Sibling Growth Patterns in Great tits: Does Increased Selection on Last-hatched Chicks Favour an Asynchronous Hatching Strategy?

Abstract

In birds, competitive abilities of siblings in relation to their sex and the magnitude of hatching delay are still poorly understood. We compared the sex-specific growth of the last-hatched, competitively disadvantaged chicks with that of synchronously hatched chicks in two successive years. Sons exhibited higher growth rates than daughters in a year with delayed onset of breeding, and this sex-related difference was more pronounced among the asynchronously hatched chicks. Females apparently do not selectively allocate more resources to the last-laid eggs because neonatal body mass and the growth rate of asynchronously hatched chicks did not differ between years, despite the fact that in one of the years, asynchronous chicks hatched from replaced eggs taken randomly from other nests and not from eggs laid last by the incubating female. Among chicks that survived, moderately asynchronous siblings grew at a lower rate than synchronous ones, whereas no difference in mass gain was revealed between synchronous and strongly asynchronous siblings irrespective of their contrasting competitive abilities. We suggest that selection of the fittest asynchronous chicks may improve the overall quality of asynchronous broods, thus favouring the maintenance of asynchronous hatching strategies in variable environments.

Insulin‐like growth factor 1 and growth rate in nestlings of a wild passerine bird

“…Hence, we cannot exclude the possibility that IGF‐1 levels were already in the declining phase in the middle of nestling period. However, we have previously shown that great tit nestling growth rate is highest in the middle of the nesting period (days 6–7 post‐hatch) and decreases substantially towards fledging (Tilgar & Mänd ). This indicates that age‐related changes in IGF‐1 levels from day 8 to day 15 post‐hatch follow the respective changes in somatic growth rate.…”

“…Given that increased energy intake sustains higher IGF‐1 secretion to mediate increased growth rate (Ross & Buchanan ; Breier ), we predicted that fledglings in the reduced broods should have higher IGF‐1 levels and therefore more rapid development (increased post‐natal growth rate, better body condition) than those in control and enlarged broods. Also, it has been shown previously that nestling growth rate is highest in the middle of the fledging period (Tilgar & Mänd ); therefore, we expected that plasma IGF‐1 on day 8 post‐hatch would be higher than on day 15, to support more rapid development.…”

Long-term consequences of early ontogeny in free-living Great Tits Parus major

“…Moreover, given that sex differences can exist in the sensitivity of offspring to prenatal (Gorman and Nager 2004) and post-hatching developmental conditions (e.g. Kilner 1998;Oddie 2000;Naguib et al 2006;Tilgar and Mänd 2006), it is surprising that only one study in wild birds has raised the question of whether rearing conditions have sex-specific effects on individuals' future reproductive performance (Gustafsson 1989).…”

Birthing asynchrony is not a consequence of asynchronous offspring development in a non‐avian vertebrate, the Australian skink Egernia whitii

“…This may be of specific importance to the Egernia system, as being born last may represent a fitness disadvantage, such as reduced survival or growth rate compared with siblings (Massemin et al . 2002; Tilgar & Mand 2006). This would potentially facilitate a parent–offspring conflict with regard to birth order.…”