Growth Of Nestlings Research Articles

IGF-1 receptor inhibitor OSI-906 reduces growth in nestlings of a wild passerine

Young animals need to grow to a large body size fast to maximise their survival prospects until sexual maturity. However, body size varies substantially in wild populations, and neither the selection pressures maintaining this variation, nor the regulatory mechanisms are well understood. IGF-1 administration has been shown to accelerate growth, but this does not necessarily imply that natural variation in growth rate is IGF-1 dependent. To test the latter we administered OSI-906 to pied flycatcher Ficedula hypoleuca nestlings, which has an inhibitory effect on IGF-1 receptor activity. We performed the experiment in two breeding seasons to test the prediction that blocking the IGF-1 receptor downregulates growth. As predicted, OSI-906 treated nestlings had lower body mass and reached a smaller structural size than siblings receiving a vehicle only, with the mass difference being most profound at the age preceding the highest body mass growth rate. The IGF-1 receptor inhibition effect on growth varied with age and year of study, and we discuss possible explanations. The OSI-906 administration results indicate that natural variation in growth rate is regulated by IGF-1, and constitutes a novel tool to study causes and consequences of growth variation, but details of the underlying mechanism still need to be resolved.

The Role of Food Quantity and Prey Type in Nestling Development of American Kestrels

ABSTRACT Diet is an important component of life history that can vary with, and ultimately determine, individual variation in phenotypically plastic traits. American Kestrels (Falco sparverius) generally have low post-fledging survival rates, which suggests a need to better understand what ecological factors, such as diet, influence nestling maturity. The generalist diet of kestrels makes it unlikely that all nestlings in a population receive the same diet. We investigated how breeding phenology and nestling sex ratio interact with diet metrics (diet diversity, percentage of prey types, rate of prey biomass delivery) and relate to nestling maturity (mass, tarsus length, wing length, hematocrit, hemoglobin concentration). We hypothesized that: (1) phenology and nestling sex ratio would correlate with inter-nest diet variation; (2) diet metrics would be predictive of nestling development; and (3) the manipulation of food quantity through food supplementation would lead to nestlings with greater developmental maturity. We found that inter-nest variation in diet was correlated with breeding phenology and nestling sex ratio, independently. However, the variation in diet was unrelated to nestling maturity. In response to food supplementation, kestrel parents decreased their food-provisioning rate, indicating that food quantity regulates parental care. Male nestlings appeared to benefit from supplementation while females did not. Our data demonstrated high inter-brood variation in nestling diet, and suggested that diet variation interacts with sex to influence growth and development of nestlings, which could potentially be linked to population decline.

Are brood sex ratios adaptive?-The effect of experimentally altered brood sex ratio on nestling growth, mortality and recruitment.

Brood sex ratios (BSRs) have often been found to be nonrandom in respect of parental and environmental quality, and many hypotheses suggest that nonrandom sex ratios can be adaptive. To specifically test the adaptive value of biased BSRs, it is crucial to disentangle the consequences of BSR and maternal effects. In multiparous species, this requires cross-fostering experiments where foster parents rear offspring originating from multiple broods, and where the interactive effect of original and manipulated BSR on fitness components is tested. To our knowledge, our study on collared flycatchers (Ficedula albicollis) is the first that meets these requirements. In this species, where BSRs had previously been shown to be related to parental characteristics, we altered the original BSR of the parents shortly after hatching by cross-fostering nestlings among trios of broods and examined the effects on growth, mortality and recruitment of the nestlings. We found that original and experimental BSR, as well as the interaction of the two, were unrelated to the fitness components considered. Nestling growth was related only to background variables, namely brood size and hatching rank. Nestling mortality was related only to hatching asynchrony. Our results therefore do not support that the observed BSRs are adaptive in our study population. However, we cannot exclude the possibility of direct effects of experimentally altered BSRs on parental fitness, which should be evaluated in the future. In addition, studies similar to ours are required on various species to get a clearer picture of the adaptive value of nonrandom BSRs.

Home range size and habitat quality affect breeding success but not parental investment in barn owl males

Life-history theory predicts that parents should balance their limited resources to maximize lifetime fitness, limiting their investment in current reproduction when the fitness value of current progeny is lower than that gained by producing offspring in the future. Here, we examined whether male barn owls (Tyto alba) breeding in low-quality habitats increased their parental effort to successfully complete offspring rearing or limited their investment by paying a fitness cost while saving energy for the future. We equipped 128 males with GPS devices between 2016 and 2020 to collect information on home range size, habitat composition, food provisioning rate to the brood and nightly distances covered. We also recorded nestlings’ growth and survival, as well as males’ body mass variation and future reproductive success. Males living in lower-quality habitats exploited bigger home ranges compared to individuals whose nests were settled in prey-rich habitats. They fed their brood less frequently, while covering longer nightly distance, resulting in a slower growth of late-hatched nestlings and ultimately in a lower fledging success. As males did not differ in body mass variation or future reproductive success our findings suggest that males hunting in home ranges with less prey-rich structures do not jeopardize future reproduction by investing disproportionately larger resources to compensate for their current low home range quality.

Ectoparasites exposure affects early growth and mouth colour in nestlings of a cavity-nesting raptor

Ectoparasites exposure affects early growth and mouth colour in nestlings of a cavity-nesting raptor

Can diet composition estimates using stable isotope analysis of feathers predict growth and condition in nestling mountain bluebirds (Sialia currucoides)?

Insectivorous birds breeding in seasonal environments provision their dependent young during periods when prey diversity and abundance vary. Consequently, the composition and nutritional value of diets parents feed to their offspring may differ within and among broods, potentially affecting the condition of nestlings. In a population of mountain bluebirds (Sialia currucoides), we used two methods to estimate diet composition for individual nestlings: direct observation of provisioning using video recordings at 5 and 9 days post‐hatch, and stable isotopes of the δ13C and δ15N in nestling feathers and prey followed by analysis with mixing models. We determined the macronutrient content (% fat and lean mass) and estimated the metabolized energy from each type of prey. We evaluated whether different methods of estimating diet composition would produce similar results, and whether the types of prey nestlings ate at one or both ages affected their morphology, growth rates, or blood ketone concentration. We found that bluebirds fed their young 5 main types of prey: beetles, cicadas, grasshoppers, insect larvae, and spiders. Both observational and mixing model estimates of diet composition indicated that larvae are traded off with grasshoppers and that fewer larvae are provided to nestlings as the season progresses. In evaluating how diet influences individual growth and condition, estimates from direct observations had greater explanatory power than those from mixing models, indicating that diets rich in the most energy‐dense prey (greatest fat content; cicadas and larvae) were associated with larger size and higher body condition, and faster rate of mass gain and growth of tarsus. Lower value prey had more limited, specific effects on nestlings, but may still be important dietary components. While isotopic methods produced estimates of diet composition that were generally informative, when applied to explain the growth and condition of nestlings they proved less useful.

A comparison of the diet of urban and forest great tits in a Mediterranean habitat

The low breeding performance and body condition of nestling passerine birds in urban environments has been attributed to the poor quality and low abundance of food in these settings. However, detailed data on prey provided by parents to their chicks in the urban habitat is scarce. Here we used video cameras set in nest boxes to compare the diet of urban and forest great tits Parus major when provisioning their chicks in a Mediterranean area. We additionally analysed brood size and fledgling success. Breeding success of urban great tits was lower than that of forest birds. Urban parents displayed a lower average hourly feeding rate per nestling than forest parents. Among the three prey item categories, the percentage of spiders did not vary according to habitat. However, the percentage of caterpillars delivered to the nest by great tit parents was higher in the forest than in the urban habitat while the percentage of ‘other’ prey showed a reverse pattern. ‘Other’ prey were mainly adult butterflies and wasps in the urban habitat. Our paper adds to the view that the low feeding rates and scarcity of caterpillars in urban environments may be the underlying cause constraining the growth of great tit nestlings in these areas.

The Impacts of Parasitic Flies (Philornis spp.) on Nestlings of Three Passerines in a Southern Temperate Forest of Argentina

Philornis flies exert strong negative selection on avian host life histories. However, their possible influence at the southern limits of parasite distribution remains poorly studied. We collected data on Philornis parasitism (prevalence, parasite load and latency) during three consecutive breeding seasons of the Masked Gnatcatcher Polioptila dumicola, Vermilion Flycatcher Pyrocephalus rubinus and Blue-and-yellow Tanager Pipraeidea bonariensis in a south temperate forest in Argentina. We hypothesised that Philornis parasitism would negatively influence both the growth and survival of infected nestlings. Regarding body size differences among nestlings of these three host species, we predicted that nestling survival in parasitised nests and parasite load per nestling would be greater the larger the species, and that the number of growth parameters that differ between parasitised and non-parasitised nestlings would be greater the smaller the species. We monitored 564 nests and found prevalence to be 16.7% for the Masked Gnatcatcher, 30.1% for the Vermilion Flycatcher, and 37.9% for the Blue-and-yellow Tanager. Parasitism increased during the season for all three species. As predicted, for the three species, nestling survival was lower in parasitised nests (∼30%) than in non-parasitised nests (∼90%) and for each species, one or more growth parameters were significantly lower for parasitised nestlings compared to non-parasitised nestlings. Our prediction relating to host body size was partially supported since differences in parasite load only emerged in nests that produced fledglings, as well as in the extent of growth parameters affected. This is the first study providing detailed data of Philornis fly parasitism and how this parasite affects the fitness of three bird hosts at the southern limit of the parasite's distribution.—Gonzalez, E., Jauregui, A. & Segura, L.N. (2022). The impacts of parasitic flies (Philornis spp.) on nestlings of three passerines in a southern temperate forest of Argentina. Ardeola, 69: 3-20.

Early life neonicotinoid exposure results in proximal benefits and ultimate carryover effects

Neonicotinoids are insecticides widely used as seed treatments that appear to have multiple negative effects on birds at a diversity of biological scales. Adult birds exposed to a low dose of imidacloprid, one of the most commonly used neonicotinoids, presented reduced fat stores, delayed migration and potentially altered orientation. However, little is known on the effect of imidacloprid on birds growth rate despite studies that have documented disruptive effects of low imidacloprid doses on thyroid gland communication. We performed a 2 times 2 factorial design experiment in Zebra finches, in which nestling birds were exposed to a very low dose (0.205 mg kg body hbox {mass}^{-1}) of imidacloprid combined with food restriction during posthatch development. During the early developmental period, imidacloprid exposure resulted in an improvement of body condition index in treated nestlings relative to controls. Imidacloprid also led to compensatory growth in food restricted nestlings. This early life neonicotinoid exposure also carried over to adult age, with exposed birds showing higher lean mass and basal metabolic rate than controls at ages of 90–800 days. This study presents the first evidence that very low-dose neonicotinoid exposure during early life can permanently alter adult phenotype in birds.

Sex-specific growth of nestlings of the Whiskered Tern Chlidonias hybrida, a species with sexual size dimorphism and female brood desertion

The growth of a nestling during the first few weeks after hatching is crucial for its further life and is sensitive to the conditions experienced during this period. Among species exhibiting Sexual Size Dimorphism (SSD), one of the most important factors influencing growth parameters is the sex of the hatchlings. In this study, we tested whether sex and other factors (hatching date, egg volume, brood size and sex ratio in the brood) were related to sex on growth parameters in Whiskered Tern (Chlidonias hybrida) chicks. In this species, males are 3%–10% larger than females when adult. At hatching, only the total head length was greater in males than females, but values close to fledging were greater for most of the measured parameters (3%—tarsus, 4%—total head length, 13%—body mass). Moreover, the maximum body mass growth rate was higher in males but females entered the rapid growth phase somewhat earlier and achieved the final total head length sooner. Nestling growth was also significantly correlated with brood size and hatching date, though mostly negatively (e.g. lower tarsus and total head length asymptote, lower maximum growth rate and growth rate constant of total head length in bigger and later clutches). The occurrence of SSD during the nestling period and higher maximum growth rate of body mass in males indicates that the costs of raising sons may be higher than of raising daughters. However, we did not find evidence of either sex-biased nestling mortality or skewed sex ratio.

Growth Curves of Ryukyu Scops Owl Nestlings, an Owl Species with Asynchronous Hatching and Reversed Sexual Dimorphism

The growth pattern of an organism is influenced by multiple factors, such as its sex, parental investment, food abundance and sibling competition. For species showing sexual size dimorphism, the sex of siblings is expected to affect the growth of nestlings through differences in competitive hierarchies mediated by an asymmetry in body size. This study describes growth curves of Ryukyu Scops Owl Otus elegans nestlings, based on the analysis of body mass, wing length and tarsus length in 99 individuals. We subsequently tested for effects of sex of the nestlings, having an older male sibling, hatching order and brood size on the growth curves. Overall, we detected significant effects of several predictor variables, although differences between groups were often small. Males and later hatched chicks had lower body mass throughout their growth. The smaller the brood, the heavier the chicks were on days 0–1 and 30–32, although this relationship was reversed on days 8–13. Males and later hatched chicks also had shorter wings. Males had shorter tarsi than females from day 0 to day 4, however, dimorphism in this trait was reversed afterwards. Chicks in a large brood had longer tarsi. Earlier hatched chicks had longer tarsi in the first half of the growth period, but some of this difference had disappeared by the second half. From day 0 to day 3, individuals with an older brother had longer tarsi than those with an older sister. However, halfway through the growth period and thereafter, individuals with an older brother had shorter tarsi than those with an older sister. The fact that older brothers had an effect on the tarsus growth of their siblings was not due to sexual body size asymmetry, as in fact males were smaller than females in wing length and body size. We suggest that sexual differences in aggression mediated by androgens may have played a role.

Growth and Development of Nestlings of White-Throated Kingfisher, Halcyon Smyrnensis (Linnaeus, 1758)

The growth and development of the nestlings of white-throated kingfisher (Halcyon smyrnensis) showed that at the hatching day, the mean body weight was 14.6±1.0 g which gained up to 69.9±3.0 g during fledging. The mean length of the body, wing, beak, head, tarsus and feet were 59.4±12.4, 19.4±5.7, 3.1±1.2, 11.7±1.4, 8.5±1.8 and 17.3±3.6 mm, respectively at the hatching day and 203.5±14.1, 105.7±5.8, 40.4±1.3, 29.1±1.1, 14.9±0.4 and 29.8±0.7 mm, respectively during fledging day. The primaries, rectrices and the claw was started to grow from 3rd day hatching and grew up to 67.8±5.6, 27.7±3.4 and 5.2±0.2 mm, respectively during fledging time.
 Asiat. Soc. Bangladesh, Sci. 46(1): 71-80, June 2020

Parental dependence on the nest's spatial cues in offspring recognition decreases with nestling growth in the azure-winged magpie.

In altricial birds, to address which cues are used by parents to recognize their offspring, and when they switch between cues during reproduction, it has not been well determined. In this study, we address this question in a Tibetan population of the azure-winged magpie Cyanopica cyanus, by examining the dependence of parents on a nest’s spatial position in offspring recognition. During the egg and nestling phases, azure-winged magpie nests were translocated to new positions across various distances from their original site, and parental responses to the translocated nests were investigated. Our findings show that a nest’s spatial position is not connected with the survival of its young, but might be used as a cue in parental offspring recognition. When nests are translocated to a new position within a certain distance, parents could recognize their nests and returned to resume their parenting behaviors. Parental dependence on the nest’s spatial position in offspring recognition is higher during the egg phase than during the nestling phase, and it decreases with the growth of nestlings. After nestlings reach a certain age, the nest’ s spatial position was no longer used by parents as the single cue for offspring recognition. These findings suggest that azure-winged magpies switch their cues in offspring recognition during the different stages of reproduction. After parent–offspring communication has been established, the offspring’s phenotypic traits may become a more reliable cue than the nest’s spatial position in offspring recognition.

Nestling growth and body condition of critically endangered Orange-bellied Parrots Neophema chrysogaster

ABSTRACT Intervening when bird nestlings are performing poorly relative to the population mean may be a management priority if individuals are of high-conservation value. Assessing body condition may enable identification of potential problems before they cause mortality. We aimed to provide a tool for conservation managers to identify underperforming nestlings in a severely threatened bird population. We develop models of nestling growth and empirically quantify nestling body condition of critically endangered Orange-bellied Parrots Neophema chrysogaster, which have declined to only a single population in southwestern Tasmania, Australia. Using census data on growth of nestlings born over four years into the contemporary wild population, we test whether a body condition index is influenced by sex, hatch order, year of birth, brood size, whether one or both parents were captive bred, and fledging date. The best model describing body condition in Orange-bellied Parrot nestlings included additive effects of year of birth and hatch order. Nestling body condition was lowest in 2013, where first hatched nestlings were 2.5 g lighter than those born in 2016, and > 4.2 g lighter than in 2017/18. Nestlings that hatched either first or in the middle of the brood were respectively 4.8 g and 3.8 g heavier than last-hatched birds. Our body condition index provides a repeatable, rapid and cheap way to assess body condition of wild Orange-bellied Parrot nestlings. This represents a step towards accurate evaluation of management actions aimed at improving reproductive outcomes for this species, and provides a framework for developing hypotheses to test using an empirical and measurable index of individual quality.

Effects of environmental metal pollution on reproduction of a free-living resident songbird, the tree sparrow (Passer montanus)

Anthropogenic metal pollution is known to adversely affect bird reproduction; however, few systematic studies are available on the effects of metal pollution on breeding performance and parental investment in a common resident songbird, the tree sparrow (Passer montanus). We conducted this study in two sites, a long-term heavy metal polluted site (Baiyin [BY]) and a relatively unpolluted site at approximately 110 km distance (Liujiaxia [LJX]), to assess the potential effects of environmental metal contamination on breeding parameters (clutch size, hatching success, fledging success, and growth of nestlings) and parental investment. The results showed smaller clutch size, lower fledging success, and differences in incubation behaviors of tree sparrows in BY than in LJX. Although there was no difference in parental body condition (residual body mass) between the two study sites, the parents responded differently with respect to reproduction due to varying metal levels in their habitats and bodies. Higher Cd levels in the primary feathers of females in BY were associated with lower clutch sizes. Parental investment including incubation duration and feeding rates showed no significant difference between the two sites during the incubation and nestling periods, but the frequencies of incubation visits were higher in BY. Parental behavior during the incubation period was also negatively affected by the parental Pb and Cd levels. Although the nestling growth patterns were relatively similar between the two sites, the nestlings were smaller, had lower body weight, and fledged later and fledging rate was also lower in BY than in LJX. Metal concentrations were higher in nestling organs and feces in BY. Taken together, metal pollution might adversely affect nestling growth condition. Our results suggest a negative response in the reproduction of tree sparrows to long-term environmental metal pollution.

Tree Swallow (Tachycineta bicolor) nest success and nestling growth near oil sands mining operations in northeastern Alberta, Canada

Industrial development and contaminant exposure may affect reproductive success and food quality for birds. Tree Swallows (Tachycineta bicolor (Vieillot, 1808)) nesting near oil sands development in northern Alberta (Canada) potentially experience elevated environmental stressors that could influence reproduction. We measured reproductive and growth endpoints in Tree Swallows, predicting reduced reproductive success and nestling growth near oil sands operations compared with reference sites. We also identified the invertebrate prey in the stomach contents of nestlings to understand variability in the diet and its potential effect on growth and survival of nestlings. From 2012 to 2015, clutch initiation varied among years but was not influenced by proximity to oil sands operations. Hatching and fledging success decreased in response to increased precipitation, regardless of location. Measurements of nestling growth reflected the variation associated with nestling sex and possibly asynchronous hatching. The composition of the nestling diet was significantly different; birds near oil sands development consumed Odonata, whereas birds at reference sites consumed Ephemeroptera. Nestlings from all sites consumed relatively high quantities of terrestrial insects. Our results demonstrate that factors such as weather conditions, diet, hatching order, and nestling sex are important when interpreting the potential effects of oil sands development on nest success and nestling growth.

Oxidative status and telomere length are related to somatic and physiological maturation in chicks of European starlings (Sturnus vulgaris).

Telomere length can be considered as an indicator of an organism's somatic state, long telomeres reflecting higher energy investment in self-maintenance. Early-life is a period of intense investment in somatic growth and in physiological maturation but how this is reflected in telomere length remains unclear. Using European starling chicks we tested: (i) how telomere length measured at asymptotic mass is related to proxies of somatic growth and physiological maturity in 17-day-old nestlings; (ii) how telomere length measured at 17 days then predicts the changes in somatic and physiological maturity occurring in fledglings (between 17 and 21 days); (iii) how growth and telomere length co-vary when chicks are under experimentally good (fed) growth conditions. Depending on environmental conditions, our data suggest links between somatic growth, physiological maturation and body maintenance parameters (positive with oxidative stress and negative with telomere length) in nestlings. Telomere length measured at day 17 predicted a subsequent change in physiological maturation variables observed in fledglings, but only in second-brood chicks: chicks with shorter telomeres had a higher pre-fledging rate of increase in haematocrit and haemoglobin content and a greater decrease in reticulocyte count. Finally, food supplementation of chicks did not change telomere length compared with that in control siblings. Our results suggest that physiological maturation prior to fledging may occur at the expense of telomere length but only when environmental conditions are sub-optimal.

GPS tracking during parental care does not affect early offspring development in lesser black-backed gulls

Tracking devices are increasingly used to monitor individual movement patterns continuously and in high resolution. However, carrying a device could potentially compromise an individual’s physiology or behaviour, thereby making tracking data unreliable for detailed behavioural measurements. To this end, we assessed the possible consequences of the application of GPS devices on offspring development in an opportunistic seabird species, the lesser black-backed gull (Larus fuscus), by comparing the growth and survival of nestlings of which none, one or both parents were equipped with a GPS device. We found that the developmental trajectories of the nestlings were not affected, and there were no differences in skeletal size and body mass at the fledging stage. A lack of negative effects on offspring development strongly suggests that the parental behaviour, and thus likely the foraging behaviour, did not differ between tagged and non-tagged individuals. The evidence that GPS data can be used to reliably study parental care, as well as other aspects of the bird’s behaviour, opens up new possibilities to study behavioural and evolutionary ecological questions in ever-increasing resolution.

Gut microbes limit growth in house sparrow nestlings (Passer domesticus) but not through limitations in digestive capacity.

Recent research often lauds the services and beneficial effects of host‐associated microbes on animals. However, hosting these microbes may come at a cost. For example, germ‐free and antibiotic‐treated birds generally grow faster than their conventional counterparts. In the wild, juvenile body size is correlated with survival, so hosting a microbiota may incur a fitness cost. Avian altricial nestlings represent an interesting study system in which to investigate these interactions, given that they exhibit the fastest growth rates among vertebrates, and growth is limited by their digestive capacity. We investigated whether reduction and restructuring of the microbiota by antibiotic treatment would: (i) increase growth and food conversion efficiency in nestling house sparrows (Passer domesticus); (ii) alter aspects of gut anatomy or function (particularly activities of digestive carbohydrases and their regulation in response to dietary change); and (iii) whether there were correlations between relative abundances of microbial taxa, digestive function and nestling growth. Antibiotic treatment significantly increased growth and food conversion efficiency in nestlings. Antibiotics did not alter aspects of gut anatomy that we considered but depressed intestinal maltase activity. There were no significant correlations between abundances of microbial taxa and aspects of host physiology. Overall, we conclude that microbial‐induced growth limitation in developing birds is not driven by interactions with digestive capacity. Rather, decreased energetic and material costs of immune function or beneficial effects from microbes enriched under antibiotic treatment may underlie these effects. Understanding the costs and tradeoffs of hosting gut microbial communities represents an avenue of future research.

Experimentally increased nest temperature affects body temperature, growth and apparent survival in blue tit nestlings

The thermal environment experienced by birds during early postembryonic development may be an important factor shaping growth and survival. However, few studies have directly manipulated nest temperature (Tn) during the nestling phase, and none have measured the consequences of experimental heat stress on nestlings’ body temperature (Tb). It is therefore not known to what extent any fitness consequences of development in a thermally challenging environment arise as a direct, or indirect, effect of heat stress. We, therefore, studied how experimentally increasedTnaffectedTbin 8–12 d old blue titCyanistes caeruleusnestlings, to investigate if increased thermoregulatory demands to maintain normothermicTbinfluenced nestling growth and apparent long‐term survival. Nestlings in heated nest‐boxes had significantly higherTbcompared to unheated nestlings during most of the experimental period. Yet, despite facingTn 50°C (as measured in the bottom of the nest cup below the nestlings), the highest nestlingTbrecorded was 43.8°C with nestlings showing evidence of controlled facultative hyperthermia without any increased nestling mortality in heated nests. However, body mass gain was lower in these nestlings compared to nestlings from control nest‐boxes. Contrary to our prediction, a larger proportion of nestlings from heated nest‐boxes were recaptured during their first winter, or subsequently recruited into the breeding population as first‐ or second‐year breeders. This result should, however, be treated with caution because of low recapture rates. This study highlights the importance of the thermal environment during nestling development, and its role in shaping both growth patterns and possibly also apparent survival.