Laying‐sequence variation in yolk carotenoids and egg characteristics in the red‐winged blackbird Agelaius phoeniceus

In many bird species with asynchronous hatching, smaller, later‐hatched nestlings are out‐competed for food by their larger, earlier‐hatched siblings and therefore suffer increased mortality via starvation. It is thought that female birds can either maintain or reduce the survival disadvantage of later‐hatched nestlings by differentially allocating maternal resources across the eggs of a clutch. Carotenoid pigments are an example of resources that female birds allocate differentially when producing a clutch, but laying sequence patterns for these pigments remain poorly studied in North American songbirds. We examined intraclutch variation in yolk carotenoids and egg metrics in 27 full clutches of red‐winged blackbird Agelaius phoeniceus eggs collected from eight wetlands in central Alberta, Canada. We predicted that carotenoids would decrease across the laying sequence, as in this species, later‐hatched, marginal nestlings suffer greater mortality than earlier‐hatched, core nestlings. We found nine carotenoid pigments in red‐winged blackbird egg yolks, including two that have never been described from avian yolks: α‐doradexanthin and adonirubin. As predicted, concentrations and amounts of most carotenoids decreased across the laying sequence, suggesting that female red‐winged blackbirds depleted their carotenoid resources as they laid more eggs. However, egg mass and yolk mass both increased across the laying sequence, suggesting that female red‐winged blackbirds may use other maternal resources to compensate for the size and survival disadvantage experienced by later‐hatched, marginal nestlings.     

No effect of carotenoid supplementation on phytohemagglutinin response or body condition of nestling house wrens

Carotenoids are an essential and often limiting resource in animals and play important roles in immune system function. In birds, the period shortly after hatching is an energetically demanding stage characterized by rapid growth in body size and organ systems, including the immune system. Availability of carotenoids for the growing nestlings may be of particular importance and potentially limiting at this stage of development. We tested the hypothesis that the availability of carotenoids for the embryo in the egg and in the diet of nestlings limits the condition and immune responses of nestling house wrens (Troglodytes aedon Vieillot 1809), a species with melanin-based plumage pigments. In one experiment, nestlings within females' second broods were randomly assigned to receive either a control or a lutein supplement (2008); in a second experiment, females, before their first broods, were either induced to lay additional eggs or not induced, and nestlings within both kinds of broods were supplemented as in the first experiment (2009). There were no significant effects of lutein supplementation on nestling condition or phytohemagglutinin response. There was a significant effect of lutein supplementation on nestling mass in 2008, but the difference was opposite to that predicted. Moreover, even when breeding females were stressed by inducing them to lay supernumerary eggs, lutein supplementation of nestlings had no effect on the size or condition of nestlings hatching from these eggs. These results suggest that maternally derived lutein in the egg and that provided in the diet of nestlings are not limiting to normal development and to the components of the immune system involved in the phytohemagglutinin response of nestling house wrens.     

Experimental manipulation of egg carotenoids affects immunity of barn swallow nestlings

The yolk of bird eggs contains maternal carotenoids that may act as antioxidants thus influencing offspring performance and survival. However, to our knowledge, this hypothesis has not been subjected to experimental tests and the function of transmission of carotenoids to the egg is largely unknown. We directly manipulated the concentration of the main carotenoid (lutein) in the eggs of barn swallows (Hirundo rustica) and analysed the effect of experimental manipulation on growth of nestlings and two fundamental components of their acquired immunity. Nestlings hatched from lutein-inoculated eggs had larger T-cell-mediated immune response compared with those of two control groups. T-cell-mediated immune response predicted nestling survival until fledging. However, lutein inoculation did not affect antibody response to an immunogen, body mass, tarsus length or plumage development. Nestling body mass and plumage development declined with egg laying order, but the effects of lutein inoculation were independent of egg laying order for all traits. Our results show that maternal yolk carotenoids can have a major effect in promoting a fundamental component of immunity that predicts offspring survival and suggests that adaptive early maternal effects can be mediated by transmission of antioxidants to eggs.     

Sex-related effects of maternal egg investment on offspring in relation to carotenoid availability in the great tit

1. Maternal carotenoids in the egg yolk have been hypothesized to promote maturation of the immune system and protect against free radical damages. Depending on availability, mothers may thus influence offspring quality by depositing variable amounts of carotenoids into the eggs. Sex allocation theory predicts that in good quality environments, females should invest into offspring of the sex that will provide larger fitness return, generally males. 2. In a field experiment we tested whether female great tits bias their investment towards males when carotenoid availability is increased, and whether male offspring of carotenoid-supplemented mothers show higher body condition. We partially cross-fostered hatchlings to disentangle maternal effects from post-hatching effects, and manipulated hen flea Ceratophyllus gallinae infestation to investigate the relationship between carotenoid availability and resistance to ectoparasites. 3. As predicted, we found that carotenoid-supplemented mothers produced males that were heavier than their sisters at hatching, while the reverse was true for control mothers. This suggests that carotenoid availability during egg production affects male and female hatchlings differentially, possibly via a differential allocation to male and female eggs. 4. A main effect of maternal supplementation became visible 14 days after hatching when nestlings hatched from eggs laid by carotenoid-supplemented mothers had gained significantly more mass than control nestlings. Independently of the carotenoid treatment, fleas impaired mass gain of nestlings during the first 9 days in large broods only and reduced tarsus length of male nestlings at an age of 14 days, suggesting a cost to mount a defence against parasites. 5. Overall, our results suggest that pre-laying availability of carotenoids affects nestling condition in a sex-specific way with potentially longer-lasting effects on offspring fitness.     

Egg yolk carotenoids in relation to habitat and reproductive investment in the great tit Parus major

Maternal allocation of antioxidants to egg yolk has been shown to affect early embryonic development and nestling survival. In environments with high levels of anthropogenic pollution, antioxidants (such as carotenoids) are important to protect the body from elevated oxidative stress. Thus, female allocation of antioxidants to yolk may be traded off against self-maintenance. Here we investigate maternal reproductive investment with respect to yolk carotenoid content and composition in relation to subsequent female condition and carotenoid status in urban and rural great tits Parus major. We found no differences between the urban and rural populations in total yolk carotenoids, egg mass, clutch size, hatching success, or female carotenoid status. Interestingly, however, rural eggs contained more zeaxanthin, a more potent antioxidant than lutein, which suggests that rural embryos have better antioxidant protection than urban embryos. Whether rural females actively transfer more zeaxanthin to the yolk or whether it passively reflects differences in dietary access or uptake needs to be further investigated. This highlights the importance of carotenoid identity and composition in future studies of carotenoid physiology, ecology, and signaling.     

Maternally derived yolk testosterone enhances the development of the hatching muscle in the red-winged blackbird Agelaius phoeniceus

Hatching asynchrony in avian species often leads to the formation of a size hierarchy that places last-hatched nestlings at a significant disadvantage. The hatching muscle (musculus complexus) is responsible for breaking the shell during hatching and for dorsal flexion of the neck during begging. An increase in its strength in last-hatched nestlings could mitigate the effects of hatching asynchrony by reducing the time required for hatching or enhancing the effectiveness of begging for parentally delivered food or both. We have previously found that yolk testosterone concentration increases with laying order in the red-winged blackbird Agelaius phoeniceus. In this study, we investigated the hypothesis that yolk testosterone has anabolic effects on the development of the complexus, thereby influencing competition among asynchronously hatched nestlings. We found that both yolk testosterone concentration and relative complexus mass (complexus mass/nestling body mass) increased with laying order and that these two variables were positively correlated in both newly hatched nestlings and in two-day-old broods. Moreover, direct injections of testosterone into egg yolks resulted in an increase in relative complexus mass, while injections of flutamide, a testosterone antagonist, resulted in a decrease in relative complexus mass. Neither yolk testosterone concentration nor relative complexus mass differed between male and female nestlings.     

Effects of carotenoid availability during laying on reproduction in the blue tit

Carotenoids are antioxidant pigments involved in several physiological processes and signalling in animals that cannot synthesise them and therefore must acquire them from food. We experimentally investigated the effects of carotenoid availability in the diet during egg laying on antioxidant deposition in egg yolk and the related effects on nestling condition, female body condition and parental investment in the blue tit (Parus caeruleus). Carotenoid supplementation of egg-laying females resulted in a significant increase in carotenoid concentration in egg yolk, but not in vitamin E or A concentration. There was no relationship between yellow plumage colour of adult females and carotenoid deposition in eggs, and no differential effect of feeding treatment depending on female colour. Nestlings from eggs laid by carotenoid supplemented females had longer tarsi, had faster development of the immune system as reflected by leukocyte concentration in blood, and grew brighter yellow feathers than nestlings from control females. However, nestlings from the two groups did not differ significantly in body mass, plasma antioxidants or plumage colour hue. At the time of chick rearing, carotenoid-fed females had increased plasma vitamin E levels compared to controls. However, females from the two treatment groups did not differ significantly in body condition or feeding rate. These results suggest that carotenoid availability is limiting during egg laying, and that females may have to balance the benefits of investing in egg quality against the potential costs of impairing their own future antioxidant protection. In addition, there may be considerable variation in carotenoid availability not only across seasons, but also among different stages of the breeding season.     

Phenotypic variation in nestlings of a bird of prey under contrasting breeding and diet conditions

Environmental conditions often vary in space and time, and this may explain variation in the expression of phenotypic traits related to individual quality, such as ornamental coloration. Furthermore, the direction and strength of the relationship between coloured trait expression and individual quality might vary under contrasting conditions. These issues have been explored in adult birds but much less so in nestlings, which are more likely to experience different selective pressures and different physiological trade‐offs than adults. Here, we empirically investigated the effects of contrasting breeding and diet conditions on the expression of carotenoid‐based colour traits displayed by marsh harrier (Circus aeruginosus) nestlings. We studied the variation in coloration, body condition, and immune responsiveness of nestlings in four populations over a 5‐year period. We characterized spatiotemporal differences in rearing conditions experienced by C. aeruginosus nestlings in terms of breeding (laying date, clutch size, and number of nestlings hatched and fledged) and diet (percentage of mammal in diet and prey diversity) conditions. We found that breeding conditions influenced the co‐variation between coloration and immune responsiveness in female nestlings, and that diet conditions influenced the condition‐dependence of nestling coloration in later‐hatched nestlings. In addition, breeding conditions influenced nestling body condition and immune responsiveness, whereas diet conditions influenced nestling coloration and body condition. Our study highlights that nestling phenotype (levels of signalling, circulating carotenoids, and immunity) varies both spatially and temporally, and that some of this variation is related to differences in breeding and diet conditions. Moreover, under contrasting conditions, the direction of the relationships between nestling carotenoid‐based coloration and nestling quality may also vary. In order to fully understand the evolution and maintenance of colour traits in nestling birds, studies and experiments should ideally be replicated under contrasting rearing conditions. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Hatching asynchrony and brood reduction influence immune response in Common Kestrel Falco tinnunculus nestlings

The onset of incubation before the end of laying imposes asynchrony at hatching and, therefore, a size hierarchy in the brood. It has been argued that hatching asynchrony might be a strategy to improve reproductive output in terms of quality or quantity of offspring. However, little is known about the mediating effect of hatching asynchrony on offspring quality when brood reduction occurs. Here, we investigate the relationship between phenotypic quality and hatching asynchrony in Common Kestrel Falco tinnunculus nestlings in Spain. Hatching asynchrony did not increase breeding success or nestling quality. Furthermore, hatching asynchrony and brood reduction had different effects on nestlings’ phytohaematogglutinin (PHA)‐mediated immune response and nestling growth. In asynchronous and reduced broods (in which at least one nestling died), nestlings showed a stronger PHA‐mediated immune response and tended to have a smaller body size compared with nestlings raised in synchronous and reduced broods. When brood reduction occurred in broods hatched synchronously, there was no effect on nestling size, but nestlings had a relatively poor PHA‐mediated immune response compared with nestlings raised in asynchronous and reduced broods. We suggest that resources for growth can be directed to immune function only in asynchronously hatched broods, resulting in improved nestling quality, as suggested by their immune response. We also found that males produced a greater PHA‐mediated immune response than females only in brood‐reduced nests without any effect on nestling size or condition, suggesting that females may trade off immune activities and body condition, size or weight. Overall, our results suggest that hatching pattern and brood reduction may mediate resource allocation to different fitness traits. They also highlight that the resolution of immune‐related trade‐offs when brood reduction occurs may differ between male and female nestlings.     

Sex-biased nestling mortality in the Montagu's harrier Circus pygargus

I evaluate causes and patterns of nestling mortality in a sexually dimorphic species, the Montagu's harrier Circus pygargus , and their relationship with sex and condition. Starvation was apparently the main reason for nestling death. Condition of birds that died was lower than those that survived. Both probability of nestling death and the proportion of nestlings that died within a brood increased with the number of hatched nestlings in a brood, and with increasing hatching date. For the nestlings that died after being sexed, when controlling for brood effects, probability of death was significantly related to nestling sex, with smaller males having a higher probability of dying. The probability of nestling death if hatched late in the season was relatively greater for males than for females. There was also a significant interaction between sex and hatching date on nestling condition: the decline in condition if hatched late in the season was steeper for males than for females. Males did not have a higher probability of death when having more sisters: neither the probability of brood reduction nor the proportion of nestlings that died were significantly related to within-brood sex ratio. Results suggest that mortality may partly result from sibling competition: females, being the larger sex, might be better able to compete for food within a brood than their male siblings. Additionally, smaller males may be less able to recover from periods of declining body weight.     

Maternal compensation for hatching asynchrony in the collared flycatcher Ficedula albicollis

Several hypotheses have been proposed to explain the adaptiveness of hatching asynchrony for the parents, but delayed hatching is generally detrimental for the late hatched young. These offspring often experience competitive disadvantage and delayed development. If hatching asynchrony has a reason other than producing competitive differences among offspring, it would be advantageous, not only for the offspring but even for the parents, to compensate for its detrimental effects. In some species, increasing investment into later laid eggs has been reported and discussed as a compensation mechanism, but its effect on nestling growth and fledging size has not been examined in details. In this study we investigated nestling growth and size at fledging in terms of body mass and length of primaries in relation to the accurate laying and hatching order in collared flycatcher Ficedula albicollis broods. We found that females laid larger eggs at the end of the laying sequence, and this helped to decrease the disadvantages for the last offspring. The last offspring had lower body mass growth rate and fledged with shorter feathers, but in both cases the larger the last egg was, the smaller the lag of the offspring was. We conclude, that even if females were not able to fully compensate for the detrimental effects of hatching asynchrony, larger eggs may improve the survival prospects of late hatched nestlings.     

Benefits of extra food to reproduction depend on maternal condition

The amount of food resources available to upper‐level consumers can show marked variations in time and space, potentially resulting in food limitation. The availability of food resources during reproduction is a key factor modulating variation in reproductive success and life‐history tradeoffs, including patterns of resource allocation to reproduction versus self‐maintenance, ultimately impacting on population dynamics. Food provisioning experiments constitute a popular approach to assess the importance of food limitation for vertebrate reproduction. In this study of a mesopredatory avian species, the lesser kestrel Falco naumanni, we provided extra food to breeding individuals from egg laying to early nestling rearing. Extra food did not significantly affect adult body condition or oxidative status. However, it increased the allocation of resources to flight feathers moult and induced females to lay heavier eggs. Concomitantly, it alleviated the costs of laying heavier eggs for females in poor body condition, and reduced their chances of nest desertion (implying complete reproductive failure). Extra food provisioning improved early nestling growth (body mass and feather development). Moreover, extra food significantly reduced the negative effects of ectoparasites on nestling body mass, while fostering forearm (a flight apparatus trait) growth among highly parasitized nestlings. Our results indicate that lesser kestrels invested the extra food mainly to improve current reproduction, suggesting that population growth in this species can be limited by food availability during the breeding season. In addition, extra food provisioning reduced the costs of the moult–breeding overlap and affected early growth tradeoffs by mitigating detrimental ectoparasite effects on growth and enhancing development of the flight apparatus with high levels of parasitism. Importantly, our findings suggest that maternal condition is a major trait modulating the benefits of extra food to reproduction, whereby such benefits mostly accrue to low‐quality females with poor body condition.     

Maternal Condition Affects the Offspring Testosterone Concentration in Early Nestling Period: An Experimental Test on Tawny Owls

This study addressed the question of whether maternal condition during egg laying or the rearing environment has a greater effect on offspring testosterone levels. We tested this in field experiments on a population of tawny owls Strix aluco in Duna‐Ipoly National Park, Hungary. In the experiments with females of poor condition, when broods were supplied with extra food none of the nestlings died, whereas in control broods, which were not supplied with extra food, some nestlings did die. Large differences in testosterone levels were correlated positively with hatching order both in experimental and control broods. However, it was only in control broods that the later‐hatched nestlings with low testosterone concentrations, died. In the experiments with females of good condition, the males were removed and females and their broods were supplied with restricted amounts of food. In these broods starving nestlings, whose growth had stopped, were considered as having died and were removed from the nest and hand‐reared. In control broods all nestlings fledged, and both in experimental and control broods testosterone concentrations were more even between siblings. Both types of trials confirmed a maternal influence on offspring testosterone concentration: large between‐sibling differences in concentrations in the broods of females of poor condition, where some nestlings died, could not be reduced with increased food supply, and the more even concentrations in the broods of females in good condition, where all nestlings survived, could also not be increased by restricting the food supply.     

Male parental care promotes early fledging in an open-nester, the Willow Warbler Phylloscopus trochilus

The importance of male parental care to female reproductive success was investigated in the monogamous Willow Warbler Phylloscopus trochilus by removing the male parent at two different stages of the breeding cycle. Females that were widowed at the start of egg-laying continued breeding and managed to raise their brood on their own with no apparent reductions in numbers fledged or fledgling body-mass. The widowed females compensated for the loss of male assistance by increasing their own food provisioning rate as compared with control females. However, widows spent less time brooding the small young, and the growth rate of nestlings was reduced. In nests where the male parent was removed 7 days after the eggs hatched, the subsequent growth rate of nestlings was still affected, which suggests that male care is influential throughout the nestling period. On average, broods reared by widows fledged 2 days later than did broods of control females. An extension of the nestling period may appreciably affect reproductive success, since 68% of nests failed due to predation, mostly during the nestling period. We suggest that the main role of male parental care in the Willow Warbler is to assure a high growth rate of nestlings, which leads to early fledging and hence a reduced risk of nest predation.     

Egg yolk antioxidant deposition as a function of parental ornamentation,age, and environment in great tits Parus major

Females can modify phenotype of their offspring through the deposition of biologically active compounds into eggs, including carotenoids, vitamins and other antioxidants. Understanding patterns of deposition is critical for better insight into the significance of maternal effects. Here we investigated how egg yolk antioxidants (lutein, zeaxanthin, β‐carotene, vitamin A and E) related to environmental conditions and parental characteristics in great tits Parus major using data from three breeding seasons. Male and female traits included condition, age and multiple feather ornaments, both carotenoid‐ and melanin‐based (carotenoid and UV chroma of yellow breast feathers, area of black breast band, white cheek immaculateness). Yolk mass increased with ambient temperature during laying, laying date, and the area of male black breast band. Lutein, zeaxanthin, and vitamin E increased with laying date. Total antioxidants increased with female age, immaculateness of female white cheek patch, and UV chroma of carotenoid‐based yellow breast feathers of the social mate. These patterns were thus consistent with 1) environmental effects on yolk mass and composition, 2) higher quality females depositing more antioxidants, and 3) differential allocation of resources in females in relation to male ornamentation. Overall, environmental factors, female traits, and male traits all had an influence on egg yolk characteristics in this socially monogamous songbird.     

The effects of diet quality and quantity on plumage colour and growth of great tit Parus major nestlings: a food manipulation experiment along a pollution gradient

The yellow carotenoid-based plumage coloration of great tit Parus major nestlings is found to be paler in polluted and urban environments. Because carotenoid pigmentation is often considered to be a condition dependent trait in birds we wanted to find out whether food-limitation and poor nestling condition could explain the pale plumage colour in a polluted area. P. major nestlings were supplemented with variable diets along a well known heavy metal pollution gradient around a copper smelter: two food treatments with carotenoids, one food treatment with little carotenoid and one unsupplemented control. Our field experiment showed that nestlings in the polluted area grew better with carotenoid rich diets, while such effect was not found in the unpolluted area. Nestlings showed higher plasma carotenoid (lutein) levels and higher plumage carotenoid chroma values in the unpolluted area than in the polluted area. However, plasma lutein levels or plumage colour were not associated with heavy metal levels in nestling faeces (a proxy for dietary exposure). Our results provide only weak evidence for carotenoid-based colouration to be condition-dependent in great tit nestlings as we found a positive relationship between body mass and carotenoid chroma in the non-supplemented control group only. The positive relationship between body mass and plumage colour intensity is more likely to be produced by the fact that good availability of caterpillars, an important food source for P. major, also means a good availability of carotenoids to nestlings. Our results suggest that main reason for pale nestling plumage in the polluted area is lower quality invertebrate food, and not nutrition-related oxidative stress.     

Age‐related prenatal maternal effects and postnatal breeding experience have different influences on nestling development in an altricial passerine

Reproductive success and nestling performance are related to the age of parents across several vertebrate taxa. However, because breeding experience and prenatal maternal investment in reproduction often covary, the source of these age‐related differences can be difficult to determine. In this study, we evaluated the influence of prenatal maternal effects and postnatal breeding experience on the performance of nestling tree swallows Tachycineta bicolor by conducting a carefully controlled partial cross‐fostering experiment. We swapped half‐broods of nestlings between the nest of a young first‐time breeding female and the nest of a female known to have previously raised and fledged young. Our manipulation did not influence the within‐brood nestling hierarchies, and controlled for the effects of egg laying order. We found that nestlings of older females were heavier just prior to fledging regardless of the breeding experience of the attending female. In addition, fledglings raised by experienced females grew their flight feathers faster, and had greater probability of fledging. Our study demonstrates that prenatal investment in reproduction by older females can have long‐term consequences on nestling mass, and suggests limited potential for compensatory mass gains prior to fledging. Because our analyses controlled for feeding rates, our results also suggest that foraging quantity and quality are not the only benefits nestlings gain by being raised by an experienced female.     

Phenotypic correlates of yolk and plasma carotenoid concentration in yellow-legged gull chicks

Abstract Carotenoids perform important biological actions in animal tissues, including contributing antioxidant protection. However, the function of transmission of maternal carotenoids to bird eggs is still largely unknown. We made a yolk biopsy of yellow-legged gull (Larus michahellis) eggs and found that the concentration of lutein declined with laying date and across the laying order and increased with egg mass. The concentration of all the main carotenoids (lutein, zeaxanthin, and dehydrolutein) pooled also declined with date and increased with egg mass. We also performed a partial reciprocal cross-fostering of eggs between clutches and investigated the covariation between morphology, T cell-mediated immunity, and plasma carotenoid concentrations of the chicks and carotenoid concentrations in their original eggs. Absolute plasma carotenoid concentrations did not covary with those in the yolk, whereas a positive covariation was found for relative concentrations. Yolk and absolute plasma carotenoid concentrations positively predicted chick body mass and size but not the intensity of the cell-mediated immune response. Thus, yolk carotenoid concentrations may affect chick carotenoid profile and growth, possibly mediating early maternal effects. However, rearing conditions also contributed to determining relative concentrations of circulating carotenoids. Since yolk or plasma antioxidant capacity did not correlate with carotenoid concentrations, future studies of maternal effects mediated by antioxidants should integrate information on carotenoids with information on other components of the antioxidant systems.     

Yolk carotenoids increase fledging success in great tit nestlings

Avian mothers can influence offspring phenotype through the deposition of different compounds into eggs, such as antibodies, hormones and antioxidants. The concentration of carotenoids in yolk is larger than in maternal plasma, suggesting an important role of these compounds for offspring development. Since carotenoids have to be acquired from the diet, they may be available in limiting amounts to the mothers. Here, we investigated the role of egg carotenoids for offspring growth by experimentally increasing the concentration of yolk lutein, the main carotenoid in great tit (Parus major) yolk. We subsequently measured body condition, oxidative stress, immune response, plumage colouration and fledging success. Lutein increased body mass soon after hatching and fledging success, but did not affect tarsus length, oxidative stress, immune response and plumage colouration. The higher content of yolk lutein could have increased body mass by reducing oxidative stress caused by high metabolic rates of rapidly growing embryos or by promoting cell differentiation and proliferation. The positive effect of lutein on fledging success seems to be mediated by its influence on body mass 3 days post-hatch, since these two traits were correlated. The finding that our treatment did not affect traits measured later in the nestling period, except for fledging success, suggests that yolk lutein has short-term effects that are essential to increase survival until fledging. Our study shows the positive effect of yolk lutein on offspring survival in the great tit, and therefore suggests an important role of carotenoid-mediated maternal effects.     

Seasonal variation in food availability influences the breeding strategy of White‐collared Blackbirds Turdus albocinctus on the Tibetan Plateau

Parent birds show a continuous spectrum of breeding strategies, ranging from a low‐fecundity and high‐survival pattern to a high‐fecundity, low‐survival pattern. Investigations of parental breeding strategies under variable environmental conditions can illustrate how parents trade‐off the benefits and costs of these two extreme strategies. White‐collared Blackbirds Turdus albocinctus can breed twice a year on the Tibetan Plateau. We show that both life‐history traits and parental feeding behaviour differ between these two breeding attempts. In the first attempt, the birds produced small clutches and fledged a small number of nestlings of high body condition. In the second attempt, they produced larger clutches and fledged more nestlings of lower body condition. Males made greater contributions to brood provisioning compared with females in the first attempt but there was no sex difference in brood provisioning in the second attempt. In the first attempt, producing smaller clutches can shorten the nestling period, and the increased male contribution to brood provisioning can protect the energy reserves of females. Thus, females can begin a second attempt sooner and produce larger clutches. During the second nesting attempt, when conditions are warmer and wetter, parents rely on a broader array of food types (both invertebrates and plant material, primarily berries) than during the first attempt, which includes only animal food such as arthropods and annelids. We suggest that this difference in breeding strategies between nesting attempts and sexes is in part influenced by marked seasonal variation in food availability.