Sibling Rivalry in Gulls: Effects of Asynchronous Hatching and Egg-Characteristics on Offspring Growth, Survival and Behaviour. A Study on Lesser Black-Backed Gulls (Larus fuscus)

In many bird species, eggs within a clutch differ in their size and content composition (egg characteristics). Additionally, in asynchronously hatching broods, chicks also differ in age and relative size due to staggered hatching. The last-hatched chick within a brood generally has the shortest incubation duration, is socially subordinate to its older siblings and has the lowest fledging success. Both hatching rank and egg characteristics have been shown to correlate with the duration of incubation, chick growth, chick survival and chick behaviour. Many studies have investigated the significance of those differences and whether it is the hatching rank or egg characteristics which causes the differences between siblings, however, these two factors are often confounded in natural broods. The last-hatching offspring often also hatches from an egg different from its older siblings. Therefore it is hard to distinguish between the two alternative explanations. Recent studies suggest that mainly hatching asynchrony affects offspring success. Chicks of last-laid eggs lag behind in growth and experience and are therefore disadvantaged in competitive situations. The egg characteristics of the last-laid egg, though, might actually bear some advantages for its chick to overcome the disadvantage given by the last hatching rank. In several species, last-laid eggs were found to contain a higher level of testosterone, which has been suggested to increase the embryonic and neonatal growth of the chick as well as the aggressive behaviour in order to enable the last-hatched chick to catch up with its siblings. However, other studies have shown that a high testosterone level might also decrease the chick' success by weakening the immune system, delaying hatching and decreasing growth rates of the last-hatched chick. These results show that it still remains unclear whether the hatching rank or the egg characteristics affect incubation duration, chick growth, chick survival and chick behaviour, i.e. fledging success. Lesser black-backed gulls {Larus fuscus) generally lay a clutch of three eggs with the last egg being smaller and showing differences in egg composition (e.g. nutrients, hormones) compared with the other two eggs of the same clutch. Chicks hatching from last-laid eggs have been shown to differ in terms of size, growth, immune response, condition and survival, compared to their siblings. I manipulated the hatching order relative to the laying order of lesser black-backed gull broods with clutches of three eggs in such a way that chicks of first-laid eggs were the last chick to hatch within a brood, and chicks of last-laid eggs were the first chick to hatch within a brood. This experimental design separated the effects of hatching rank from the effects of egg characteristic and therefore enabled me to investigate whether it is the hatching rank or the egg characteristic which affects incubation duration, chick growth, chick survival and chick behaviour. Emphasis was laid on chicks of first-laid eggs (A-chicks) and of last-laid eggs (C-chicks). When hatching last, C-chicks had a shorter incubation duration than A-chicks due to a shorter hatching duration. When hatching first, A- and C-chicks did not differ in hatching duration. These results indicate that it is the characteristics of C-eggs which enables these offspring to respond to the presence of older chicks in the nest by hatching more quickly. C-chicks had a poorer condition at hatching than A-chicks independent of hatching rank, thus, the differences in condition between A- and C-chicks was due to the same differences in characteristics of A- and C-eggs. Nevertheless, there were no differences in growth, fledging success and cause of mortality between A-and C-chicks. C-chicks begged more intense than A-chicks, independent of hatching rank, and within the last hatching rank, only C-chicks were able to monopolise food and to address begging behaviour towards their nestmates. Within control nests. C-chicks were fed as frequent as A-chicks, and when hatching last C-chicks were ted even more frequently than A-chicks. When hatching first, however, A-chicks were fed more often than C-chicks. These results suggest that C-chicks are more adapted to the last hatching rank within a brood. Many studies explain the third chick disadvantage as a mechanism to facilitate brood reduction. In the case of scarce resource abundance, parental birds could adjust the brood size to the low resource abundance by expelling the chicks in which they have invested the least and which has the lowest survival probability. Alternatively, last-laid eggs might be an insurance strategy. In the case that one or all of the first chicks die very soon after hatching. The lack of a distinct third chick disadvantage in this study contradicts the brood reduction hypothesis. The predictions of the insurance hypothesis can not be confirmed either since last-hatched chicks did not show a significant insurance value within a brood, i.e. they did not survive significantly more often when either one or both of the earlier hatched chicks died than when both of the older nestmates survived. The results of this study give evidence of a co-evolution of hatching asynchrony and differences in egg characteristics