early lactation, females fasted and devoted 54.9% of total energy loss to milk production. Later in lactation 30.5% more energy was devoted to milk production and evidence suggested that larger females fed more during lactation than smaller females. It appears that Weddell seals may exhibit a flexible strategy to adjust reproductive investment to local resource levels by taking advantage of periods when prey are occasionally abundant, although it is restricted to larger females possessing the physiological capacity to dive for longer and exploit different resources during lactation. This supports the assumption that although body mass and phylogenetic history explain most of the variation in lactation patterns (20–69%), the remaining variation has likely resulted from physiological adaptations to local environmental conditions. Our study confirms that Weddell seals use a mixed capital–income breeding strategy, and that considerable intraspecific variation exists. Questions remain as to the amount of energy gain derived from the income strategy, and the consequences for pup condition and survival. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

No effect of partner age and lifespan on female age‐specific reproductive performance in blue tits

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.     

Hormone levels predict individual differences in reproductive success in a passerine bird

Hormones mediate major physiological and behavioural components of the reproductive phenotype of individuals. To understand basic evolutionary processes in the hormonal regulation of reproductive traits, we need to know whether, and during which reproductive phases, individual variation in hormone concentrations relates to fitness in natural populations. We related circulating concentrations of prolactin and corticosterone to parental behaviour and reproductive success during both the pre-breeding and the chick-rearing stages in both individuals of pairs of free-living house sparrows, Passer domesticus. Prolactin and baseline corticosterone concentrations in pre-breeding females, and prolactin concentrations in pre-breeding males, predicted total number of fledglings. When the strong effect of lay date on total fledgling number was corrected for, only pre-breeding baseline corticosterone, but not prolactin, was negatively correlated with the reproductive success of females. During the breeding season, nestling provisioning rates of both sexes were negatively correlated with stress-induced corticosterone levels. Lastly, individuals of both sexes with low baseline corticosterone before and high baseline corticosterone during breeding raised the most offspring, suggesting that either the plasticity of this trait contributes to reproductive success or that high parental effort leads to increased hormone concentrations. Thus hormone concentrations both before and during breeding, as well as their seasonal dynamics, predict reproductive success, suggesting that individual variation in absolute concentrations and in plasticity is functionally significant, and, if heritable, may be a target of selection.