Corticosterone levels in host and parasite nestlings: Is brood parasitism a hormonal stressor?

Parasite chicks from non-evictor species usually try to monopolize host parental care, thereby increasing considerably the level of food competition in the nest. Here, we propose that brood parasitism is an important stressor for host and parasite nestlings and explore this hypothesis in the non-evictor great spotted cuckoo (Clamator glandarius) and its main hosts, the same-sized black-billed magpie (Pica pica) and the larger carrion crow (Corvus corone). We experimentally created 3-nestling broods of different brood compositions (only cuckoo chicks, only host chicks, or cuckoo and host chicks together) and measured baseline corticosterone levels of nestlings along their developmental period (early, middle and late). We found that brood parasitism increased corticosterone levels in magpie nestlings in the mid and late nestling period compared to those raised in unparasitized nests. Interestingly, carrion crow nestlings from parasitized nests only increased their corticosterone levels in the mid nestling period, when the competition for food with the cuckoo nestling was highest. Our results suggest that brood parasitism could be a potential physiological stressor for host nestlings, especially during the developmental stages where food requirements are highest. Conversely, cuckoo nestlings could be physiologically adapted to high competition levels since they did not show significant differences in corticosterone levels in relation to brood composition.     

Intraseasonal effects of clutch manipulation on parental provisioning and residual reproductive value of eastern phoebes (Sayornis phoebe)

Summary First clutches of double-brooded eastern phoebes Sayornis phoebe were manipulated (up two eggs, down 2 eggs or no change) to test for intraseasonal reproductive tradeoffs and to test whether size of first brood influenced food delivery rates to nestlings and nestling quality in second broods.Considering all nests from both broods, rate of feeding nestlings increased linearly with brood size but nestling mass per nest decreased with increasing brood size. High nestling weights in small broods may have resulted from parents delivering better quality food, but we did not test this.Among treatment groups in first broods, nestlings from decreased broods weighed more than those in control or increased broods. Treatment did not influence the likelihood that second nests would be attempted after successful first nests nor did it alter the interval between nests. Nestlings of parents that renested weighed more than those of parents that did not, regardless of treatment, suggesting that post-fledging care may preclude renesting. Mass of individual females did not change between broods, regardless of brood size. Clutch sizes of second attempts were not affected by manipulations of first broods but increasing first broods reduced the number of nestlings parents were able to raise to day 11 in their second broods. However, manipulation of first broods did not affect mean nestling mass per nest of nestlings that survived to day 11.In phoebes, parents of small first broods are able to raise nestlings in better condition. We predict that in harsh years, parents of small first broods would be more likely to renest. Parents of enlarged first broods sacrificed quality of offspring in second broods, which seems a reasonable strategy if nestlings from second broods have lower reproductive value.     

Parental Alarm Calls of the White‐Crowned Sparrow Fail to Stimulate Corticosterone Production in Nest‐Bound Offspring

Alarm calling by parents is widespread among animals and has strong implications for parent and offspring fitness, yet it is virtually unknown whether parental alarm calls can initiate a corticosterone response in offspring. We investigated whether parental alarm calls of the white‐crowned sparrow, Zonotrichia leucophrys, activated the corticosterone response of their nest‐bound young, as such a response might prepare older nestlings for premature fledging and increase their survival when contacted by a predator at the nest. We conducted an experiment in which nestlings were either exposed to parent alarm calls (treatment) or experienced a period without parental alarm calls (control) immediately prior to blood sampling. We then sampled nestlings to measure corticosterone levels within 4 min of first contact (baseline corticosterone) and 60 min later (handling‐induced corticosterone). Young nestlings (i.e. 3–4 d post‐hatch) did not exhibit a corticosterone response to parental alarm calls or to handling, as mean corticosterone levels were similar in the control and treatment groups for both baseline and 60‐min post‐baseline samples. Against our predictions, there was no difference in mean levels of baseline corticosterone between control and treatment groups in older nestlings (i.e. 7?8 d post‐hatch) that were capable of surviving out of the nest. However, we did find a significant increase in mean levels of corticosterone after handling in both groups, which indicated that older nestlings were able to mount a functional corticosterone response when confronted with a potential predator. Why older nestlings did not initiate a corticosterone response after exposure to parental alarm calls is unclear but may have occurred because the costs of mounting such a response outweighed the benefits, perhaps because of growth or developmental costs.     

The process of fledging in the Mountain Bluebird

Fledging is a critical event in the avian breeding cycle, but remains unstudied in almost all species. As a result, little is known about factors that cause nestlings to leave nests. We documented fledging behavior in a box‐nesting population of Mountain Bluebirds (Sialia currucoides) using radio‐frequency identification. We attached a passive integrated transponder (PIT tag) to the leg of each nestling in 40 nests. An antenna checked for the presence of a transponder signal (i.e., a nestling) at nest‐box entrances every 2 s. The time of last detection of a nestling was taken as the time that nestling fledged. We found that fledging began when the oldest nestlings were 15–22 d old. Broods that were ahead in development, as measured by primary feather length, fledged at relatively younger ages. All nestlings fledged on the same day at 33 nests (83%) and over 2 d at remaining nests. When all nestlings fledged on the same day, fledging usually began in the morning and median time between the first and last fledging was 55 min (range = 2.3 min–10.6 h). When young fledged over 2 d, fledging always began >8 h after sunrise and usually just one nestling fledged the first day, suggesting that this fledging may have been accidental. Clutches in our population often hatch asynchronously, which sets up a hierarchy within broods in developmental state, size, and competitive ability. In such situations, fledging may be initiated by one of the most‐developed and hence most‐competitive nestlings in a brood, presumably when it reaches a certain threshold state of development. Alternatively, fledging may begin when a less‐developed, less‐competitive, and probably hungrier nestling leaves the nest, presumably to gain better access to food. We used the proportion of time that a nestling was able to occupy the nest‐box entrance late in the nestling stage, waiting to intercept parents with food, as an index of nestling competitive ability. Assuming that the number of nest entrance detections reliably indicates nestling competitive ability, we found that the most‐competitive nestling fledged first at over half of all nests, supporting the notion that fledging usually begins when oldest nestlings reach a threshold state of development.     

Sudden weather deterioration but not brood size affects baseline corticosterone levels in nestling Alpine swifts

While evidence is accumulating that stress-induced glucocorticoid responses help organisms to quickly adjust their physiology and behaviour to life-threatening environmental perturbations, the function and the ecological factors inducing variation in baseline glucocorticoid levels remain poorly understood. In this study we investigated the effects of brood size by experimentally manipulating the number of nestlings per brood and the effect of weather condition on baseline corticosterone levels of nestling Alpine swifts (Apus melba). We also examined the potential negative consequences of an elevation of baseline corticosterone on nestling immunity by correlating corticosterone levels with ectoparasite intensity and the antibody production towards a vaccine. Although nestlings reared in enlarged broods were in poorer condition than nestlings reared in reduced broods, they showed similar baseline corticosterone levels. In contrast, nestling baseline corticosterone levels were higher immediately after cold and rainy episodes with strong winds. Neither nestling infestation rate by ectoparastic flies nor nestling antibody production against a vaccine was correlated with baseline corticosterone levels. Thus, our results suggest that altricial Alpine swift nestlings can quickly modulate baseline corticosterone levels in response to unpredictable variations in meteorological perturbation but not to brood size which may be associated with the degree of sibling competition. Apparently, short-term elevations of baseline corticosterone have no negative effects on nestling immunocompetence.     

The Process and Causes of Fledging in a Cavity-Nesting Passerine Bird, the House Wren (Troglodytes aedon)

Little is known about the process or causes of fledging or nest‐leaving in passerine birds because researchers can rarely predict when fledging will occur in a given nest. We used continuous videotaping of nests to both document the process of fledging in the house wren, Troglodytes aedon, a small, cavity‐nesting songbird, and test hypotheses as to what might cause fledging to begin. Fledging began any time from 14 to 19 d after hatching commenced. Slower‐developing broods fledged later than faster‐developing broods. Fledging typically began within 5 h of sunrise and over 80% of all nestlings fledged before noon. All nestlings fledged on the same day at 65% of nests and over two consecutive days in most other nests. We found no evidence that fledging was triggered by changes in parental behaviour. Parental rate of food delivery to nestlings did not decline during a 3‐h period leading up to the first fledging, nor was the rate of feeding just prior to the first fledging lower than the rate at the same time the day before. Moreover, parents did not slow the rate of food delivery to nests after part of the brood had fledged. Hatching is asynchronous in our study population which creates a marked age/size hierarchy within broods. At most nests, the first nestling to fledge was the most well‐developed nestling in the brood or nearly so (as measured by feather length). This suggests that fledging typically begins when the most well‐developed nestlings in the brood reach some threshold size. However, at about one‐fifth of nests, the first nestling to fledge was only moderate in size. At these nests, severe competition for food may have caused smaller, less competitive nestlings to fledge first to increase their access to food. We found no strong support for the suggestion that the oldest nestlings delay fledging until their least‐developed nestmate reaches some minimum size, although further experimental work on this question is warranted.     

Ectoparasitism in marsh tits: costs and functional explanations

Among hole-nesting birds, the blood-sucking hen flea is a commonparasite affecting both nestlings and parents. By adding fleasto marsh tit (Parus palustris) nests, I aimed to investigatethe effect of fleas on nestling growth rate and parental effortas well as evaluating a potential mechanism by which fleas affectnestlings (i.e., resting metabolic rate; RMR). Nestlings fromflea-infested broods were lighter than nestlings from controlbroods. This reduced growth rate was evident as soon as 3 daysafter adding extra fleas to the nest, but the reduction didnot increase after this initial drop in mass. Parents did notalter their feeding frequency in response to the manipulation;thus the small size of nestlings in manipulated nests seemsto be directly caused by the fleas. Mass-specific RMR was significantlyhigher in nestlings from flea-infested nests compared to nestlingsfrom control nests. I used the results to evaluate the suggestedmechanisms for parasite-related decrease in host growth rate.The increase in RMR and the very rapid reduction in nestlinggrowth rate after experimental addition of fleas can be explainedby an immune reaction, mainly by the innate immune system, tosubstances in the saliva of the fleas.     

Siblicide, starvation and nestling growth in the laughing kookaburra

I examined the growth of surviving nestlings in broods of the cooperatively breeding laughing kookaburra Dacelo novaeguineae , which has complex patterns of brood reduction. Laughing kookaburras usually lay three eggs that hatch asynchronously. Brood reduction occurs in nearly half of all broods and always affects the youngest nestling. In most cases, the youngest nestling is killed within a few days of hatching by aggressive attacks from its older siblings. In a smaller proportion of nests, the youngest nestling dies from starvation, rather than physical attack, much later in the nestling period when nestling growth rates and adult feeding rates peak (about 20 days post-hatching). These mechanistically and temporally distinct episodes of brood reduction were associated with very different patterns of growth in the senior nestlings. Seniors that killed their youngest sibling reached higher asymptotic weights than seniors that did not commit siblicide. In contrast, if the youngest nestling was not killed by its older siblings, but later starved to death, the surviving seniors were skeletally smaller and had retarded feather development compared to seniors from other broods. These differences in nestling growth may have longer-term fitness consequences, because kookaburra fledging weight is positively associated with both juvenile survival and successful recruitment into the breeding population. Therefore, although parents of broods without mortality produce the highest number of fledglings and also the highest number of independent juveniles, if parents are unable to raise a full brood, early siblicide may represent the best brood reduction option. Early siblicide is at least associated with high quality young that have enhanced survival and recruitment prospects. In contrast, the poor growth of seniors in broods where the youngest nestling starved suggests that parents overestimated the size of the brood they could provision.     

Assessing the effects of repeated handling on the physiology and condition of semi‐precocial nestlings

Repeated exposure to elevated levels of glucocorticoids during development can have long‐term detrimental effects on survival and fitness, potentially associated with increased telomere attrition. Nestling birds are regularly handled for ecological research, yet few authors have considered the potential for handling‐induced stress to influence hormonally mediated phenotypic development or bias interpretations of subsequent focal measurements. We experimentally manipulated the handling experience of the semi‐precocial nestlings of European Storm Petrel Hydrobates pelagicus to simulate handling in a typical field study and examined cumulative effects on physiology and condition in late postnatal development. Neither baseline corticosterone (the primary glucocorticoid in birds), telomere length nor body condition varied with the number of handling episodes. The absence of a response could be explained if Storm Petrels did not perceive handling to be stressful or if there is dissociation of the hypothalamic–pituitary–adrenal axis from stressful stimuli in early life. Eliciting a response to a stressor may be maladaptive for cavity‐dwelling young that are unable to escape or defend themselves. Furthermore, avoiding elevated overall glucocorticoid exposure may be particularly important in a long‐lived species, in which accelerated early‐life telomere erosion could impact negatively upon longevity. We propose that the level of colony‐wide disturbance induced by investigator handling of young could be important in underlining species‐specific responses. Storm Petrel nestlings appear unresponsive to investigator handling within the limits of handling in a typical field study and handling at this level should not bias physiological and morphological measurements.     

Supplementary feeding increases nestling feather corticosterone early in the breeding season in house sparrows

Several studies on birds have proposed that a lack of invertebrate prey in urbanized areas could be the main cause for generally lower levels of breeding success compared to rural habitats. Previous work on house sparrows Passer domesticus found that supplemental feeding in urbanized areas increased breeding success but did not contribute to population growth. Here, we hypothesize that supplementary feeding allows house sparrows to achieve higher breeding success but at the cost of lower nestling quality. As abundant food supplies may permit both high‐ and low‐quality nestlings to survive, we also predict that within‐brood variation in proxies of nestling quality would be larger for supplemental food broods than for unfed broods. As proxies of nestling quality, we considered feather corticosterone (CORT_f), body condition (scaled mass index, SMI), and tarsus‐based fluctuating asymmetry (FA). Our hypothesis was only partially supported as we did not find an overall effect of food supplementation on FA or SMI. Rather, food supplementation affected nestling phenotype only early in the breeding season in terms of elevated CORT_f levels and a tendency for more variable within‐brood CORT_f and FA. Early food supplemented nests therefore seemed to include at least some nestlings that faced increased stressors during development, possibly due to harsher environmental (e.g., related to food and temperature) conditions early in the breeding season that would increase sibling competition, especially in larger broods. The fact that CORT_f was positively, rather than inversely, related to nestling SMI further suggests that factors influencing CORT_f and SMI are likely operating over different periods or, alternatively, that nestlings in good nutritional condition also invest in high‐quality feathers.     

Brood size and begging intensity in nestling birds

Theoretical models suggest that sibling competition should selectfor conspicuous begging signals. If so, begging intensity mightbe expected to increase with the number of competitiors. Thepurpose of our study was to examine the relationship betweenbegging intensity and brood size using nestling tree swallows(Tachycineta bicolor) as our model. Over 2 years, we videotapedbegging behavior in unmanipulated broods of different sizes.We found that begging intensity increased with brood size. Theaverage weight of nestlings in each brood did not vary withbrood size, but feeding rate per nestling decreased with broodsize, suggesting that nestlings in larger broods begged moreintensively, possibly because they were hungrier. We also conductedan experiment to examine the effect of nest mates on beggingin different-sized broods. We found that nestlings with similarweights, previous competitive environments, and food deprivationbegged more intensively in large broods than in small broods.Overall, our study indicates that begging intensity increaseswith brood size in tree swallows. This relationship may resultfrom interactions among brood mates rather than from lower feeding rates to individual nestlings in larger broods.     

Optimal brood size and its limiting factors in the Rufous Turtle DoveStreptopelia orientalis

Optimal brood size and its limiting factors of the Rufous Turtle Dove,Streptopelia orientalis, were studied at the campus of the University of Tsukuba, Japan, during the breeding season in 1990–92. The dove usually lays two eggs in a nest. I made nests of a brood size of one and three by transferring a hatchling from one nest to the other, and compared their fledging success, factors of breeding failure, weight and tarsus length at fledging, growth rate and nestling period with those of a brood of two. The index of fitness (fledgling weight multiplied by average number of fledglings per nest) was almost the same in broods of two and three. However, the highest variation in fledging weight within the brood and the extension of nestling period were observed in broods of three, which caused the extension of inter-brood interval and consequently the smaller number of broods in the total breeding season. Therefore, broods of three would not have an advantage in producing more offspring than broods of two. Crop milk production had an effect on the growth of nestlings in the early phase of the nestling period, but the rapid growth in the granivorous phase compensated for the growth delay of the smallest nestling in broods of three. Small brood size and a large number of broods in a season would also be more effective under high predation pressure.     

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.     

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.     

Chick loss from mixed broods reflects severe nestmate competition between an evictor brood parasite and its hosts

Hatchlings of the obligate brood parasite common cuckoo Cuculus canorus typically evict eggs and nestmates but, rarely, host and parasite nestlings may grow up together. As part of previous experiments, we manipulated host clutches by inducing two great reed warbler Acrocephalus arundinaceus and one parasite young to share a nest from 4 days posthatch, when the cuckoo's eviction behaviour is thought to cease. We documented that in mixed broods typically at least one nestling eventually fell out of nest during the period of 5-10 days posthatch. In 83% of nests one or two host chicks disappeared, and in 17% of nests parasite chicks were lost. All nestlings remained in control broods of three hosts or one parasite. These results imply strong physical competition for space in mixed broods. We suggest that continued foster care for parasitized broods may occasionally be beneficial because host nestlings have some chance to escape the costs of parasitism, even when their parents fail to reject the parasite's egg and the parasite hatchling fails to evict nestmates. Conversely, evictor parasite chicks benefit not only through improved growth, as reported before, but also through the elimination of nestmate competition for space and the risk of displacement from mixed broods.     

Weight differences, brood reduction, and sibling competition among nestling Stonechats Saxicola torquata (Aves: Turdidae)

Weight differences within broods of nestling birds are commonly assumed to be the outcome of a parental strategy of selectively starving one or more offspring so as to enhance the survival of the remaining chicks. This study examined patterns of development of nestling Stonechats to reveal the significance of weight hierarchies in determining the relative growth and survival of siblings. Although Stonechat broods display obvious weight hierarchies, the results show that a low rank does not strongly influence survival within the nest, especially early in the nestling period, and in small broods. Low ranking chicks were more likely to leave the nest at lighter weights, however. A proximate mechanism for the establishment of weight hierarchies was investigated, involving competition among nestlings for advantageous positions in the nest, in which the heavier siblings were more successful.     

Removal of nestling radio-transmitters by adult Sprague’s Pipit (Anthus spragueii)

Fledgling birds are notoriously difficult to find and capture because of their cryptic behavior. As a result, researchers usually affix transmitters to nestlings to study aspects of post-fledging ecology. However, parents may attempt to remove nestling transmitters, which could negatively impact nestlings. We attached radio-transmitters to 95 Sprague’s Pipit (Anthus spragueii) nestlings using a modified Rappole and Tipton leg harness. Within 1–2 days after transmitter attachment, we recorded six cases of parents removing transmitters from nestlings and depositing the transmitters outside the nest. At one of these nests, we videotaped parents pecking and pulling at nestlings that had transmitters and also dragging a nestling out of the nest by the transmitter. Of 12 video-monitored nests where we attached transmitters to nestlings, 33% had cases of parents attempting to remove transmitters by pecking and pulling at nestlings. Our observations highlight the need to closely monitor nestlings with transmitters so that researchers have the opportunity to reattach transmitters should parents remove them and to monitor the health of nestlings. Video-monitoring nests could also help to identify any unseen, negative impacts on nestlings due to transmitter attachment.     

Shade seeking by Common Grackle (Quiscalus quiscula) nestlings at the scale of the nanoclimate

Solar radiation is a major source of heat for open-cup passerine nestlings, and a key variable influencing parental nest site selection. Although temperature, like food, is critical for nestling growth and survival, the use of thermal and insolation gradients when describing passerine nestling orientation and movement within the open-cup nest has not been explored. Our study used the Common Grackle (Quiscalus quiscula) as a model system to directly test the hypothesis that passerine nestlings mitigate thermal extremes behaviourally. Pairs of nestlings were tested in shaded (homogeneous) and exposed (heterogeneous) nests, movements within nests were tracked and nest temperatures recorded. Our results show that nestlings are able to detect and respond to nanoclimate heterogeneity, and can mitigate short-term radiant heat loads through shade-seeking.     

Within-brood distribution of ectoparasite attacks on nestling blue tits: a test of the tasty chick hypothesis using inulin as a tracer

Blow fly ( Protocalliphora spp.) larvae are nest ectoparasites that compete with nestlings for resources by feeding on blood. Their main direct consequences on nestlings are a reduction in growth, blood parameters, and physiological performance. The impact of Protocalliphora larvae on nestling development can be extremely variable between nests within years and between years due to varying food availability. However, there is also considerable within-nest variation in nestling growth and development which has been attributed to an aggregation of ectoparasites on less resistant chicks; the tasty chick hypothesis. We tested for aggregated attacks of Protocalliphora within broods of nestling blue tits using radio-labelled inulin to trace plasma, and hence blood, turnover. To estimate blood removal by Protocalliphora, we compared inulin turnover in naturally parasitized and experimentally deparasitized nestlings. Inulin turnover, and hence the blood loss attributed to Protocalliphora attacks, was significantly correlated with chick mass and size rank within the brood, with the smallest and lowest-ranking chicks showing greatest inulin turnover. This is the first study demonstrating non-random blood-feeding by Protocalliphora larvae, a phenomenon which can render the statistical detection of their impact problematical by increasing within-brood variance in developmental and physiological measures.     

Fitness-related consequences of egg mass in nestling house wrens

The fitness-related consequences of egg mass, independent of confounding influences associated with parental quality, remain poorly understood for wild birds in general and for passerines in particular. We performed cross-fostering experiments to test the hypothesis that egg mass, independent of parental quality, is the primary determinant of fitness-related traits in nestling house wrens (Troglodytes aedon), an insectivorous passerine. Nestling mass was significantly correlated with the mass of the eggs from which nestlings hatched early but not late in the nestling period in early-season broods. In contrast, in late-season broods, nestling mass was correlated with egg mass until nestlings achieved asymptotic mass. Neither nestling growth nor survival to nest leaving was related to egg mass in either early- or late-season broods; however, nestlings in late-season broods grew more slowly than did nestlings in early-season broods. We propose that nestling mass and egg mass remained correlated throughout the nestling period in late-season broods because decreased arthropod food resources late in the breeding season constrain parents'' ability to provision nestlings. We conclude that female house wrens in this population trade-off clutch size for greater egg mass to maximise reproductive success in late-season broods.