Brood Size Manipulation Research Articles

Feeding Effort of Male Tawny OwlsStrix alucoFollows a Fixed Schedule: A Field Experiment in the Early Nestling Period

The ‘fixed schedule hypothesis’ proposes that parental investment is independent of offspring needs in order to maximize adult survival or to prevent reduced future fecundity. We tested this hypothesis on Tawny Owls Strix aluco by experimental manipulation of brood size. We measured the call frequency of begging chicks, feeding frequency of males, overnight weight gain of nestlings and parental body condition in reduced or enlarged broods, and in control broods. In reduced or enlarged broods nestling begging calls reflected the respective decrease or increase in demand, but the frequency with which the males fed did not differ between the reduced and enlarged broods. Consequently, chicks gained body weight more rapidly in reduced broods, whereas those in enlarged broods grew more slowly. Male body weight did not change during the early nestling period when they delivered food to reduced or enlarged broods, but the condition of females worsened in the enlarged broods. As the males did not increase the feeding frequency in the enlarged broods, the females were under pressure to pass on more of the prey to nestlings begging in these broods. Male Tawny Owls regulated their feeding effort according to a fixed schedule, independently of the chicks' needs, to maintain their body condition. Nevertheless, the females whose condition deteriorated during the feeding of enlarged broods did not stop brooding earlier, nor did they suffer mortality in the breeding period.

The glucocorticoid stress response is attenuated but unrelated to reproductive investment during parental care in a teleost fish

We investigated whether circulating glucocorticoids and androgens are correlated with reproductive investment in smallmouth bass ( Micropterus dolomieu), a teleost fish with sole paternal care. Circulating cortisol and androgens prior to and 25 min following a standardized 3 min emersion stressor were quantified for non-reproductive and parental fish across the parental care period. To experimentally investigate the influence of reproductive investment on endocrine parameters, we manipulated brood size (reduced, enlarged, sham-treated, or unmanipulated) 24 h prior to sampling parental fish. We predicted that fish guarding offspring would exhibit increased androgens and baseline cortisol levels, and an attenuated cortisol response to the stressor when compared with non-reproductive individuals. We further predicted that these effects would scale with reproductive investment. As predicted, parental care-providing fish exhibited lower post-stress plasma cortisol concentrations than non-reproductive fish. This difference was strongest early during parental care. However, no differences in baseline or post-stress cortisol concentrations were detected among parents guarding offspring with varying brood sizes. There was, however, a trend for parental fish to exhibit an increased cortisol response following brood manipulation, regardless of the direction of change in brood size, a response that likely reflected disturbance. No differences were found in baseline cortisol concentrations. Circulating androgens were found to be highest during early parental care, and no differences were found among parents guarding manipulated broods. Collectively, these findings demonstrate that the endocrine stress response is affected by reproductive status, but the response in this model species does not appear to be scaled according to reproductive investment as predicted by life-history theory.

Context-dependent effects of parental effort on malaria infection in a wild bird population, and their role in reproductive trade-offs

Although trade-offs between reproductive effort and other fitness components are frequently documented in wild populations, the underlying physiological mechanisms remain poorly understood. Parasitism has been suggested to mediate reproductive trade-offs, yet only a limited number of parasite taxa have been studied, and reproductive effort-induced changes in parasitism are rarely linked to trade-offs observed in the same population. We conducted a brood size manipulation experiment in blue tits (Cyanistes caeruleus) infected with malaria (Plasmodium) parasites, and used quantitative PCR to measure changes in parasitaemia. In one of two years investigated, parasitaemia increased as a result of brood enlargement, and was also positively associated with two other indicators of reproductive effort: clutch size and single parenthood. These associations between both experimental and naturally varying reproductive effort and parasitaemia suggest that immune control of chronic malaria infections can be compromised when parents are working hard. Brood size manipulation significantly affected the number of independent offspring produced, which was maximised when brood size was unchanged. Moreover, when parents were infected with one of two common Plasmodium species, the shape of this trade-off curve was more pronounced, suggesting that parasitic infection may exacerbate the trade-off between quantity and quality of offspring. Although the involvement of parasites in survival costs of reproduction has received much attention, these results suggest their role in other commonly documented reproductive trade-offs, such as that between number and quality of offspring, warrants further study.

Flexibility in the Foraging Behavior of Blue Tits in Response to Short‐Term Manipulations of Brood Size

AbstractPrevious work suggests that short‐term changes in feeding rate are usually produced by the parent‐offspring interaction. However, few studies have properly tested this assumption. In this study, we attempt to explore the short‐term consequences of daily (within‐pair) brood size manipulations (reduced, original, and enlarged) on feeding behavior (provisioning rates, prey size, and prey type) of Mediterranean blue tits Cyanistes caeruleus. Total provisioning rates were lowest when broods were reduced in size and greatest when broods were enlarged. Mean prey size was also affected by the brood size changes: parents tended to bring larger prey when confronted with low brood demand reinforcing the view that a trade‐off exists between minimizing foraging time and maximizing food quantity. Such differences in feeding frequencies and the load sizes delivered may be explained by changes in the parents’ foraging tactic. Increase of brood size compelled parents to work harder and be less selective in prey choice; we found that stressed birds with a high level of feeding responsibility (hungry nestlings) opted to concentrate on more readily available food items (Tortricids). On the other hand, their immediate reaction when faced with a low level of feeding responsibility was to decrease this prey type in the diet, so that the percentage of other preys (Noctuids) in the diet increased. There was no intersexual difference in the way in which parents responded to the manipulation. In sum, our results revealed a flexibility in foraging strategies of blue tits to cope with changing scenarios, which supports the idea that provisioning behavior is largely governed by nestling demand.

Dissecting Carotenoid from Structural Components of Carotenoid‐Based Coloration: A Field Experiment with Great Tits (Parus major)

Carotenoid-based yellowish to red plumage colors are widespread visual signals used in sexual and social communication. To understand their ultimate signaling functions, it is important to identify the proximate mechanism promoting variation in coloration. Carotenoid-based colors combine structural and pigmentary components, but the importance of the contribution of structural components to variation in pigment-based colors (i.e., carotenoid-based colors) has been undervalued. In a field experiment with great tits (Parus major), we combined a brood size manipulation with a simultaneous carotenoid supplementation in order to disentangle the effects of carotenoid availability and early growth condition on different components of the yellow breast feathers. By defining independent measures of feather carotenoid content (absolute carotenoid chroma) and background structure (background reflectance), we demonstrate that environmental factors experienced during the nestling period, namely, early growth conditions and carotenoid availability, contribute independently to variation in yellow plumage coloration. While early growth conditions affected the background reflectance of the plumage, the availability of carotenoids affected the absolute carotenoid chroma, the peak of maximum ultraviolet reflectance, and the overall shape, that is, chromatic information of the reflectance curves. These findings demonstrate that environment-induced variation in background structure contributes significantly to intraspecific variation in yellow carotenoid-based plumage coloration.

Testing the developmental stress hypothesis in canaries: consequences of nutritional stress on adult song phenotype and mate attractiveness

The complex songs of songbirds are thought to have evolved through sexual selection. Sexually selected signals must be associated with costs in order to ensure their honesty as indicator of male quality. Costs may relate to the development of the neural substrate underlying song learning, which develops already very early in life. Song may, therefore, serve as an indicator of the early developmental history. This nutritional stress hypothesis has initially been confirmed for a variety of species, but recent studies using zebra finches as a model species reported somewhat inconsistent effects, and the functional consequences of changes in adult song phenotype remain unclear. We tested the nutritional stress hypothesis in canaries by manipulating either the brood size or the food quality postfledging. The brood size manipulation had a significant effect on early development, and low food quality postfledging led to a transient reduction in body mass. However, we did not find evidence that any of the song traits measured reflected the early developmental conditions, which is in conflict with the nutritional stress hypothesis. Canaries may be less vulnerable to nutritional stress or are able to compensate stressful conditions during early development. However, if males compensated, this compensation may have come at a survival cost. Female mate choice decisions were independent of the developmental history of a male. Instead, females preferred males singing longer song bouts, a trait that may contain a heritable component.

Males are sensitive — sex-dependent effect of rearing conditions on nestling growth

The sex-dependent effect of environmental conditions on nestlings has been extensively studied in size dimorphic birds. Whether males or females are more sensitive to poor conditions is not yet clear; however, the degree of sexual size-dimorphism, brood size and their interactions seem to influence the pattern. Much less is known about sex-dependent environmental sensitivity in size-monomorphic species, even though it may result in biased sex allocation. We altered the rearing conditions by brood size manipulation in the size-monomorphic collared flycatcher and then examined the sex-specific development of the nestlings. In all analyses, we controlled for the effect of paternity, because one may expect extra-pair young to be of better genetic quality and perform better at least under poor conditions. However, this was not the case, because we did not find any difference in growth rate or fledging size between extra- and within-pair young. We found that male nestlings had the potential for faster growth under favourable conditions, but suffered more under poor conditions. We found no sex × environment interaction for fledging size probably because the growth curves level off before fledging, and the disadvantaged nestlings can catch up with their siblings. The larger sensitivity of males does not explain the previously found seasonal shift in brood sex ratios and contradicts previous findings in another size-monomorphic species where females were more sensitive. This suggests that even in size-monomorphic species, no general rule exists, which determines the more sensitive sex.

Sex‐specific heritability of cell‐mediated immune response in the blue tit nestlings (Cyanistes caeruleus)

Here, we aimed at estimating sex-specific heritabilities of cell-mediated immune response (CMI) in the blue tit nestlings (Cyanistes caeruleus). To separate genetic and environmental components of the phenotypic variance in CMI (measured using phytohaemagglutinin assay), we performed a cross-fostering experiment. Additionally, controlled environmental variation was introduced by enlarging some broods. Our analyses revealed a significant genetic component (as approximated by the nest-of-origin term) of the phenotypic variance in immune response. More importantly, these genetic effects differed between sexes and experimentally manipulated brood sizes, as indicated by significant genotype-by-sex and genotype-by-environment interactions. We discuss possible causes of such sexual dimorphism in gene expression and suggest that sex- and environment-specific genetic interactions may contribute to the maintenance of genetic variability in traits related to immune functions.

Does oxidative stress mediate the trade‐off between growth and self‐maintenance in structured families?

Summary1. Life‐history theory predicts a trade‐off between growth and self‐maintenance. However, little is known about the underlying mechanisms. Oxidative stress, the imbalance between the production of reactive oxygen species (ROS) during normal metabolic processes and the level of antioxidant defences, is a potential mechanism.2. In many altricial birds asynchronous hatching creates different castes of progeny as a hedge against developmental and environmental uncertainty (a ‘structured’ family). Older, ‘core’ offspring and later hatched ‘marginal’ offspring consequently experience contrasting levels of resource availability during development. To be adaptive the phenotypic handicap incurred by marginal offspring must be reversible when conditions allow.3. Dietary‐derived antioxidants are an important component of antioxidant defences, which modulate oxidative damage arising from normal metabolic processes such as growth. Dietary antioxidants could therefore be important determinants of trade‐offs during growth, with the balance of these trade‐offs differing between core and marginal offspring.4. To investigate this, we manipulated brood size and structure as well as antioxidant availability (by vitamin and mineral supplementation), then measured growth and oxidative damage in nestling red‐winged blackbirdsAgelaius phoeniceus.5. We found that individuals supplemented with dietary antioxidants allocated these extra resources to increasing growth rate, rather than reducing oxidative damage. Irrespective of hatch rank, nestlings that received supplementary antioxidants grew faster than controls, despite no difference in their begging behaviour or parental provisioning rates. There was no overall difference in the level of lipid peroxidation (malondialdehyde; MDA) in the blood plasma of supplemented and control chicks, but among marginal offspring those that received an antioxidant supplement had lower plasma MDA. Antioxidant supplementation did not affect nestling survival to fledging age.6. These novel results support the idea that oxidative stress may play a central role in the trade‐off between growth and self‐maintenance. Moreover, they further highlight the importance of physiological costs in the evolution of life‐histories and propose a role for oxidative stress in the adaptive basis of the structured family.

Effect of sibling competition and male carotenoid supply on offspring condition and oxidative stress

Early developmental conditions have major implications for an individual’s fitness. In species where offspring are born simultaneously, the level of sibling competition for food access is intense. In birds, high sibling competition may subject nestlings to decreased growth rate as a result of limited food and increased levels of oxidative stress through high metabolic activity induced by begging behaviors. We manipulated the level of sibling competition in a natural population of great tits and assessed the consequences for nestling body condition and resistance to oxidative stress. In a full factorial design, we both augmented brood size to increase sibling competition and supplemented the male parents with physiological doses of carotenoids thereby doubling the natural carotenoid intake, aiming at increasing the males’ investment in current reproduction and thereby decreasing sibling competition. Nestling body mass was reduced by the brood enlargement and enhanced by the carotenoid supplementation of fathers. Nestling resistance to oxidative stress, measured as total antioxidant defenses in whole blood, was not influenced by the treatments. Because nestlings experience high metabolic activities, an absence of an effect of sibling competition on free radicals production seems unlikely. Nestling body mass decreased and resistance to oxidative stress tended to increase with initial brood size, and hence these correlational effects suggest a trade-off between morphological growth and development of the antioxidant system. However, the result of the experimental treatment did not support this trade-off hypothesis. Alternatively, it suggests that nestling developed compensatory mechanisms that were not detected by our antioxidant capacity measure. Key words: antioxidant capacity, body condition, brood size manipulation, carotenoid supplementation, Parus major, sibling competition. [Behav Ecol]

Parental investment and brood value in tree swallows, Tachycineta bicolor

Summary Parent birds are predicted to provide greater care to broods with greater value. Begging vocalizations may provide proximate cues of brood value. Finally, parental quality may also affect care. We assessed parental care in nestling tree swallows (Tachycineta bicolor) relative to brood value in both natural broods, and in those where brood size and flea loads (Ceratophyllus idius) were manipulated. We also quantified parental quality (based on their size and mass), nestling sex and age (natural broods only), and begging vocalizations (manipulated broods only). Feeding rates were not related to brood value in natural brood sizes (range 2‐6), but were positively related over the range (1‐8) of manipulated brood sizes. Defence intensity was greater for larger and heavier nestlings, independent of age, in natural broods, but not in manipulated broods. Although measures of parental investment were not significantly related to flea load, flea manipulations did not result in higher flea loads when measured following fledging. Parental feeding rates were not associated with variation in parental morphology, but defence intensity was higher by females with longer tails and by heavier males. In our population of tree swallows, only a few of the predicted relationships from parental investment theory were supported.

Experimental manipulation of the rearing environment influences adult female zebra finch song preferences

In songbirds, sensory and social learning processes in juveniles contribute to variation in male song and female preferences. The developmental stress hypothesis proposes that suboptimal early development affects the costly brain structures involved in male song learning and, as a consequence, song quality. As an extension of this hypothesis we tested in this study whether developmental conditions also modulate female song preference acquisition. We tested song preferences in adult female zebra finches, Taeniopygia guttata, originating from a brood size manipulation experiment that had induced differences in mass, condition, immune response and levels of plasma testosterone at the early nestling stage. During the song-learning phase, juvenile birds were housed in small mixed-treatment groups with unrelated adult male song tutors. Adult females' song preferences were tested in an operant set-up where females could trigger different song playbacks by pecking different response keys. When females could choose between their own and an unfamiliar tutor's song they preferred their tutors' songs independent of experimental brood size. However, when females chose between two unfamiliar songs there was a significant effect of experimental brood size on preference strength: females from small broods showed significantly stronger preferences than those from medium and large broods. Hence, both females' rearing environment and sensory learning processes appear to contribute to variation in the direction and strength of female preference for male mating signals.

Heritability of resting metabolic rate in a wild population of blue tits

We report the first study with the aim to estimate heritability in a wild population, a nest box breeding population of blue tits. We estimated heritability as well as genetic and phenotypic correlations of resting metabolic rate (RMR), body mass and tarsus length with an animal model based on data from a split cross-fostering experiment with brood size manipulations. RMR and body mass, but not tarsus length, showed significant levels of explained variation but for different underlying reasons. In body mass, the contribution to the explained variation is mainly because of a strong brood effect, while in RMR it is mainly because of a high heritability. The additive variance in RMR was significant and the heritability was estimated to 0.59. The estimates of heritability of body mass (0.08) and tarsus length (0.00) were both low and based on nonsignificant additive variances. Thus, given the low heritability (and additive variances) in body mass and tarsus length the potential for direct selection on RMR independent of the two traits is high in this population. However, the strong phenotypic correlation between RMR and mass (0.643 +/- 0.079) was partly accounted for by a potentially strong, although highly uncertain, genetic correlation (1.178 +/- 0.456) between the two traits. This indicates that the additive variance of body mass, although low, might still somewhat constrain the independent evolvability of RMR.

Adjustment of brood size and androgen levels in a teleost species with exclusive male parental care

In maximizing reproductive success, individuals face a trade-off between parental care for their current offspring and investing in androgen-dependent sexual traits to produce further offspring. It has thus been proposed that parental effort would suppress androgen levels. Here, we studied parental effort by manipulating brood size in the peacock blenny, Salaria pavo, a littoral fish species in which males show long periods of paternal care. We focused on the effects of brood size on female spawning rate (measured as number of eggs received in the nest by focal males) and androgen levels. We found a positive linear relationship between brood size and the number of eggs received subsequently. Accordingly, spawning rate increased for males with experimentally enlarged broods while it decreased for males in which brood size was reduced. However, over a longer time interval, brood sizes of both treatments returned to the nonmanipulated brood size, suggesting an effect of additional factors such as male quality. Brood size did not show the expected negative relationship with levels of testosterone and 11-ketotestosterone. On the contrary, testosterone levels were positively correlated with brood size. However, in agreement with the prediction, changes in the level of parental care by manipulation of brood size showed an inverse relationship with testosterone levels. As with brood size, given time, testosterone levels tended to return to nonmanipulated levels. Such changes suggest that males adjust their brood size to an individual value through androgen modulation of courtship or other traits influencing female spawning rate.

Sex‐specific effects of altered competition on nestling growth and survival: an experimental manipulation of brood size and sex ratio

1. An increase of competition among adults or nestlings usually negatively affects breeding output. Yet little is known about the differential effects that competition has on the offspring sexes. This could be important because it may influence parental reproductive decisions. 2. In sexual size dimorphic species, two main contradictory mechanisms are proposed regarding sex-specific effects of competition on nestling performance assuming that parents do not feed their chicks differentially: (i) the larger sex requires more resources to grow and is more sensitive to a deterioration of the rearing conditions ('costly sex hypothesis'); (ii) the larger sex has a competitive advantage in intra-brood competition and performs better under adverse conditions ('competitive advantage hypothesis'). 3. In the present study, we manipulated the level of sex-specific sibling competition in a great tit population (Parus major) by altering simultaneously the brood size and the brood sex ratio on two levels: the nest (competition for food among nestlings) and the woodlot where the parents breed (competition for food among adults). We investigated whether altered competition during the nestling phase affected nestling growth traits and survival in the nest and whether the effects differed between males, the larger sex, and females. 4. We found a strong negative and sex-specific effect of experimental brood size on all the nestling traits. In enlarged broods, sexual size dimorphism was smaller which may have resulted from biased mortality towards the less competitive individuals i.e. females of low condition. No effect of brood sex ratio on nestling growth traits was found. 5. Negative brood size effects on nestling traits were stronger in natural high-density areas but we could not confirm this experimentally. 6. Our results did not support the 'costly sex hypothesis' because males did not suffer from higher mortality under harsh conditions. The 'competitive advantage hypothesis' was also not fully supported because females did not suffer more in male-biased broods. 7. We conclude that male nestlings are not likely to be more expensive to raise, yet they have a size-related competitive advantage in large broods, leading to higher mortality of their on average lighter female nest mates.

Short‐ and long‐term consequences of early developmental conditions: a case study on wild and domesticated zebra finches

Divergent selection pressures among populations can result not only in significant differentiation in morphology, physiology and behaviour, but also in how these traits are related to each other, thereby driving the processes of local adaptation and speciation. In the Australian zebra finch, we investigated whether domesticated stock, bred in captivity over tens of generations, differ in their response to a life-history manipulation, compared to birds taken directly from the wild. In a 'common aviary' experiment, we thereto experimentally manipulated the environmental conditions experienced by nestlings early in life by means of a brood size manipulation, and subsequently assessed its short- and long-term consequences on growth, ornamentation, immune function and reproduction. As expected, we found that early environmental conditions had a marked effect on both short- and long-term morphological and life-history traits in all birds. However, although there were pronounced differences between wild and domesticated birds with respect to the absolute expression of many of these traits, which are indicative of the different selection pressures wild and domesticated birds were exposed to in the recent past, manipulated rearing conditions affected morphology and ornamentation of wild and domesticated finches in a very similar way. This suggests that despite significant differentiation between wild and domesticated birds, selection has not altered the relationships among traits. Thus, life-history strategies and investment trade-offs may be relatively stable and not easily altered by selection. This is a reassuring finding in the light of the widespread use of domesticated birds in studies of life-history evolution and sexual selection, and suggests that adaptive explanations may be legitimate when referring to captive bird studies.

The effect of parental quality and malaria infection on nestling performance in the Collared Flycatcher (Ficedula albicollis)

Plumage ornamentation often signals the quality of males and, therefore, female birds may choose elaborately ornamented mates to increase their fitness. Such mate choice may confer both direct and indirect benefits to the offspring. Males with elaborate ornaments may provide good genes, which can result in better nestling growth, survival or resistance against parasitic infections. However, these males may also provision their offspring with more food or food of better quality, resulting in nestlings growing at a higher rate or fledging in better condition. In this study, we examined if there was an association between male ornamentation and malaria infection in Collared Flycatchers (Ficedula albicollis). We also investigated offspring performance in relation to malaria infection in the parents and the quality of the genetic and rearing fathers (assessed by the size of two secondary sexual characters) under simulated good and bad conditions (using brood size manipulation). We found that secondary sexual characters did not signal the ability of males to avoid parasitic infections, and malaria infection in the genetic and the rearing parents had no effect on nestling growth and fledging size. Our results do show, however, that it may be beneficial for the females to mate with males with a large forehead patch because wing feathers of nestlings reared by large-patched males grew at a higher rate. Fast feather growth can result in earlier fledging which, in turn, could improve nestling survival in highly variable environments or under strong nest predation.

Early developmental conditions affect stress response in juvenile but not in adult house sparrows ( Passer domesticus)

The short- and long-term consequences of developmental conditions on fitness have received growing attention because the environmental conditions during early life may influence growth, condition at independence, recruitment, reproductive success or survival. We tested here, in a natural house sparrow population, if early conditions during nestling stage affected the stress response of the birds (i) shortly after fledging and (ii) next year, during their first breeding. We experimentally manipulated brood size to mimic different rearing conditions, creating reduced (−2 chicks) and enlarged broods (+2 chicks), while in a third group brood size was not manipulated. Nestling nutrition state decreased with post-manipulation brood sizes as indicated by lower body mass. Fledglings with higher body mass at the age of ten days showed lower stress response than birds that were leaner at the same age. Fledglings reared in large broods showed a higher response to stress protocol than chicks from small broods, and this effect was in significant interaction with the age of fledglings at capture. This interaction indicates that the effects of the brood size became gradually smaller as the fledglings grew older and were further from their nestling period. The effects of early conditions vanished by the next year: the stress response of adult first time breeders was unrelated to the brood size they fledged from. These results suggest that stress response may reflect the actual state of an individual, rather than its developmental history.

EXPERIMENTALLY PROLONGING THE BROOD-REARING PERIOD REVEALS SEX-SPECIFIC PARENTAL INVESTMENT STRATEGIES IN AMERICAN KESTRELS (FALCO SPARVERIUS)

Abstract Responses of parent birds to changes in demands by their offspring have typically been investigated by manipulating brood size and testing for altered patterns of provisioning. We applied an alternative method to breeding pairs of American Kestrels (Falco sparverius) by modifying the duration of their nesting periods. Rather than focusing on how hard parents can work on any given day, this novel approach tests how long they can sustain the stress of provisioning and the associated behavioral consequences. We manipulated nesting periods by exchanging nearly fledged young at the age of 20 days with offspring from other nests that were 10 days old. We found that males provisioned their second foster broods at rates similar to those for their original broods. By contrast, females brooded offspring in second foster nests less and also delivered less food to them. As a consequence, male offspring in second nests gained less mass than those in first nests, and nestlings of both sexes had reduced feather...

Experimental reduction of incubation temperature affects both nestling and adult blue tits Cyanistes caeruleus

Incubation was for a long time considered to be a period of decreased activity and low cost for parents. It was therefore ignored as a potential factor affecting life‐history trade‐offs in birds. Lately this view has started to change, and studies now show that there might be considerable costs connected to incubation. We experimentally reduced the nest temperature during incubation in blue tits Cyanistes caeruleus, thus increasing the energetic cost of incubation, to test the importance of incubation as a component of reproductive costs and for nestling quality. While most other studies use brood size manipulation to manipulate reproductive costs, we were able to separate treatment effects acting during the incubation period from those acting on later reproductive performance by applying a cross‐foster design. We were also able to isolate the effects of decreased incubation temperature on the nestlings from treatment effects acting on incubating females. We found no experimental effect on the length of the incubation period or on hatching success. The lower temperature during incubation, however, caused lower growth rates in nestlings and reduced chick rearing capacity in adults. We conclude that incubation is a costly period, with the potential to affect both the trade‐off between current and future reproduction and the one between parental effort and offspring quality within the current breeding attempt.