sexual dimorphism in egg-mass biased by laying order
We found that females may potentially adjust reproductive investment by differentially allocating resources to eggs according to the sex of the embryo. To our knowledge, only five other studies have reported sexual size-dimorphism in size of avian eggs (Mead, Morton & Fish 1987; Anderson et al. 1997; Cordero et al. 2000, 2001; Cunningham & Russell 2001). Furthermore, we found evidence that female kestrels may allocate resources to eggs differentially depending simultaneously on sex and laying order. That is, initial eggs bearing a male embryo were heavier than initial eggs bearing a female embryo, but no differences in mass associated to sex were found for eggs laid later in the laying sequence. To our knowledge, this striking result provides the first evidence of sexual egg dimorphism biased by laying order.
Parental favouritism expressed in differential resource allocation to different-sex eggs may be a mechanism to adjust allocation for both the primary and secondary sex-ratio of clutches in order to increase parental and offspring fitness (Cordero et al. 2000, 2001). We found that differential investment in eggs bearing sons or daughters was not related to the proportion of the sexes in the first vs. subsequent eggs, and that sex-ratio was not biased by laying order. Other studies have shown that first-laid eggs are only significantly sex-biased or more biased than later-laid eggs (Emlen 1997). Laying order may be biased towards the larger (Leroux & Bretagnolle 1996; Arnold et al. 2001) or the smaller sex (Bednarz & Hayden 1991; Blanco et al. 2002) in sexually size-dimorphic birds. Furthermore, opposite trends have been found in different populations of the same species (Clotfelter 1996; Kilner 1998) and between years with contrasting food conditions (Dzus, Bortolotti & Gerrard 1996). Our results suggest that differential investment in offspring may also be reached by differential provisioning of eggs according to the sex of the first-laid egg, rather than biasing sex ratios of initial eggs, although we cannot discard that female kestrels could be able to choose the sex of first-laid eggs.
sex of initial eggs and egg-mass hierarchies within clutches
Intraclutch egg-mass variation may result from physiological or nutritional constraints on the laying female, but it may also be an adaptive response to enhance the parental control of sibling competition (Slagsvold et al. 1984; Viñuela 1997). These alternatives are not mutually exclusive for explaining within-clutch egg mass variation between species, as it may depend on how the resources for laying are gathered (Viñuela 1997). Different strategies of intraclutch egg-mass variation might also occur within species if individuals differ in how and how many resources for laying are gathered and allocated within the clutch, as suggested by experimental food supplementation before and during laying altering egg-mass hierarchies within clutches (Simmons 1994; Wiebe & Bortolotti 1996; Aparicio 1999). Our results indicate that females starting the clutch with an egg bearing a daughter increased mass investment on eggs laid subsequently in the clutch, while females laying a first egg bearing a son invested a decreasing amount of nutrients in subsequent eggs. This is the first evidence suggesting that different individuals of a given population may adopt different strategies of intraclutch egg-mass variation depending on the sex of initial eggs.
Constraints such as ‘gearing up’ physiologically for starting egg production may explain relatively small initial eggs (Parsons 1976; Leblanc 1987), while prolactin influence on developing follicles during incubation or depletion of reserves at the end of laying may explain relatively small last-laid eggs (Leblanc 1987; Arnold 1991). However, these proximate mechanisms cannot explain why a relatively large or small initial egg should be associated with a particular sex and furthermore, why this association should influence the pattern of investment on subsequent laid eggs in the clutch. Our results suggest that resources invested on initial eggs bearing daughters would be partially reabsorbed or differentially provisioned when passing through the oviduct to result in relatively small eggs, and saved resources should be then invested in relatively larger eggs laid subsequently in the laying sequence independently of their sex. On the other hand, resources invested in relatively large initial eggs bearing sons would be not restricted and then reserves should be increasingly reduced due to their depletion during the laying period.
the dilemma of the initial-egg sex and its consequences for offspring
Different strategies of resource allocation according to egg sex and laying order may be different forms of investment in the progeny by influencing the degree of sibling competition. According to the ‘brood-survival’ hypothesis, females may allocate a relative large amount of resources on eggs laid in last order to decrease the probability that the last-hatched nestling will starve. Alternatively, females may invest a relatively larger amount of nutrients on initial eggs to enhance some mortality due to the hierarchy imposed by hatching asynchrony under unpredictable food shortages (‘brood-reduction’ hypothesis). In our study, different patterns of intraclutch egg-mass variation were not found being translated to differences in egg losses, hatching asynchrony or offspring mortality. Brood reduction in this population of kestrels is relatively rare (< 10% of hatched chicks died), due probably to high food availability and optimal hunting habitat, and thus the possible effects of intraclutch egg mass variation on brood reduction may be difficult to detect. Alternatively, our results suggest that parents may be able to invest in their progeny through different egg-mass hierarchies, depending on the sex of the first-laid eggs, and this may allow to reduce possible nonadaptive mortality induced by increasing sexual dimorphism in the mass of chicks along growth (Bortolotti 1986).
Regarding brood composition, we found that clutches initiated with an egg bearing a male fledged more sons than those initiated with an egg bearing a female. The opposite trend, albeit not reaching significance, was found for the number of daughters. Even if these trends influenced the distribution of sexes among fledglings from clutches initiated with different-sex eggs, sex-ratios at fledgling did not differ from parity in both kinds of nests, and perhaps these non-significant results could be caused by the relatively small sample size in these analyses (see West & Sheldon 2002). The small difference between the number of sons and daughters raised in both kinds of nests may be a simple effect of considering the sex of the initial egg as the grouping variable. Alternatively, these patterns may be due to differences in environmental or parental features, such as condition, age, breeding experience or the onset of incubation start, influencing the ability to raise broods with different sex ratios. We lack data on parental characteristics to test these predictions, but a negative relationship between female condition and proportion of male offspring (the cheaper sex) have been found in this species (Korpimäki et al. 2000) and in the closely related American kestrel (Wiebe & Bortolotti 1992). Therefore, it remains possible but untested that different sex allocation strategies associated with different egg-mass hierarchies and sex of initial eggs depend on parental characteristics.
A fine-tuned egg mass vs. laying sequence strategy has been suggested to be unlikely to evolve in American kestrels because females seem not to have the ability to always hatch eggs in the same order they are laid (Bortolotti & Wiebe 1993). Eurasian kestrels have been suggested to be able to hatch eggs in the order they were laid (Wiebe et al. 1998). However, the possibility that the lack of matching between laying and hatching orders would enhance the adjustment of parental effort when eggs of different sex are provisioned differently has been not addressed before. We found that daughters from initial eggs, which were relatively lighter than the remaining eggs in the clutch, hatched earlier than sons from initial eggs, which were relatively heavier than the remaining eggs in the clutch, due probably to differences in the duration of incubation associated to sex (Blanco et al. in press). Therefore, females may be adjusting egg-mass provisioning to initial eggs of different sex in an attempt to control sex-sequence at hatching and then possible mass hierarchies within the brood (Bortolotti 1986; Blanco et al. 2002; Krebs et al. 2002). Thus, first-laid eggs bearing daughters would be relatively lighter, to compensate for faster growth rate of female chicks if hatched in first order (see Blanco et al. 2002). In this case, daughters from initial eggs may not take advantage of the brood hierarchy due to the mass of the eggs from which they hatch. In contrast, initial eggs bearing sons would be relatively heavier in an attempt to compensate for their lower growth rate and delayed hatching.
Different strategies of egg provisioning and hatching patterns were associated, respectively, with mass hierarchies and the lack of mass hierarchies without differential mortality in broods initiated with eggs bearing sons and daughters. Thus, fledgling mass decreased with hatching order in nests initiated with eggs bearing sons, while fledging mass of chicks hatched in different orders did not differ in nests initiated with eggs bearing daughters. Overall, mean mass of both male and female fledglings did not differ between both kinds of broods. However, fledgling mass differences due to hatching order in nests initiated with a male egg could result in differential mortality during breeding seasons with a food shortage or after fledgling. We do not know if females were able to control the sex of initial eggs, but we have showed that they were apparently able to identify their sex and provision them accordingly to presumably maintain the subsequent control of investment. This control may be achieved due mainly to the clear influence that egg mass has on hatchling mass (Williams 1994; Christians 2002), and then the potential influence that egg-mass allocation within the clutch may have on the brood mass hierarchy (Slagsvold et al. 1984; Viñuela 1997). In addition, this control may be achieved through the influence that first-hatched chicks may have on growth and survival of their siblings in sexually dimorphic species with variable degrees of hatching asynchrony (Bortolotti 1986; Blanco et al. 2002; Krebs et al. 2002).
In conclusion, the combination of small initial eggs and the larger mass of subsequent eggs laid in the clutch may smooth intrabrood mass hierarchy when the first egg laid bears an embryo of the larger-size sex. In contrast, relatively heavier initial eggs, and the lightest mass of final eggs in the clutch, enhances intrabrood mass hierarchy when the first egg laid bears an embryo of the smaller-size sex. These strategies may potentially be adopted to (1) compensate differences in mass induced by genetically determined sexual differences in growth rate, thus avoiding possible maladaptive intrabrood mass hierarchies induced by asynchronous hatching, while (2) maintaining a given degree of intrabrood mass hierarchy favouring early hatched chicks. However, the outcome of these strategies may vary depending on environmental or parental conditions, or both, which need be investigated further. The main conclusion of this study reinforces the evidence that egg mass may potentially be controlled depending on the female ability both to identify egg sex and to differentially allocate resources accordingly to adjust reproductive investment. More research is needed on the role of the combination of sex and mass of eggs laid in particular orders, especially initial eggs, in the adjustment of reproductive effort in birds with marked sexual dimorphism in mass.