First-hatched Chicks Research Articles

Age Difference, Not Food Scarcity or Sibling Interactions, May Drive Brood Reduction in Wild Scarlet Macaws in Southeastern Peru

Avian brood reduction was initially thought to be driven by insufficient food supply. Now it is more commonly considered a consequence of asynchronous hatching and resulting siblicide, direct filial infanticide (where parents kill specific chicks) or indirect filial infanticide (where parents starve specific chicks). In psittacines, brood reduction has been reported, but the mechanisms and causes remain unexplored. In this paper, we test the hypotheses that Scarlet Macaw chick starvation is driven by (1) sibling aggression, (2) food scarcity, and (3) parental food allocation based on (a) chick hatch weight and (b) chick age differences. We documented wild Scarlet Macaw behavior in lowland Peru in 37 nests over 19 seasons using morphological measurements and nest videos. Chick starvation was the leading cause of chick mortality (27% of all second-hatched chicks starve, and nearly all third- and fourth-hatched chicks starve). We found no evidence that starvation was caused by (1) sibling conflicts or (2) food availability. We did find parental food distribution favors first-hatched chicks, with larger age differences increasing the chances of second chick starvation. This study offers insights into brood reduction among Neotropical cavity-nesting birds and enhances our capacity to develop scientifically informed management strategies to support endangered psittacines.

Parental overproduction allows siblicidal bird to adjust brood size to climate-driven prey variation.

Parental overproduction is hypothesized to hedge against uncertainty over food availability and stochastic death of offspring and to improve brood fitness. Understanding the evolution of overproduction requires quantifying its benefits to parents across a wide range of ecological conditions, which has rarely been done. Using a multiple hypotheses approach and 30 years of data, we evaluated the benefits of overproduction in the Blue-footed booby, a seabird that lays up to three eggs asynchronously, resulting in an aggressive brood hierarchy that facilitates the death of last-hatched chicks under low food abundance. Results support the resource-tracking hypothesis, as low prey abundance (estimated from sea surface temperature and chlorophyll-a concentration) led to rapid brood reduction. The insurance hypothesis was supported in broods of three, where last-hatched chicks' survival increased after a sibling's death. Conversely, in broods of two, results suggested that parents abandoned last-hatched chicks following first-hatched chicks' deaths. No direct evidence supported the facilitation hypothesis: the presence of a last-hatched chick during development did not enhance its sibling's fitness in the short or long term. The value of last-hatched offspring to parents, as "extra" or "insurance" varied with indices of food abundance, brood size, and parental age. Ninety percent of overproduction benefits came from enabling parents to capitalize on favorable conditions by fledging additional offspring. Our study provides insight into the forces driving overproduction, explaining the adaptiveness of this apparently wasteful behavior and allowing us to better predict how overproduction's benefits might be modified by ocean warming.

Autumn–winter breeding by the Spotted Harrier Circus assimilis in southern Victoria

The breeding cycle of a pair of Spotted Harriers Circus assimilis was observed in southern Victoria, at 38°S, in autumn–winter 2017 for 42 hours over 22 days from incubation to the post-fledging period. From hatching on 10 June and fledging on 19 July, laying was inferred to have commenced on c. 8 May. Three chicks fledged from an initial brood of four; the nestling period was 38 days for the first-hatched chick. Rodents dominated a small sample of identified prey during a plague of House Mice Mus musculus. The male prey-delivery rate to the incubating female was 0.32 item/h, whereas during the nestling period the male brought 0.71 item/h and the female 0.11 item/h for a combined effective delivery rate of 0.64 item/h to nestlings. The combined parental feeding rate to new fledglings was 0.65 item/h. Parental behaviour is described. Juveniles were dependent for 6 weeks after fledging.

Interplays between pre- and post-natal environments affect early-life mortality, body mass and telomere dynamics in the wild.

Early-life conditions are crucial determinants of phenotype and fitness. The effects of pre- and post-natal conditions on fitness prospects have been widely studied but their interactive effects have received less attention. In birds, asynchronous hatching creates challenging developmental conditions for the last-hatched chicks, but differential allocation in last-laid eggs might help to compensate this initial handicap. The relative importance and potential interaction between pre- and post-hatching developmental conditions for different fitness components remains mostly unknown. We manipulated hatching order in wild pied flycatchers (Ficedula hypoleuca), creating three groups: natural asynchrony (last-laid eggs hatching last), reversed asynchrony (last-laid eggs hatching first) and hatching synchrony (all eggs hatching at once). We examined the effects of these manipulations on early-life survival, growth and telomere length, a potential cellular biomarker of fitness prospects. Mortality was mostly affected by hatching order, with last-hatched chicks being more likely to die. Early-life telomere dynamics and growth were influenced by the interplays between laying and hatching order. Last-laid but first-hatched chicks were heavier but had shorter telomeres 5 days after hatching than their siblings, indicating rapid early growth with potential adverse consequences on telomere length. Synchronous chicks did not suffer any apparent cost of hatching synchronously. Impaired phenotypes only occurred when reversing the natural hatching order (i.e. developmental mismatch), suggesting that maternal investment in last-laid eggs might indeed counterbalance the initial handicap of last-hatched chicks. Our experimental study thus highlights that potential interplays between pre- and post-natal environments are likely to shape fitness prospects in the wild.

Parental sex allocation and sex-specific survival drive offspring sex ratio bias in little owls

Although biased offspring sex ratios are common in species with sexual size dimorphism, the proximate causes are often unresolved. This is because two general mechanisms operating in different ways and in various periods of reproduction can lead to the bias: sex-biased survival or parental sex-allocation. We examined nestling sex ratio patterns between hatching and fledging, sexual size dimorphism, and factors affecting nestling survival using growth and survival data of 846 individual little owl Athene noctua nestlings with known sex from 307 broods from Germany, the Netherlands and Denmark. Nestling sex ratio was female-biased, mainly due to a significant female bias in the first-hatched chicks. Females showed a higher body weight than male nestlings at ringing and body weight of nestlings decreased with hatching sequence. Nestling survival was higher in females (Φ = 0.91) than in males (Φ = 0.85), and survival rates were positively related to body mass and negatively to brood size. Although the observed lower survival of males can cause an overall female-biased sex ratio, the sex dimorphism and survival patterns found here are unlikely to explain the conspicuous sex ratio pattern with a female bias in the first-hatched nestlings and the increase in female bias across the season. Thus, these results point towards interacting mechanisms of parental sex allocation strategies and sex-specific survival. As the female bias was allocated to the first rank that is most likely to survive, the female bias will increase under suboptimal breeding conditions. We therefore suggest that under suboptimal ecological conditions, higher investment into females is adaptive in little owls. Biased sex ratios can have severe effects on the social behaviour and population dynamics of endangered species. However, the existence of subtle sex ratio bias is often unknown and its proximate mechanisms and ultimate consequences often remain unclear. Small sample sizes make the detection of subtle effects unlikely and often fail to disentangle diverging mechanisms such as sex-biased survival and parental sex allocation. We used a large dataset of 846 little owl nestlings from 307 broods from three countries to investigate offspring sex ratio patterns, sexual size dimorphism and nestling survival simultaneously. Our findings suggest interacting mechanisms of parental sex allocation strategies and sex-specific survival to drive biased offspring sex ratios in little owls. The context dependence of the sex ratio bias indicates that offspring sex ratio bias in little owls is both, a consequence of—and an adaptation to—suboptimal breeding conditions.

Multiple Stressors in a Top Predator Seabird: Potential Ecological Consequences of Environmental Contaminants, Population Health and Breeding Conditions

Environmental contaminants may have impacts on reproduction and survival in wildlife populations suffering from multiple stressors. This study examined whether adverse effects of persistent organic pollutants (POPs) increased with poor population health and breeding conditions in three colonies (60–74°N) of great skua (Stercorarius skua) in the north-eastern Atlantic (Shetland, Iceland and Bjørnøya [Bear Island]). POPs (organochlorines [OCs] and polybrominated diphenyl ethers [BDEs]) were measured in plasma of incubating birds (n = 222), concentrations differing nearly tenfold among colonies: Bjørnøya (2009) > Bjørnøya (2010) > Iceland (2009) > Shetland (2009). Reproductive success (hatching success and chick survival) showed that breeding conditions were favourable in Shetland and at Bjørnøya (2010), but were very poor in Iceland and at Bjørnøya (2009). Biomarkers indicated that health was poor in the Shetland population compared to the other populations. Females whose chicks hatched late had high POP concentrations in all colonies except at Bjørnøya (2010), and females losing their eggs at Bjørnøya (2009) tended to have higher concentrations than those hatching. Moreover, there was a negative relationship between female POP concentrations and chick body condition at hatching in Iceland and at Bjørnøya (2010). Supplementary feeding experiments were conducted, and in Iceland where feeding conditions were poor, significant negative relationships were found between female POP concentrations and daily growth-rate in first-hatched chicks of control nests, but not in food supplemented nests. This suggests that negative impacts of POPs were mitigated by improved feeding conditions. For second-chicks, there was a strong negative relationship between the female POP concentrations and growth-rate, but no effects of supplementary feeding. Lowered adult return-rate between breeding seasons with increasing POP loads were found both at Bjørnøya (2009) and in Shetland, especially related to BDEs. This indicates stronger fitness consequences of POPs following seasons with very poor breeding conditions and/or high reproductive effort. This study suggests that the impacts of POPs may differ depending on population health and breeding conditions, and that even low concentrations of POPs could have ecological consequences during adverse circumstances. This is important with regard to risk assessment of biomagnifying contaminants in marine ecosystems.

Parental resource allocation among offspring varies with increasing brood age in Black-legged Kittiwakes Rissa tridactyla

Capsule Black-legged Kittiwakes Rissa tridactyla breeding at a North Sea colony allocated more resources to younger chicks with increasing brood age.Aims Examine how feeding, attendance and resource allocation change with increasing brood age and how allocation of feeds affects growth rate and fledging success.Methods Broods of two were observed on Coquet Island to compare feeding rates and fledging success between chicks of different hatching order.Results Growth and feeding rates were similar between chicks of different hatching order. The relationship between growth and feeding rate may have differed between siblings, although this relationship was not strong. Feeding rate per brood and nest attendance decreased nonlinearly as brood age increased. First-hatched chicks were fed more frequently at the beginning of multiple feeds and received a higher proportion of feeds during early chick-rearing. However, during late chick-rearing second-hatched chicks received proportionally more feeds.Conclusion Parents reduced overall feeding rate as brood age increased, while increasing the proportion of resources allocated to younger offspring. This may explain general similarities in growth rate and fledging success between chicks of different hatching order. By considering resource allocation throughout development we can better understand parental investment strategies in asynchronous species.

Nestling Development of Ring-necked Parakeets (Psittacula krameri) in a Nest Box Population

Chick development of a population of non-native Ring-necked Parakeets (Psittacula krameri) (RNP) has been investigated between 2006 and 2008 in Heidelberg, Germany. In parrots hatching asynchrony is common. RNP chicks are under natural selection to fledge synchronously with their siblings, as parents return less often to the nest after the first chick has fledged and remaining chicks may starve. Female nestlings apparently outperform the males by about one d in body mass gain, but also had higher measurements in tarsus, wing and tail growth. This was unexpected as adult males are generally larger than females. First-hatched chicks showed lowest biometrical values as compared to their younger siblings in several characters like body mass gain, bill, wing, and tail length. This feature may contribute to a synchronization of fledging in an otherwise asynchronous brood.

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.

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.

Body condition, hormonal correlates and consequences for survival in common tern chicks

Although sibling competition in avian species has been extensively studied, the proximate mechanisms remain largely unknown. Recent research proposed that steroid hormones, in particular testosterone (T) and corticosterone (CORT), might play a role either in promoting competitive behavioral displays or in response to chronic nutritional stress accompanied by a sustained competitive situation. Here, we examine body condition, endogenous T levels and fecal glucocorticoid metabolites (FGM) as non-invasive measures of CORT in sibling broods of wild common tern chicks (Sterna hirundo) during three post-natal developmental stages. In this species, distinct within-brood size asymmetries are imposed by an asynchronous hatching interval, and sexes show slightly different growth patterns. First-hatched (a-)chicks were in better condition than their later-hatched sibling (b-chick). FGM levels inversely covaried with condition and were elevated at the end of pre-fledging development. T levels of a- and b-chicks changed with age, although the direction of the changes differed, with b-chicks eventually having higher levels than their older siblings. Survival to fledging was not associated with FGM but with T levels, which tended to be higher in surviving chicks. Our results are discussed with regard to how plasticity in steroid hormones could be involved in mediating sibling competition in common terns.

Hosts Improve the Reliability of Chick Recognition by Delaying the Hatching of Brood Parasitic Eggs

The reliability of information that animals use to make decisions has fitness consequences. Accordingly, selection should favor the evolution of strategies that enhance the reliability of information used in learning and decision making. For example, hosts of avian brood parasites should be selected to increase the reliability of the information they use to learn to recognize their own eggs and chicks. The American coot (Fulica americana), a conspecific brood parasite, uses cues learned from the first-hatched chicks of each brood to recognize and reject parasitic chicks. However, if parasitic eggs are among the first to hatch, recognition cues are confounded and parents then fail to distinguish parasitic chicks from their own chicks. Therefore, hosts could ensure correct chick recognition by delaying parasitic eggs from hatching until after the first host eggs. Here we demonstrate that discriminatory incubation, whereby coots specifically delay the hatching of parasitic eggs, improves the reliability of parasitic chick recognition. In effect, coots gain fitness benefits by enhancing the reliability of information they later use for learning. Our study shows that a positive interaction between two host adaptations in coots--egg recognition and chick recognition--increases the overall effectiveness of host defense.

Reversed hatching order, body condition and corticosterone levels in chicks of southern rockhopper penguins ( Eudyptes chrysocome chrysocome)

In altricial and semi-altricial species, asynchronous hatching gives the first chicks to hatch an initial advantage over other siblings and often leads to the elimination of the smallest chicks. Both baseline corticosterone and acute stress-induced corticosterone levels have been shown to be higher in food deprived chicks than in chicks fed ad libitum. However, first-hatched chicks have also been shown to exhibit higher corticosterone levels than last-hatched chicks, suggesting an influence of the initial differences between eggs on corticosterone levels. We subjected single-chicks of southern rockhopper penguins Eudyptes chrysocome chrysocome to a standardised capture-stress protocol. In this species having very dimorphic two-egg clutches, we examined whether corticosterone levels were different between the two chick categories and tested for the effect of body condition controlled by the chick category. Neither body sizes, nor corticosterone levels differed between A- and B-chicks at 18 days. In contrast to baseline corticosterone levels, acute stress-induced levels of corticosterone were negatively correlated to body condition: chicks with a good body condition had lower acute stress-induced levels of corticosterone than chicks with a poor condition, whatever the chick category. Our results do not support the idea that initial differences in egg characteristics could drive the difference in corticosterone levels between siblings. On the contrary, they show that the A-egg of rockhopper penguins has, when reared alone, the same intrinsic potential to develop into a fledged chick as the B-egg. Later differences in body condition appear to lead to variation in the acute stress-induced levels of corticosterone.

Coots use hatch order to learn to recognize and reject conspecific brood parasitic chicks

Avian brood parasites and their hosts provide model systems for investigating links between recognition, learning, and their fitness consequences. One major evolutionary puzzle has continued to capture the attention of naturalists for centuries: why do hosts of brood parasites generally fail to recognize parasitic offspring after they have hatched from the egg, even when the host and parasitic chicks differ to almost comic degrees? One prominent theory to explain this pattern proposes that the costs of mistakenly learning to recognize the wrong offspring make recognition maladaptive. Here we show that American coots, Fulica americana, can recognize and reject parasitic chicks in their brood by using learned cues, despite the fact that the hosts and the brood parasites are of the same species. A series of chick cross-fostering experiments confirm that coots use first-hatched chicks in a brood as referents to learn to recognize their own chicks and then discriminate against later-hatched parasitic chicks in the same brood. When experimentally provided with the wrong reference chicks, coots can be induced to discriminate against their own offspring, confirming that the learning errors proposed by theory can exist. However, learning based on hatching order is reliable in naturally parasitized coot nests because host eggs hatch predictably ahead of parasite eggs. Conversely, a lack of reliable information may help to explain why the evolution of chick recognition is not more common in hosts of most interspecific brood parasites.

Video-monitoring helps to optimize the rescue of second-hatched chicks in the endangered Bearded VultureGypaetus barbatus

SummaryFrom 2000–2008 we used transmitting video cameras to document the breeding biology of the endangered Bearded Vulture in the Pyrenees (NE Spain), focusing the study on sibling aggression. Our goals were to study the feasibility of rescuing second-hatched chicks for conservation purposes in this species that shows obligate brood reduction. The age at which the second chick died varied between 4 and 9 days (n= 5). Prey items delivered per hour were not related to the survival time of the second chick or the aggressiveness of the first-hatched chick towards their sibling. Although sibling aggression generally began on day 1 after hatching, in two nests supplemented with food, aggression was delayed until the second and third day after hatching and the second chick survived for nine days. Our results on the death of the second chick and the test involving the rescuing of a second-hatched chick aged five days, suggest that the recommended age for intervention should be between 3 and 6 days, with 4–5 days probably being the optimal age for the rescue.

Hatching order and size-dependent mortality in relation to brood sex ratio composition in chinstrap penguins

The differential environmental sensitivity of the sexes has strong implications in the evolutionary history of species as it can alter sexual size dimorphism, population sex ratios, and the faculty of parents to manipulate offspring sex in relation to environmental conditions. We studied sexual differences in hatching patterns and evaluated sex- and size-related mortality in relation to hatching order and brood sex ratios in the chinstrap penguin Pygoscelis antarctica, a moderately size-dimorphic species, with a modal clutch size of 2 eggs. We found that male, second-hatched, and large eggs showed shorter hatching periods than female, first-hatched, and small eggs. We also found a male-biased mortality of nestlings in the colony. However, male mortality patterns differed depending on the brood sex ratio composition. Mortality of male chicks in all-male broods was higher than in mixed broods and higher than female mortality in all-female broods. Contrary, females from mixed brood showed higher mortality than their male nest mates and higher too than females in all-female broods. Second-hatched chicks also suffered from higher mortality than first-hatched chicks. Our results indicate that both the superior competitive capacity and the higher energy demand of the larger sex constitute 2 causal factors explaining patterns of sex-biased mortality. Both factors occur in the same species and in different situations of sibling competition shaped by brood sex ratio composition. This study constitutes a good example of how patterns of sex-related mortality can vary depending on nest environmental circumstances. Furthermore, our study suggests that hatching period can be a mechanism underlying sexual differences in the embryonic period of birds.

Serum immunoglobulin G levels are positively related to reproductive performance in a long-lived seabird, the common tern (Sterna hirundo)

The evolution of longevity requires a low risk of mortality from extrinsic factors, relative to intrinsic factors, so that individuals that differentially invest in physiological self-maintenance and minimize their annual reproductive costs will maximize lifetime fitness through a prolonged reproductive lifespan. The trade-off between reproductive effort and self-maintenance, as measured by immune function, has been well documented in short-lived birds, but is difficult to demonstrate in long-lived birds. To assess self-maintenance in a long-lived seabird, we measured serum protein levels, including immunoglobulin G (IgG = IgY), in 30 breeding pairs of common terns (Sterna hirundo) and their first-hatched (A) chicks. Most parents were of known age from banding as hatchlings; our sample was selected to contrast young breeders (6-9 years) with very old birds (17-23 years). Body-mass of the parents declined by 5% during the chick-rearing period, while serum protein levels were stable. Serum IgG levels were higher in parents of offspring with faster growth rates, while IgG levels were lower in parents whose broods were reduced by starvation. A-chicks in broods of two had higher IgG levels than singleton chicks. Albumin levels were not related to reproductive performance. Thus, despite adequate statistical power, we could find no evidence for a trade-off between reproduction and self-maintenance in common terns, even in old age. The results are consistent with life-history predictions for long-lived vertebrates, in which selection favors sustained self-maintenance across the reproductive lifespan. The positive relationships between IgG levels and reproductive performance indicate that IgG can be used as an index of parental "quality."

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) × 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.

Sex-biased initial eggs favours sons in the slightly size-dimorphic Scops owl (Otus scops)

We assessed the potential control of female Scops owls (Otus scops) over the production of daughters and sons in relation to their order in the laying sequence, and investigated the possible adaptive value of such control on the survival and growth of nestlings. The population sex ratio at fledging was not significantly biased. A relationship between ovulation order and egg sex was found, which was mostly due to the strong male biased sex ratio among initial eggs, as 17 of 18 first-hatched chicks (94%) from first-laid eggs were males. Asynchronous laying and subsequent asynchronous hatching resulted in a decreasing survival of chicks with increasing hatching order. Female chicks fledged with a higher mass than males and fledging mass decreased with hatching order due to male biased hatching order and its influence in the growth of the single first-hatched female. These results suggest that female Scops owls may be able to adjust the sex of their offspring at egg production to invest differentially in their survival and growth. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 76, 1–7.

Sex-biased initial eggs favours sons in the slightly size-dimorphic Scops owl (Otus scops)

We assessed the potential control of female Scops owls (Otus scops) over the production of daughters and sons in relation to their order in the laying sequence, and investigated the possible adaptive value of such control on the survival and growth of nestlings. The population sex ratio at fledging was not significantly biased. A relationship between ovulation order and egg sex was found, which was mostly due to the strong male biased sex ratio among initial eggs, as 17 of 18 first-hatched chicks (94%) from first-laid eggs were males. Asynchronous laying and subsequent asynchronous hatching resulted in a decreasing survival of chicks with increasing hatching order. Female chicks fledged with a higher mass than males and fledging mass decreased with hatching order due to male biased hatching order and its influence in the growth of the single first-hatched female. These results suggest that female Scops owls may be able to adjust the sex of their offspring at egg production to invest differentially in their survival and growth.