Egg temperature and initial brood patch area determine hatching asynchrony in Magellanic penguin Spheniscus magellanicus

In birds, the adaptive significance of hatching asynchrony has been under debate for many years and the parental effects on hatching asynchrony have been largely assumed but not often tested. Some authors suggest that hatching asynchrony depends on the incubation onset and many factors have been shown to influence hatching asynchrony in different species. Our objective was to analyze the exact timing of the onset of incubation and if this affects hatching asynchrony; and, in addition, which other factors (brood patch development, incubation position, adult body condition, intra‐clutch egg dimorphism, laying date and year) affect hatching asynchrony in Magellanic penguins Spheniscus magellanicus. We first estimated the eggshell temperature at which embryo development starts, with a non‐destructive and novel method. We then recorded individual egg temperatures in 61 nests during incubation, and related them, and other breeding parameters, to hatching asynchrony. We also observed incubation positions in 307 nests. We found a significant positive relationship between hatching asynchrony and the temperature that the first‐laid egg experienced during egg laying and between hatching asynchrony and the initial brood patch area. We also found a negative relationship between hatching asynchrony and the difference in temperature between second and first‐laid eggs within a clutch, measured after the egg‐laying period was finished. We ruled out position of the eggs during incubation, adult body condition, egg volume, laying date, and study year as factors influencing hatching asynchrony. The egg temperature during laying and the difference in temperature between eggs of a clutch are determinants of hatching asynchrony in Magellanic penguins.     

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.     

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.     

Sex,growth rate,rank order after brood reduction,and hatching date affect first‐year survival of long‐lived Herring Gulls

Among most species of birds, survival from hatching throughout the first year of life is generally lower than subsequent survival rates. Survival of young birds during their first year may depend on a combination of selection, learning, unpredictable resources, and environmental events (i.e., post‐fledging factors). However, knowledge about post‐fledging development in long‐lived species is usually limited due to a lengthy immature stage when individuals are generally unobservable. Therefore, pre‐fledging characteristics are often used to predict the survival of young birds. We assessed effects of nestling growth rates, hatching date, hatching asynchrony, brood size and rank order after brood reduction, and sex on first‐year survival of 137 fledglings using a mark‐resighting analysis. We found that the survival probability (Φ_1yr = 0.39) of first‐year Herring Gulls (Larus argentatus) in our study colony located at the outer port of Zeebrugge (Belgium) was lower than that of older individuals (Φ_>1yr = 0.75). All 10 models best supported by our data included nestling growth rate, suggesting that variability in first‐year survival may be linked primarily to individual variation in growth. First‐year survival was negatively correlated with hatching date and rank order after brood reduction. Hence, carry‐over effects of breeding season events such as timing of breeding, early development, and social status had an influence on survival of Herring Gulls after fledging. Furthermore, we found sex‐biased mortality in first‐year Herring Gulls, with females (Φ_1yr = 0.45) surviving better than males (Φ_1yr = 0.38). Although adult survival is generally regarded as the key parameter driving population trajectories in long‐lived species, juvenile survival has recently been acknowledged as an important source of variability in population growth rates. Thus, increasing our knowledge of factors affecting age‐specific survival rates is necessary to improve our understanding of population dynamics and ultimately life‐history variation.     

Differential effects of egg albumen content on barn swallow nestlings in relation to hatch order

In diverse animal taxa, egg mass variation mediates maternal effects with long-term consequences for offspring ontogeny and fitness. Patterns of egg mass variation with laying order differ considerably among birds, but no study has experimentally investigated the function of variation in albumen or yolk egg content in the wild. In barn swallows (Hirundo rustica), absolute and relative albumen mass increased with egg laying order. Experimental albumen removal delayed hatching, had larger negative effects on growth of late-hatched nestlings, and reduced nestling survival. Laying order positively predicted hatch order. Because nestling competitive ability depends on size, and albumen egg content influences hatchling size, present results suggest that by increasing albumen content of late eggs mothers reduce hatching asynchrony and enhance growth particularly of late-hatched nestlings. Thus, variation in albumen mass with laying order may function to mitigate the negative phenotypic consequences of hatching late in species that adopt a 'brood-survival' strategy.     

Early maternal effects and antibacterial immune factors in the eggs,nestlings and adults of the barn swallow

Transfer of immune factors via the egg may represent a maternal adaptation enhancing offspring survival. Lysozyme is a major component of maternal antibacterial immunity which is transferred to the eggs in birds. In a population of barn swallows (Hirundo rustica), lysozyme activity declined during the prelaying and laying periods in females but not in males. Egg hatching failure decreased with maternal lysozyme activity. The first eggs in a clutch contained more lysozyme and produced nestlings with larger lysozyme activity when 5 days old than last‐laid ones. In a cross‐fostering experiment where brood size was manipulated, nestling origin but not post‐manipulation brood size affected lysozyme activity. Hence, maternal lysozyme varies during the breeding season and may differentially enhance antibacterial immune defence of the eggs and nestlings in relation to laying order. These findings suggest that offspring innate immunity is influenced by early maternal effects.     

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.     

Lesser double‐collared sunbirds Nectarinia chalybea do not compensate for hatching asynchrony by adjusting egg mass or yolk androgens

Substantial amounts of maternal androgens are found in birds’ eggs and have been shown to benefit offspring development. Within‐clutch patterns of increasing androgen concentrations over the laying sequence are often hypothesized to compensate for the negative effects of hatching asynchrony. However, detrimental effects to offspring fitness of exposure to high yolk androgen levels have also been demonstrated. This suggests that mothers should forego these costs to their offspring when the need for compensation for hatching asynchrony is low or when alternative compensatory strategies, e.g. in terms of increasing egg mass, are available. Here we show that in the south‐temperate lesser double‐collared sunbird Nectarinia chalybea, a species with hatching asynchrony but also with high survival of last‐hatched chicks, mothers do not deposit resources differentially in terms of either yolk androgen concentration or egg mass across the laying sequence. We discuss to what extend this challenges the original explanation of within‐clutch variation in these egg parameters and offer some explanation for their between‐clutch variation which was related to female body mass.     

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.     

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.     

Egg size and offspring performance in the collared flycatcher (<Emphasis Type="Italic">Ficedula albicollis</Emphasis>): a within-clutch approach

Adaptive within-clutch allocation of resources by laying females is an important focus of evolutionary studies. However, the critical assumption of these studies, namely that within-clutch egg-size deviations affect offspring performance, has been properly tested only rarely. In this study, we investigated effects of within-clutch deviations in egg size on nestling survival, weight, fledgling condition, structural size and offspring recruitment to the breeding population in the collared flycatcher (Ficedula albicollis). Besides egg-size effects, we also followed effects of hatching asynchrony, laying sequence, offspring sex and paternity. There was no influence of egg size on nestling survival, tarsus length, condition or recruitment. Initially significant effect on nestling mass disappeared as nestlings approached fledging. Thus, there seems to be limited potential for a laying female to exploit within-clutch egg-size variation adaptively in the collared flycatcher, which agrees with the majority of earlier studies on other bird species. Instead, we suggest that within-clutch egg-size variation originates from the effects of proximate constraints on laying females. If true, adaptive explanations for within-clutch patterns in egg size should be invoked with caution.     

Do group size and laying order influence maternal deposition of testosterone in smooth-billed ani eggs?

The avian egg contains maternal hormones that affect behavior, growth, morphology, and offspring survival. Evidence to date suggests that patterns of yolk androgen deposition could provide females with a means to manipulate sibling competition and, thereby, increase their fitness. We examined yolk testosterone (T) concentrations in eggs of the smooth-billed ani (Crotophaga ani) to understand patterns of androgen deposition in eggs of this plural-breeding joint-nesting cooperatively breeding species. We tested the hatching asynchrony adjustment hypothesis, which states that increases in yolk androgen levels over the laying sequence function to mitigate the disadvantage of being a later-hatched chick in species without adaptive brood reduction. We also investigated the effect of group size on yolk T deposition to test the hypothesis that females in multi-female groups could give a competitive edge to their own chicks by depositing higher T levels in their eggs. Predictions of the hatching asynchrony adjustment hypothesis were supported in both single- and multi-female groups as yolk testosterone levels increased from early- to late-laid eggs. This suggests that ani females can influence nestling competition and chick survival by within-clutch differential T allocation. Unexpectedly, we did not observe an effect of group size on yolk T deposition. Yolk testosterone concentrations may not be a mere reflection of a female's hormonal status as female plasma circulating levels of T did not vary in the same direction as yolk T levels. Results of this study therefore support the idea that females may adaptively manipulate chick behavior through hormonal deposition in eggs.     

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.     

Larval sensory abilities and mechanisms of habitat selection of a coral reef fish during settlement

In most animals, siblings from a given reproductive event emerge over a very short period of time. In contrast, many species of birds hatch their young asynchronously over a period of days or weeks, handicapping last-hatched chicks with an age and size disadvantage. Numerous studies have examined the adaptive significance of this atypical hatching pattern, but few have attempted to explain the considerable intrapopulation variation that exists in hatching asynchrony. I explored proximate determinants of hatching asynchrony by monitoring 112 Burrowing Owl (Athene cunicularia) nests in the grasslands of southern Saskatchewan, Canada, over 4 years. Age disparities between first- and last-hatched siblings (i.e., hatching spans) varied considerably, ranging between 1 and 7 days (mode = 4 days). These hatching spans increased with increased hatching success. Hatching spans also increased with larger clutches, but the increase was less than predicted given the increased time required to lay more eggs. Hatching span was unrelated to number of prey cached in the nest during egg laying (an index of food availability), and was unaltered by a year of super-abundant prey. Furthermore, pairs given extra food during laying had hatching spans equal to those of unsupplemented control pairs. These results were inconsistent with both the energy constraint and facultative manipulation hypotheses, which predict that hatching asynchrony should vary with the level of food during laying, when incubation onset is determined. Burrowing Owls were apparently free of food limitation early in breeding, yet may not have been able to optimize hatching spans because food conditions during laying were largely unrelated to food conditions during brooding. Thus, one of the premises for facultative manipulation of hatching asynchrony—that laying females are able to forecast post-hatch food conditions—may not have been met for this population of Burrowing Owls.     

BREEDING SUCCESS AND POPULATION GROWTH IN A COLONY OF HERRING AND LESSER BLACK-BACKED GULLS LARUS ARGENTATUS AND L. FUSCUS

This paper describes the results of investigations into the factors affecting breeding success of the Herring and Lesser Black-backed Gulls Larus argentatus and L. fuscus, in the large colony on Walney Island, northwest Lancashire, between 1962 and 1965. These investigations were concerned with the incubation period, and the first ten days after hatching. The survival of chicks to ten days is 67% in Herring Gulls, and 56% in Lesser Black-backs. Most of these losses occur in the period just after hatching and are due to “cannibalism” by other gulls. This form of predation does not appear to be masking any effects from starvation or disease. The following factors contribute to egg or chick mortality:breeding too late (and, to a much smaller extent, too early); breeding in the open, as opposed to amongst cover; the facts that eggs in small clutches have a lower hatching success than those in large ones and that Herring Gull (but not Lesser Black-back) chicks in small broods are less likely to survive to ten days than are those in large broods. Chick mortality after the first ten days is not certainly known. About 30% of the eggs laid gave rise to fledged young— or about one fledged chick per pair. In the Herring Gull, the average clutch size (2.56) is lower than that of the Lesser Black-back (2.76). Both species show a seasonal decline in clutch size—this occurs earlier in the Herring Gulls than in the Lesser Black-backs. The Walney population, which stood at about 700 pairs in 1950, had reached 12,000 in 1957, and is at present about 18–19,000 pairs. It is suggested that this increase may be linked to the greater availability, or exploitation, of human garbage in the Morecambe Bay area. The population explosion between 1950 and 1957 must have been partly due to massive immigration and could not have come about through natural increase alone. The possible influences of the gulls' behaviour on the population growth are discussed. There is no evidence of any “shock disease”, although the Walney colony is very crowded. “Cannibalism” is regarded, not as evidence of a failing food supply, but as an extension of the normal hunting behaviour of these omnivorous gulls; it will be an economical means of obtaining food only in a large, dense colony, such as Walney. It may be offset by increased breeding efficiency due to social factors.     

Testing the relationship between clutch size and brood size in the Coot (Fulica atra)

The time between egg laying and chick fledging is of crucial importance for the survival of young birds. I analyzed breeding output at consecutive phases of growth of young Coots (Fulica atra) relative to the clutch size and laying date. Considering the specific breeding biology of the Coot, I tested whether chick survival reveals clutch size-dependent variability. Clutch size did not affect hatching success; it only affected brood size, and that merely temporarily. During the first 20 days after hatching, i.e. during the time of the highest chick mortality, birds with larger clutches lost chicks at a higher rate. As a result, the number of fledged chicks was independent of the initial number of chicks, and pairs with different clutch sizes had a similar number of fledglings. The laying date had no effect. This pattern of age-related chick survival points to the greater role of the type of chick growth (semi-precocial) and behavior in their survival.     

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

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

Maternal compensation for hatching asynchrony in the collared flycatcher Ficedula albicollis

Several hypotheses have been proposed to explain the adaptiveness of hatching asynchrony for the parents, but delayed hatching is generally detrimental for the late hatched young. These offspring often experience competitive disadvantage and delayed development. If hatching asynchrony has a reason other than producing competitive differences among offspring, it would be advantageous, not only for the offspring but even for the parents, to compensate for its detrimental effects. In some species, increasing investment into later laid eggs has been reported and discussed as a compensation mechanism, but its effect on nestling growth and fledging size has not been examined in details. In this study we investigated nestling growth and size at fledging in terms of body mass and length of primaries in relation to the accurate laying and hatching order in collared flycatcher Ficedula albicollis broods. We found that females laid larger eggs at the end of the laying sequence, and this helped to decrease the disadvantages for the last offspring. The last offspring had lower body mass growth rate and fledged with shorter feathers, but in both cases the larger the last egg was, the smaller the lag of the offspring was. We conclude, that even if females were not able to fully compensate for the detrimental effects of hatching asynchrony, larger eggs may improve the survival prospects of late hatched nestlings.     

Discrepancy between factors affecting nestling growth and survival and maternal success in Common Grackles

ABSTRACT Multiple factors potentially affect nestling survival and maternal reproductive success. However, little is known about the relative importance of different factors when operating simultaneously or whether the same factors are important for nestlings and their mothers. We determined the effect of hatching asynchrony, individual egg size, mean egg size, nestling sex, and clutch initiation date on the survival of individual nestlings and on maternal reproductive success in Common Grackles (Quiscalus quiscula) from 2004 to 2006 in central Illinois. Factors most important to maternal success differed from those important for individual nestling growth and survival. Hatching asynchrony had the greatest within‐nest influence on the fate of nestlings; the earlier a nestling hatched relative to siblings, the greater its mass and likelihood of fledging. Clutch size had the greatest influence on maternal reproductive success, with females with larger clutches fledging more young. Thus, both nestling survival and maternal success were largely determined by a single, albeit different, factor. A possible explanation for the apparent unimportance of most factors we measured in determining maternal success is that we did not consider variation among females. Individual variation in maternal attributes such as condition, size, age, experience, or mate quality may result in females tailoring clutch attributes (i.e., egg size, sex, and degree of hatching asynchrony) in ways that allow them to maximize their reproductive success. The discordance between factors that benefited mothers versus their offspring illustrates the importance of considering the maternal consequences of any factor that appears to affect offspring survival. Factors that increase the mass and survival of some offspring may not result in increased maternal reproductive success.