Influence of hatching order on growth rate and resting metabolism of kestrel nestlings

Hatching asynchrony in altricial birds may result in a competitive disadvantage for the youngest nestlings compared to older siblings. We studied the effects of a size hierarchy on the growth rate of Eurasian kestrels Falco tinnunculus chicks in nests with and without access to supplemented food in western Finland. Body mass stopped increasing on the 19th day after hatching while body size, estimated by a combination of bone and feather lengths continued to increase at least until fledging at 26 days. Body condition, reflecting muscle and fat, did not change markedly during the growth period from the 12th day to fledging. Body temperature and resting metabolism were usually lower in nestlings 12 days old than in nestlings at fledging. Growth of body mass, size and condition, and resting metabolism were delayed in last-hatched nestlings aged 19 days. Just before fledging, last-hatched nestlings attained a similar body mass and size, and had a similar resting metabolism to those of older siblings. At fledging, only in nests without access to supplemented food was the body condition of last-hatched chicks lower than that of its siblings, but in nests with access to supplemented food no such difference was detected. Our results highlight that the level of lipids in the last-hatched nestling can be affected by the food restriction imposed by hatching order.     

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

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

Plasma corticosterone in American kestrel siblings: effects of age,hatching order,and hatching asynchrony

Although it is well documented that hatching asynchrony in birds can lead to competitive and developmental hierarchies, potentially greatly affecting growth and survival of nestlings, hatching asynchrony may also precipitate modulations in neuroendocrine development or function. Here we examine sibling variation in adrenocortical function in postnatally developing, asynchronously hatching American kestrels (Falco sparverius) by measurements of baseline and stress-induced levels of corticosterone at ages 10, 16, 22, and 28 days posthatching. There was a significant effect of hatching order on both baseline and stress-induced corticosterone levels during development and these effects grew stronger through development. First-hatched chicks exhibited higher baseline levels than later-hatched chicks throughout development and higher stress-induced levels during the latter half of development. Furthermore, there was significant hatching span (difference in days between first- and last-hatched chicks) x hatching order interaction on both baseline and stress-induced corticosterone levels during development. Hatching span was also positively correlated with both measures of corticosterone and body mass in first-hatched chicks, but was negatively correlated with these factors through most of the development in last-hatched chicks. It is known that hatching asynchrony creates mass and size hierarchies within kestrel broods and we suggest that hierarchies in adrenocortical function among siblings may be one physiological mechanism by which these competitive hierarchies are maintained.     

Experimental evidence for the influence of food availability on incubation attendance and hatching asynchrony in the Australian reed warbler Acrocephalus australis

The amount of time a bird allocates to incubation is likely to be limited by energetic constraints. If food is abundant, energetic constraints may be reduced and the time spent incubating (incubation attendance) may increase. Moreover, the onset of incubation in relation to clutch completion may be advanced, resulting in a higher degree of hatching asynchrony. We measured the effect of experimentally increased food availability on incubation attendance and an estimate of hatching asynchrony in the Australian reed warbler Acrocephalus australis . Supplementary food was provided every other day, from a few days before the start of egg laying until just prior to hatching. Incubation attendance was measured with temperature loggers at nests receiving supplementary food and control nests. Hatching asynchrony was inferred from mass and size differences between siblings shortly after hatching. We found that 1) food supplementation resulted in an increase in incubation attendance, when comparing both nests receiving supplementary food to control nests as well as feeding to non-feeding days in nests receiving supplementary food, and 2) food supplementation resulted in a greater hatching asynchrony, without affecting clutch size, average egg volume or the likelihood of eggs hatching. This suggests that food availability acts in a proximate way to modify the extent of incubation attendance and hatching asynchrony. We discuss the adaptive significance of increased incubation attendance and a shift in the degree of hatching asynchrony in relation to food availability.     

HATCHING ASYNCHRONY IN ALTRICIAL BIRDS

1. The review aims to provide a simple conceptual framework on which to place recent studies of hatching asynchrony in altricial birds and to assess the evidence used in support of specific hypotheses. 2. Hatching asynchrony arises bsecause parents start incubation before laying is complete, but the precision of parental control is largely unknown. 3. Hypothesses concerning the functional significance of hatching asynchrony fall into four broad types. Hatching asynchrony might: (i) arise because of selection on the timing of events during the nesting period; (ii) facilitate the adaptive reduction in brood size; (iii) increase the energetic efficiency of raising the brood, or (iv) result from environmental or phylogenetic constraints. 4. The incubation pattern could function to minimize the losses of eggs, nestlings or adults to predators (or climatic sources of mortality), particularly in species which cannot actively defend their nest. The best evidence comes from comparative studies of hatching asynchrony. Early incubation might also be favoured if the food supply declines sharply through the breeding season, although the evidence is weak and indirect, or if there is a risk of brood parasitism. In species in which only the female incubates, early incubation could ‘force’ the male to invest more in the nestlings, but this idea remains to be tested. Males may be constrained by the risk of cuckoldry to delay incubation until laying is complete. 5. Hatching asynchrony could be adaptive by enabling the efficient reduction of brood size if food proves short after hatching (primarily because of a shortage of food in the environment or possibly because of a large proportion of ‘expensive’ nestlings in the brood in species which are sexually dimorphic). Observational evidence is often consistent with this hypothesis but few experimental studies provide adequate tests. Brood reduction could be adaptive in species (primarily eagles and pelecaniformes) which lay an extra egg to act as insurance against hatching failure, and again hatching asynchrony might facilitate brood reduction, although there are few experimental tests on such species. Hatching asynchrony might also enable sex ratio manipulation through selective brood reduction, although there is as yet no clear supportive evidence. 6. Ins species in which young have a marked peak in energy demand during the period of parental care, hatching asynchrony can reduce the peak demand of the brood, which might allow the parents to raise more healthy young. In many species such savings are likely to be small or absent. There is some behavioural evidence that hatching asynchrony can reduce fighting amongst nestlings and therefore lead to the more efficient use of energy by the brood. In general this effect seems small and the only energetic study found no difference in the energy requirements of synchronous and asynchronous broods. Other possible energetic advantages to hatching asynchrony have not been tested. 7. Environmental conditions during laying can influence both egg size and laying interval in aerial insectivores, and might directly influence incubation in this and other groups. Thus some variation in hatching asynchrony and the relative size of siblings is probably non-adaptive. The variability of incubation pattern within and across species suggests that hatching asynchrony is not under strong phylogenetic constraint. 8. The hypotheses about the adaptive significance of hatching asynchrony are complementary rather than mutually exclusive: within a species, several selective pressures could influence the optimal incubation pattern, and the relative importance of selective pressures will differ among species. Furthermore one should expect that the incubation pattern and parent–offspring interactions will be coadapted to maximize brood productivity.     

Brood Reduction via Intra-clutch Variation in Testosterone - An Experimental Test in the Great Tit

In birds, yolk androgen concentrations in eggs can increase or decrease over the laying sequence and common hypotheses hold that this serves to favour the competitive ability of either first- or last-hatched chicks depending on the prevailing conditions, and thus promote brood reduction or maintenance of original brood size respectively. Intra-clutch variation of testosterone can shift relative competitive ability of siblings and hence competitive dynamics. In a natural population of great tits, we experimentally investigated the effects and function of maternal testosterone on offspring phenotype in relation to the laying position of the egg in a context of hatching asynchrony. To this end, we created three types of clutches where either the first three or the last three eggs of a clutch were injected with testosterone (T) dissolved in sesame oil, and the remaining eggs with sesame oil only, or where all eggs of a clutch were injected with sesame oil. Increased levels of yolk T in the last-laid eggs resulted in the last-hatched chicks being significantly lighter and smaller than their siblings, while increased levels of T in the first-laid eggs had no direct effect on the first-hatched chicks, but an indirect negative effect on their siblings. Our results suggest that females can potentially adjust offspring phenotype by modulating, over the laying sequence, the amounts of T deposited in the eggs. These results are in contradiction, however, with current hypotheses and previous findings, which suggest that under good conditions higher levels of maternally derived T in the last-laid eggs should mitigate the negative effects of hatching asynchrony.     

Hatching asynchrony and brood reduction in Tengmalm's owl <Emphasis Type="Italic">Aegolius funereus</Emphasis>: the role of temporal and spatial variation in food abundance

Hatching asynchrony is the consequence of birds initiating incubation before clutch completion. It has been suggested that variation in hatching asynchrony in owls is extensive, and therefore they should be excellent objects to study the effects of spatio-temporal variation in food abundance on this phenomenon. We examined how abundance and predictability of food affected hatching asynchrony in Tengmalm's owl Aegolius funereus (Linnaeus), which mainly feeds on voles which fluctuate in 3- to 4-year cycles in northern Europe. Hatching span averaged 6-7 days (range 0-13 days) and increased with clutch size. Food supply did not directly influence levels of hatching asynchrony but it influenced indirectly via marked among-year changes in clutch size. During the decrease phase of the vole cycle the proportion of hatchlings producing fledglings decreased with asynchrony, suggesting that chick mortality was most common among asynchronous broods when food became scarce. This finding is consistent with Lack's brood reduction hypothesis, i.e. that if food becomes scarce during the nestling period the youngest nestlings would die first without endangering the survival of the whole brood.     

An experimental approach to the brood reduction hypothesis in Magellanic penguins

In many bird species, eggs in a brood hatch within days of each other, leading to a size asymmetry detrimental to younger siblings. Hatching asynchrony is often thought of as an adaptive strategy, and the most widely studied hypothesis in relation to this is the ‘brood reduction hypothesis’. This hypothesis states that when food resources are unpredictable, hatching asynchrony will allow the adjustment of the brood size maximizing fledging success and benefitting parents. The Magellanic penguin Spheniscus magellanicus is an appropriate species to test this hypothesis because it has a 2‐egg clutch that hatches over a 2‐d interval with a broad range of variation (–1 to 4 d), it shows facultative brood reduction, and food abundance between breeding seasons is variable. We performed a manipulative study at Isla Quiroga, Argentina, during three breeding seasons (2010–2012) by forcing broods to hatch synchronously (0 d) or asynchronously (2 or 4 d). Years were categorized based on estimated food abundance. Our study provided mixed results because in the low estimated food abundance year asynchronous broods did not have higher nestling survival than synchronous broods, and the second‐hatchling in asynchronous broods did not die more often than those in synchronous broods. On the other hand, younger siblings of 4‐d asynchronous broods starved earlier than those of synchronous broods, and 2‐d asynchronous broods fledged heavier young than synchronous broods. Asynchronous hatching would seem to benefit reproduction in this species, not with respect to survival, but in terms of the advantages it can accord to nestlings and, in terms of lower costs, for parents raising nestlings.     

Hatching asynchrony that maintains egg viability also reduces brood reduction in a subtropical bird

In birds, hatching failure is pervasive and incurs an energetic and reproductive cost to breeding individuals. The egg viability hypothesis posits that exposure to warm temperatures prior to incubation decreases viability of early laid eggs and predicts that females in warm environments minimize hatching failure by beginning incubation earlier in the laying period, laying smaller clutches, or both. However, beginning incubation prior to clutch completion may incur a cost by increasing hatching asynchrony and possibly brood reduction. We examined whether Florida scrub jays (Aphelocoma coerulescens) began incubation earlier relative to clutch completion when laying larger clutches or when ambient temperatures increased, and whether variation in incubation onset influenced subsequent patterns of hatching asynchrony and brood reduction. We compared these patterns between a suburban and wildland site because site-specific differences in hatching failure match a priori predictions of the egg viability hypothesis. Females at both sites began incubation earlier relative to clutch completion when laying larger clutches and as ambient temperatures increased. Incubation onset was correlated with patterns of hatching asynchrony at both sites; however, brood reduction increased only in the suburbs, where nestling food is limiting, and only during the late nestling period. Hatching asynchrony may be an unintended consequence of beginning incubation early to minimize hatching failure of early laid eggs. Food limitation in the suburbs appears to result in increased brood reduction in large clutches that hatch asynchronously. Therefore, site-specific rates of brood reduction may be a consequence of asynchronous hatching patterns that result from parental effort to minimize hatching failure in first-laid eggs. This illustrates how anthropogenic change, such as urbanization, can lead to loss of fitness when animals use behavioral strategies intended to maximize fitness in natural landscapes.     

Intra-Clutch Ratio of Yolk Progesterone Level Changes with Laying Date in Rockhopper Penguins: A Strategy to Influence Brood Reduction?

Hatching asynchrony in avian species generally leads to a size hierarchy among siblings, favouring the first-hatched chicks. Maternally deposited hormones affect the embryo and chick's physiology and behaviour. It has been observed that progesterone, a hormone present at higher levels than other steroid hormones in egg yolks, is negatively related to body mass in embryos, chicks and adults. A differential within-clutch progesterone deposition could therefore be linked to the size hierarchy between siblings and to the resulting brood reduction. We tested whether yolk progesterone levels differed between eggs according to future parental ability to feed the entire clutch in wild rockhopper penguins Eudyptes chrysocome. This species presents a unique reversed egg-size dimorphism and hatching asynchrony, with the larger second-laid egg (B-egg) hatching before the smaller first-laid egg (A-egg). Yolk progesterone levels increased only slightly with female body mass at laying. However, intra-clutch ratios were not related to female body mass. On the other hand, yolk progesterone levels increased significantly with the date of laying onset for A-eggs while they decreased for B-eggs. Early clutches therefore had proportionally more progesterone in the B-egg compared to the A-egg while late clutches had proportionally less progesterone in the B-egg. We propose that females could strategically regulate yolk progesterone deposition within clutches according to the expected food availability during chick growth, an adaptive strategy to adjust brood reduction to conditions. We also discuss these results, relating to yolk progesterone, in the broader context of other yolk steroids.     

Laying order, hatching asynchrony and nestling body mass in Tree Swallows Tachycineta bicolor

We studied the reproductive biology of a box-nesting population of Tree Swallows Tachycineta bicolor in southeastern Wisconsin, USA. We were interested particularly in the relationship between laying order and hatching order and the extent to which each was a predictor of nestling body mass. We found that laying order was a significant predictor of hatching order. Laying and hatching order were related to nestling mass at 4 days of age and to a lesser extent at 12 days of age. In addition, we investigated the effects of natural variation in hatching asynchrony. Hatching asynchrony was positively related to the range of nestling body masses within a brood at days 4 and 12. The probability that brood reduction occurred was also positively related to the degree of hatching asynchrony, though this effect was significant only at day 4. Our results suggest that laying order and hatching order have their greatest effects on nestling Tree Swallows early in the nestling period.     

Hatching asynchrony aggravates inbreeding depression in a songbird (Serinus canaria): An inbreeding–environment interaction

Understanding how the intensity of inbreeding depression is influenced by stressful environmental conditions is an important area of enquiry in various fields of biology. In birds, environmental stress during early development is often related to hatching asynchrony; differences in age, and thus size, impose a gradient in conditions ranging from benign (first hatched chick) to harsh (last hatched chick). Here, we compared the effect of hatching order on growth rate in inbred (parents are full siblings) and outbred (parents are unrelated) canary chicks (Serinus canaria). We found that inbreeding depression was more severe under more stressful conditions, being most evident in later hatched chicks. Thus, consideration of inbreeding‐environment interactions is of vital importance for our understanding of the biological significance of inbreeding depression and hatching asynchrony. The latter is particularly relevant given that hatching asynchrony is a widespread phenomenon, occurring in many bird species. The exact causes of the observed inbreeding‐environment interaction are as yet unknown, but may be related to a decrease in maternal investment in egg contents with laying position (i.e. prehatching environment), or to performance of the chicks during sibling competition and/or their resilience to food shortage (i.e. posthatching environment).     

1. A CENSUS AND STUDY OF BIRDS IN ALBANY OPEN THORNBUSHVELD

Williams, A. J. &; Cooper, J. 1983. The Crowned Cormorant: breeding biology, diet, and offspring reduction strategy. Ostrich 54:213-219.

Crowned Cormorants Phalacrocorax coronatus were studied at Dassen and Marcus Islands. The most frequent clutch was three eggs. Egg size varied within clutches with first-laid eggs being largest and heaviest and subsequent eggs progressively smaller and lighter, The mean laying interval was 2,2 days, the mean laying-to-hatching interval was 23,0 days, and the hatching interval was one day. The normal incubation period was 22.4 days. The weight of hatchings was related to the position of the originating egg in the laying sequence. Chicks were fed within 24 h of hatching. Chick development is described over the first 35 days. One chick could fly at 35 days. Hatching success was 48,2%. Hatching success was greatest in second-laid eggs, least in last-laid eggs. The mean number of chicks hatched at a nest was two. Mean diving time was 23,5 s. Most food was fish, particularly klipfish Clinidae and pipefish Syngnathus, 60–160 mm long. The number of offspring produced can be related to food availability by interaction of difference in egg size, hatching asynchrony, and the preferential feeding by adults of the strongest-begging chick. There is a trend towards producing two chicks, normally those from the first two eggs to be laid.     

Hatching asynchrony,sibling hierarchies and brood reduction in the Chinstrap penguin Pygoscelis antarctica

We studied patterns of chick growth and mortality in relation to egg size and hatching asynchrony during two breeding seasons (1991 and 1992) in a colony of chinstrap penguins sited in the Vapour Col rookery, Deception Island, South Shetlands. Intraclutch variability in egg size was slight and not related to chick asymmetry at hatching. Hatching was asynchronous in 78% (1991) and 69% (1992) of the clutches, asynchrony ranging from 1 to 4 days (on average 0.9 in 1991 and 1.0 days in 1992). Chicks resulting from oneegg clutches grew better than chicks in families of two in 1991. In 1992, single chicks grew to the same size and mass at 46 days of age as chicks of broods of two, suggesting food limitation in 1991 but not in 1992. In 1991, asymmetry between siblings in mass and flipper length was significantly greater in asynchronous than in synchronous families during the initial guard stage, but these differences disappeared during the later créche phase. In 1992, asymmetry in body mass increased with hatching asynchrony and decreased with age. Only the effect of age was significant for flipper length and culmen. Asymmetries at 15 days were similar in both years, but significantly lower in 1992 than in 1991 at 46 days of age. There were relatively frequent reversals of size hierarchies during both phases of chick growth in the two years, reversals being more common in 1991 than in 1992 for small chicks. In 1991, survivors of brood reduction grew significantly worse than chicks in nonreduced broods. In both years, chicks of synchronous broods attained similarly large sizes before fledging as both A and B chicks of asynchronous broods. In 1991, chick mortality rate increased during the guard stage due to parental desertions, decreased during the transition to crèches (occurs at a mean age of 29 days) and returned to high constant levels during the crèche stage, when it is mostly due to starvation (in total 66% of hatched chicks survived to fledging). In contrast, in 1992, mortality was relatively high immediately after hatching and almost absent for chicks older than 3 weeks (87% of chicks survived to fledging). Mortality affected similarly one- and two-chick families. In 1991, asynchronous families suffered a significantly greater probability of brood reduction than synchronous families, but this probability was not significantly related to degree of asymmetry between siblings. No association between asynchrony and mortality was found in 1992. These results show that there is food limitation in this population during the crèche phase in some years, that asynchronous hatching does not facilitate early brood reduction and that it does not ensure stable size hierarchies between siblings. Brood reduction due to starvation is not associated to prior asymmetry and does not facilitate the survival or improve the growth of the surviving chick. Asynchronous hatching may be a consequence of thermal constraints on embryo development inducing incubation of eggs as soon as they are laid.     

Within-clutch patterns of yolk testosterone vary with the onset of incubation in black-headed gulls

Hatching asynchrony in birds produces an age and size hierarchyamong siblings. Later-hatching chicks have a competitive disadvantage,and brood reduction may occur when food availability is insufficientto raise all chicks. When early-hatched chicks fail to surviveor if the circumstances allow raising all chicks, mothers shouldreverse the disadvantage to later-hatched chicks. Increasingdeposition of maternal androgens with the laying sequence hasbeen suggested to compensate for detrimental effects of hatchingasynchrony, allowing a more precise adjustment of the survivalprobabilities of each chick. Here, we show for black-headedgulls that the increase in yolk testosterone with each successiveegg is greater when the mother incubates longer before clutchcompletion, which is the major determinant of the degree ofhatching asynchrony. This finding supports the idea that yolktestosterone has a compensatory function in the context of hatchingasynchrony. Our data further show that if the time needed tocomplete a clutch is lengthened, the developmental differencesdue to incubation between the first- and the last-laid eggsincrease. In addition, the onset of incubation before clutchcompletion occurs sooner as the breeding season progresses.Both long inter-egg intervals and the seasonal shift in incubationbehavior enhance the necessity of compensation for later-hatchingchicks. Indeed, yolk levels of testosterone increased more steeplyover the laying order, if the duration of the egg-laying periodwas extended and in later-laid clutches. We suggest that prolactinplays a key role in the adjustment of testosterone allocationto the incubation pattern.     

Extirpation of alternative prey during a small rodent crash

Summary We document two episodes, in different years, of Barn Owls (Tyto alba) preying on a winter population of Burrowing Owls (Athene cunicularia) on a southern California island. The predation in each case followed a marked shift in the diet of the Barn Owls, due to the cyclic decline of their normal small mammal prey. Heavy predation in the first year resulted in the extirpation of the Burrowing Owls on the island. Such heavy predation on alternative prey species is commonly reported in cyclic predator-prey systems, however this is the first documented case of extirpation of the alternative prey. Complete elimination of any prey species by terrestrial predators is, in fact, very rare.     

Incubation prior to clutch completion accelerates embryonic development and so hatch date for eggs laid earlier in a clutch in the great tit Parus major

The ability of parents to respond to changes in food supply within a season will have a large effect on fitness through the number and quality of chicks fledged. Great tits, Parus major, attempt to synchronise their production of chicks with a seasonal food peak, but when food supply fails, hatching asynchrony of chicks provides a mechanism by which some young can be fledged because more developed chicks outcompete their less developed siblings for the reduced parental food supply. We tested whether female great tits can potentially control the degree of hatching asynchrony by using incubation before clutch completion, so that early laid eggs develop faster and hatch sooner. The temperature of an artificial egg placed in 29 nests during the laying period was measured with data loggers, and nocturnal incubation of eggs similar to incubation post clutch completion was recorded in all nests. We then demonstrated that eggs removed from the nest for 72 hour periods prior to clutch completion hatched later than eggs remaining in the nest for the entirety of the laying period. Our results show that variable pre clutch completion incubation (which was mostly nocturnal) can lead to faster embryo development and earlier hatching, so potentially providing a mechanism for adaptive female control of degree of hatching asynchrony.     

Factors that affect hatching asynchrony in the chinstrap penguin (Pygoscelis antarctica)

We examined if laying intervals and hatching asynchronies are related in a chinstrap penguin (Pygoscelis antarctica) population in the South Orkneys. The lack of association between the two variables, as well as data on brood patch development, indicated that incubation did not begin immediately after the first egg was laid. This suggests that longer laying intervals may be compensated by longer delays in the onset of full incubation. Hatching asynchrony increased with within-clutch egg size asymmetry, decreased with breeding date, and was related to the laying order of the eggs according to size. However, only egg-size asymmetry remained significant when controlling for the other variables. We conclude that more asymmetric clutches were more asynchronous, although a large part of the variation in hatching asynchrony remains unexplained.     

Growth of nestling black kites Milvus migrans: effects of hatching order, weather and season

The growth of black kites Milvus migrans was studied in 1981, 1982 and 1984 at the Doñana Biological Reserve (south-west Spain). Fifteen variables were submitted to a principal component analysis in order to characterize the growth pattern of the young. In general, in the nestlings that fledged, rapid weight gain corresponded to rapid growth of the tarsus and feathers. For nestlings that starved, tarsus growth took priority over weight gain. Within the same brood, the last-hatched sibling grew more slowly than its older sibling and the differences between them increased in proportion to the hatching asynchrony. Brood size did not affect growth as clearly as hatching order and hatching asynchrony. Increased rainfall and lower maximum temperature, factors that seem to make the search for food more difficult, had negative effects on the youngest sibling in a multiple brood and on single nestlings. The only environmental variable that had an effect on all of the nestlings was the hatching date.