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.     

Trans‐generational immune priming is constrained by the maternal immune response in an insect

Immune‐challenged vertebrate and invertebrate females can transfer immunity to their offspring. This trans‐generational immune priming (TGIP) is beneficial for the offspring if the maternal infection risk persists across generations. However, because immunity is costly, fitness consequences of TGIP have been found in primed offspring. Furthermore, transferring immunity to offspring may be costly for immune‐challenged females who are also carrying the costs of their immune response. A negative relationship between levels of immunity between mothers and offspring might therefore be expected. Consistent with this hypothesis, we show that in the insect, Tenebrio molitor, the magnitude of antibacterial immune response of immune‐challenged females negatively correlates with levels of antibacterial activity of their eggs. This negative relationship was only present in small females that are inherently of lower quality. Furthermore, female body size did not affect immune responsiveness to the challenge, indicating that small females favoured their immunity at the expenses of that of their eggs.     

Effects of intraclutch variation in egg size and hatching asynchrony on nestling development and survival in semi‐precocial Herring Gulls

Intraclutch egg size variation may non‐adaptively result from nutritional/energetic constraints acting on laying females or may reflect adaptive differential investment in offspring in relation to laying/hatching order. This variation may contribute to size hierarchies among siblings already established due to hatching asynchrony, and resultant competitive asymmetries often lead to starvation of the weakest nestling within a brood. The costs in terms of chick mortality can be high. However, the extent to which this mortality is egg size‐mediated remains unclear, especially in relation to hatching asynchrony which may operate concomitantly. I assessed effects of egg size and hatching asynchrony on nestling development and survival of Herring Gulls (Larus argentatus), where the smaller size and later hatching of c‐eggs may represent a brood‐reduction strategy. To analyze variation in egg size, I recorded the laying order and laying date of 870 eggs in 290 three‐egg clutches over a 3‐yr period (2010–2012). I measured hatchlings and monitored growth and survival of 130 chicks from enclosed nests in 2011 and 2012. The negative effect of laying date (β = ?0.18 ± SE 0.06, P = 0.002) on c‐egg size possibly reflected the fact that late breeders were either low quality or inexperienced females. The mass, size, and condition of hatchling Herring Gulls were positively related to egg size (all P < 0.0001). C‐chicks suffered from increased mortality risk during the first 12 d, identified as the brood‐reduction period in my study population. Although intraclutch variation in egg size was not directly related to patterns of chick mortality, I found that smaller relative egg size interactively increased differences in relative body condition of nestlings, primarily brought about by the degree of hatching asynchrony during this brood‐reduction period. Thus, the value of relatively small c‐eggs in Herring Gulls may lie in reinforcing brood reduction through effects on nestling body condition. A reproductive strategy Herring Gulls might have adopted to maintain a three‐egg clutch, but that also enables them to adjust the number of chicks they rear relative to the prevailing environmental conditions and to their own condition during the nestling stage.     

Context‐dependent effects of yolk androgens on nestling growth and immune function in a multibrooded passerine

Female birds may adjust their offspring phenotype to the specific requirements of the environment by differential allocation of physiologically active substances into yolks, such as androgens. Yolk androgens have been shown to accelerate embryonic development, growth rate and competitive ability of nestlings, but they can also entail immunological costs. The balance between costs and benefits of androgen allocation is expected to depend on nestling environment. We tested this hypothesis in a multibrooded passerine, the spotless starling, Sturnus unicolor. We experimentally manipulated yolk androgen levels using a between‐brood design and evaluated its effects on nestling development, survival and immune function. Both in first and replacement broods, the embryonic development period was shorter for androgen‐treated chicks than controls, but there were no differences in second broods. In replacement broods, androgen‐treated chicks were heavier and larger than those hatched from control eggs, but this effect was not observed in the other breeding attempts. Androgen exposure reduced survival with respect to controls only in second broods. Regarding immune function, we detected nonsignificant trends for androgen treatment to activate two important components of innate and adaptive immunity (IL‐6 and Ig‐A levels, respectively). Similarly, androgen‐treated chicks showed greater lymphocyte proliferation than controls in the first brood and an opposite trend in the second brood. Our results indicate that yolk androgen effects on nestling development and immunity depend on the environmental conditions of each breeding attempt. Variation in maternal androgen allocation to eggs could be explained as the result of context‐dependent optimal strategies to maximize offspring fitness.     

Predation risk affects the levels of maternal immune factors in avian eggs

Predation risk is an environmental stressor that can induce changes in prey behavior and physiology. Perception of predation risk may indirectly affect offspring traits and future fitness prospects via impacts on the condition of parents. Females may influence the survival of their offspring via maternal effects, especially when breeding in stressful conditions. We investigated the effects of continuous predation risk perceived by mothers on the maternal allocation of immune factors and carotenoids in eggs of the pied flycatcher Ficedula hypoleuca. We collected eggs from wild pied flycatchers that bred in the vicinity of a predator nest (pygmy owl Glaucidium passerinum), were exposed to cues of a mammalian nest predator (urine of least weasel Mustela nivalis), or received appropriate controls for these two groups. Pied flycatchers transferred more immunoglobulin in eggs under high predation risk in both owl and mammalian predator treatments. The presence of owl nests also lowered the level of lysozyme transferred in the eggs in one of the two study years. Predation risk did not modify egg size or overall carotenoid levels. Our results show that continuous predation risk perceived by females during egg‐laying affects egg composition. This different allocation of maternal immune factors may be an adaptive response evolved to increase the probability of offspring survival.     

Environmental and not maternal effects determine variation in offspring phenotypes in a passerine bird

Maternal and environmental effects can profoundly influence offspring phenotypes, independent of genetic effects. Within avian broods, both the asymmetric post‐hatching environment created by hatching asynchrony and the differential maternal investment through the laying sequence have important consequences for individual nestlings in terms of the allocation of resources to body structures with different contributions to fitness. The purpose of this study was to evaluate the relative importance of post‐hatching environmental and maternal effects in generating variation in offspring phenotypes. First, an observational study showed that within blue tit, Cyanistes caeruleus, broods, late‐hatched nestlings allocated resources to tarsus development, maintained mass gain and head‐bill growth and directed resources away from the development of fourth primary feathers. Second, a hatching order manipulation experiment resulted in nestlings from first‐laid eggs hatching last, thereby allowing comparison with both late and early‐hatched nestlings. Experimental nestlings had growth patterns which were closer to late‐hatched nestlings, suggesting that within‐brood growth patterns are determined by post‐hatching environmental effects. Therefore, we conclude that post‐hatching environmental effects play an important role in generating variation in offspring phenotypes.     

Azure‐winged Magpies Cyanopica cyanus trade off reproductive success and parental care by establishing a size hierarchy among nestlings

It is common in birds that the sizes of nestlings vary greatly when multiple young are produced in one nest. However, the methods used by parents to establish size hierarchy among nestlings and their effect on parental provisioning pattern may differ between species. In the Azure‐winged Magpie Cyanopica cyanus, we explored how and why parents controlled the sizes of nestlings. Asynchronous hatching was the main cause of size hierarchy within the brood, although the laying of larger eggs later in the laying sequence reduced this effect. Parents with asynchronous broods produced more eggs and fledged more nestlings than those with synchronous broods but their brood provisioning rates, food delivery per feeding bout and feeding efficiency did not differ. We performed a cross‐fostering experiment to synchronize some asynchronous broods. Provisioning rates of asynchronous broods were lower than those of synchronized broods, but the daily growth rates and fledging body mass of their nestlings were not different. Our findings indicate that parents of asynchronous broods can achieve higher reproductive success than those of synchronous broods based on the same parental care, and the same reproductive success as those of synchronized broods based on less parental care. It appears that parent birds can better trade off reproductive success and parental care by establishing a size hierarchy among nestlings.     

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.     

Brood size manipulations reveal relationships among physiological performance,body condition and plumage colour in Northern Flicker Colaptes auratus nestlings

Visual signals of quality in offspring, such as plumage colour, should honestly advertise need and/or body condition, but links between nutritional status, physiological performance and the expression of colours are complex and poorly understood. We assess how food stress during rearing affected two physiological measures (T‐cell‐mediated immune function and corticosterone level in feathers: CORT_f) and how these two variables were related to carotenoid and melanin coloration in Northern Flicker Colaptes auratus nestlings. We were also interested in how these two physiological measures were influenced by the sex of the nestling. We experimentally manipulated brood size to alter levels of food availability to nestlings during development. We measured carotenoid‐based colour (chroma and brightness) in wing feathers and the size of melanin spots on breast feathers. In agreement with our prediction, nestlings in the reduced brood treatment had better body condition and stronger immune responses than those in the control and brood enlargement treatments. This supports the hypothesis that immune responses are energetically costly. In contrast, CORT_f was not related to nestling body condition or sex and was unaffected by brood size manipulation. Nestlings of both sexes with stronger T‐cell‐mediated immune responses had larger melanin spots but only males with higher immune responses also had brighter flight feathers. Feather brightness decreased with increasing CORT_f levels. Our study is one of the few to examine the relationship between multiple physiological and plumage measures in nestlings and shows that plumage colour and immune function signalled body condition of nestlings, but that feather corticosterone levels did not.     

Maternal antibodies in a wild altricial bird: effects on offspring immunity, growth and survival

1. In many animals immunity is not fully developed until adulthood but the young still need protection against various sets of pathogens. Thus, bird nestlings are highly dependent on antibodies received from their mother (in the eggs) during their rapid early growth period. The relationship between maternal immunity and the development of neonates' own immunity has been poorly studied. 2. It has been suggested that immune function plays an important part in mediating resource competition between different life-history traits, e.g. growth and reproduction. Maternal investment of antibodies has potentially permanent effects on offspring phenotype. Thus, the trade-offs between the immune function and other important life-history traits in the offspring will also affect the fitness of the mother. 3. Our supplemental feeding experiment in the magpie Pica pica indicates that the immunoglobulin levels of offspring at hatching are dependent on a mother's nutritional condition. In addition, the amount of maternal immunoglobulins transferred to offspring increases along the laying order within a nest. 4. We also found that at the age of 8-10 days the immunoglobulin production of the offspring has already begun. Furthermore, the maternal immunoglobulin levels of the offspring at hatching were positively related to their immunoglobulin levels on day 10. 5. Maternal immunoglobulins did not significantly affect offspring growth, but there was a negative relationship between self-produced immunoglobulins and growth over the first 10 days, indicating a trade-off between these traits. Nestlings' weight, however, had a positive relationship with immunoglobulin production suggesting that the observed trade-off between growth and immunoglobulin production is due to catch-up growth of nestlings with a low hatching weight. We found that within nests nestlings with higher maternal antibody levels had higher survival rate until day 20, but between nests there was an opposite relationship. 6. Evidently, there is a trade-off, in magpies, between maternal resources, immune function and growth, shaping the evolution of maternal investment in offspring immunity.     

Pre-laying nutrition mediates maternal effects on offspring immune capacity and growth in the pied flycatcher

We have aimed at detecting prelaying maternal effects on nestling antibody defences and growth through experimental food supplementation of female pied flycatchers Ficedula hypoleuca and subsequent exchange of whole clutches with control nests. The levels of immunoglobulins and the mass and size of chicks at 12 days of age were ascertained. This is the first study controlling for maternal incubation effects by exchanging eggs rather than nestlings. Our prediction is that the females' availability of pre-laying nutritional resources affects offspring immune capacity and growth through maternal effects in the eggs when conditions during incubation and rearing are controlled for. Nestling immunoglobulin Y (IgY) levels and tarsus length were indeed positively associated with maternal food supplementation at laying. The only rearing environmental effect detected was that of mite infestation which affected both IgY levels and growth of nestlings. Nestlings that recruited to the population in the subsequent 2 years had higher IgY levels than those that did not. Maternal adaptations for allocating resources to eggs play an important role in moulding offspring phenotypes and may affect their survival prospects.     

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.     

Maternal dietary carotenoids mitigate detrimental effects of maternal GnRH on offspring immune function in Japanese quail Coturnix japonica

Maternal resources deposited in eggs can affect the development of several offspring phenotypic traits and result in trade‐offs among them. For example, maternal androgens in eggs may be beneficial to offspring growth and competitive ability, but detrimental to immunocompetence and oxidative stress. In contrast, maternal antioxidants in eggs may be beneficial if they mitigate oxidative stress and immunosuppressive effects of androgens. We investigated possible interactive effects of maternal steroids and carotenoids on aspects of offspring physiology and phenotype, by simultaneously manipulating levels of androgens (via gonadotropin‐releasing hormone, GnRH‐challenges) and carotenoids (via diet supplementation) in captive female Japanese quail Coturnix japonica during egg laying. Carotenoid supplementation of hens, which elevates yolk concentrations of carotenoid and vitamins A and E, enhanced egg hatching success, offspring survival to age 15 d, and size of the bursa of Fabricius in offspring. In contrast, repeated maternal GnRH challenges, which elevated yolk testosterone concentrations, enhanced offspring neonatal size, but negatively affected bursa size. However, interaction among the treatments suggests that the positive effect of maternal carotenoid supplementation on plasma bactericidal capacity was mediated by maternal GnRH challenges. Chicks originating from carotenoid‐supplemented hens were less immunosuppressed than those originating from carotenoid‐supplemented + GnRH‐challenged hens, which were less immunosuppressed than chicks from GnRH‐challenged females not supplemented with carotenoids. Females availability of carotenoid enriched diets allows them to enhance the development of offspring immune system via carotenoids and vitamins deposited in egg yolks and offset detrimental effects of androgens deposited by GnRH‐challenged females.     

Seasonal decline in cell-mediated immunity of collared flycatcher Ficedula albicollis nestlings: does the sex of offspring matter?

The seasonal decline in offspring performance is a frequently reported phenomenon in species breeding in a temperate zone, but the potential effect of brood sex ratio on such declines has not been studied. Here, we predicted that this decline may occur if the sex that exhibits the lower immune response or lower survival rate tends to be more frequent among late broods. The seasonal patterns of four performance parameters of collared flycatcher Ficedula albicollis nestlings have been examined during 4 years. Sex was assigned to all studied individuals using molecular techniques. We found significant seasonal decline in cell-mediated immune response, tarsus length and survival of the chicks. The lack of interactions between gender and hatching date revealed that both sexes contributed equally to the observed decline. The brood sex ratio did not vary with the laying date. On the basis of available data, we suggest that the breeding date may only exceptionally induce female-driven sex allocation in species with only slight sexual size dimorphism. In consequence, we suggest that seasonal sex ratio shifts do not account for seasonally declining fitness of nestlings in passerines.     

Unusual pattern of sex‐specific mortality in relation to initial brood sex composition in the black‐billed magpie Pica pica

In sexually size‐dimorphic species, brood sex composition may exert differential effects on sex‐specific mortality. We investigated the sex‐specific mortality and body condition in relation to brood sex composition in nestlings of the black‐billed magpie Pica pica. Neither significantly sex‐biased production at hatching nor overall sex‐biased mortality during the nestling period was found. Sex‐specific mortality as a function of brood sex composition, however, differed between female and male nestlings. We found higher mortality for females in male‐biased broods and higher mortality for males in female‐biased broods, a phenomenon that we call ‘rarer‐sex disadvantage’. As a result, fledging sex ratios became more biased in the direction of bias at hatching, a phenomenon that cannot be readily explained by previous hypotheses for sex‐specific mortality. Two temporal variables, fledging date and laying date, were also correlated with sex‐specific mortality: female nestlings in earlier broods experienced higher mortality than male nestlings whereas male nestlings in later broods experienced higher mortality. We suggest that this unusual pattern of mortality may be explained by adaptive adjustments of brood sex composition by parents, either through the effects of a slight sex difference in offspring dispersal patterns on parental fitness, or owing to sex differences as regards the benefits of early fledging.     

Brood size and environmental conditions sex-specifically affect nestling immune response in the European starling Sturnus vulgaris

In sexually size-dimorphic species, the larger sex can be more sensitive to stressful environmental conditions, often resulting in reduced growth and elevated mortality rates. Development of the immune system is regarded as highly resource dependent, and recent data suggest that nestling passerines experience a possible resource-based trade-off between growth and immunity. Given the hypothesized importance of maximizing growth for the larger sex, the corresponding immune system may also exhibit similar sensitivity to limited resources. To better understand how natural variation in brood size and resources might differentially affect growth and immune function in nestlings of a sexually size-dimorphic species, we examined the relationship between brood size and inter-sexual differences in cell-mediated immunity (CMI) and survival in European starling Sturnus vulgaris nestlings where males are larger in both mass and structural size. We hypothesized that male CMI response should be negatively impacted by increasing sibling competition (brood size), especially during periods of low resource availability. In a year of reduced parental provisioning rates and reduced chick growth rates, male offspring exhibited the predicted negative relationship, whereas female CMI response was unaffected. However, in a year of improved provisioning rates and chick growth, neither sex exhibited a negative relationship between immune response and brood size. Thus, natural variation in brood size can affect sex-specific immunity differently in offspring of a sexually size-dimorphic passerine. However, this relationship appears resource-dependent, suggesting that the hypothesized resource-based trade-off may be compensated for in years of adequate resource abundance.     

Ectoparasites and reproductive trade-offs in the barn swallow (Hirundo rustica)

Parents are predicted to trade offspring number and quality against the costs of reproduction. In altricial birds, parasites can mediate these costs because intensity of parasitism may increase with parental effort. In addition, parasites may mediate a trade-off between offspring number and quality because nestlings in large broods may have reduced anti-parasite immune defence. In this study, we experimentally analysed the effect of brood size on infestation by an ectoparasitic mite in nests of barn swallows (Hirundo rustica). Nests with an enlarged brood had larger prevalence and intensity of infestation than those with a reduced brood. Importantly, each nestling in enlarged broods was exposed to a larger number of mites, even when measured on a per nestling basis, than in reduced broods. Nestlings in enlarged broods had smaller body mass and T-cell-mediated immune response compared to reduced broods. T-cell-mediated immune response and feather growth were negatively correlated with per nestling intensity of infestation in enlarged but not in reduced broods. The results suggest that nestlings in enlarged broods have depressed immunity leading to larger per nestling mite infestation. Hence, exposure to parasites of offspring and parents increases with brood size, and parasitism can thus mediate trade-offs between reproduction and number and quality of the progeny in the barn swallow.     

Immunity, growth and begging behaviour of nestling Barn Swallows Hirundo rustica in relation to hatching order

In altricial birds, asynchronous hatching often results in a size hierarchy among nestlings. However, laying order and hatching order may affect components of offspring condition differentially. Since parasites can have a heavy impact on their hosts, strategies may have evolved that allow parents and offspring to allocate critical resources to immunity depending on hatching order. In this study of the Barn Swallow Hirundo rustica we (1) analyse the relationship between laying and hatching order of eggs, (2) compare morphological, serological and immunological traits between early- and late-hatched siblings, and (3) compare the begging behaviour of early- and late-hatched nestlings. Hatching order strictly reflected laying order. Late-hatched chicks begged more for food but had smaller body mass throughout the nestling period. Immunoglobulin concentration and intensity of T cell-mediated immunity were larger in late-hatched nestlings. Large concentration of immunoglobulins in late-hatched nestlings may reflect transfer of immunity from the mother via the egg. Alternatively, in association with larger T cell-mediated immunity, immunoglobulin production may indicate larger investment in immunity by late-hatched nestlings. Laying order and hatching order may therefore affect components of nestling condition differentially.     

Cost of reproduction, T-lymphocyte mediated immunocompetence and health status in female and nestling barn swallows Hirundo rustica

We investigate the trade-off between reproductive effort, health status and T-lymphocyte acquired immunity in female and nestling barn swallows Hirundo rustica using a brood size manipulation experiment. Maternal and total feeding effort increased with experimental brood size. Parents did not fully compensate for the increased food demand of the enlarged broods and as a consequence the per capita feeding rate of nestlings decreased with increasing experimental brood size. Body mass and a measure of T-cell mediated immunity in 12 days old nestlings also decreased with increasing experimental brood size. Different leucocyte concentrations and the heterophile/lymphocyte ratio – an index of stress – of nestlings did not change in relation to experimental brood size, suggesting that within brood competition did not affect stress to nestlings. The brood size manipulation had a significant effect on maternal T-cell mediated immunity, measured by the phytohemagglutinin skin test, but not on maternal body mass, haematocrit or differential or total white blood cell counts. Our results seem to support the prediction that under mild work stress females respond first by reducing the energetically expensive acquired immunity. Different leucocyte types and the heterophile/lymphocyte ratio appear less sensitive to parental workload.     

Individual variation in parental tradeoffs between the number and size of offspring at the pre- and post-natal stages

Life-history theory predicts that parents refer to the resources they hold to determine their breeding strategy. In multi-brooded species, it is hypothesized that single-brooded parents produce larger clutches and raise offspring with a brood survival strategy, whereas multi-brooded parents only do this under good breeding conditions. Under poor conditions, they produce smaller clutches and raise offspring with a brood reduction strategy. We tested this hypothesis in the Brown-cheeked Laughing Thrush Trochalopteron henrici, which can breed twice a year on the Tibetan Plateau, by investigating the life-history traits and provisioning behaviours of single- and double-brooded parents. Single-brooded parents laid larger clutches of smaller eggs and produced more and larger fledglings than double-brooded parents in their first brood. Double-brooded parents produced smaller clutches of larger eggs but fledged larger nestlings in their first brood than in their second brood. As single-brooded parents only need to raise one brood a year, then producing and raising as many offspring as possible (i.e. the brood survival strategy in a large brood) can maximize their reproductive success. For double-brooded parents, producing and raising fewer offspring in the first brood (i.e. the brood survival strategy in a small brood) can ensure their nesting success during a short breeding cycle. Additionally, producing more offspring but raising larger nestlings in the second brood (i.e. the brood reduction strategy in a large brood) can select for offspring of higher quality within the brood. Our findings indicate that different tradeoffs between single- and double-brooded parents in egg-laying and nestling-raising may be an adaptation to the seasonal variation in environmental conditions.