Sibling rivalry in black-legged kittiwakes (Rissa tridactyla)

Chicks of several species compete with their siblings for parental provisioning of resources and care. This competition is mainly manifested by begging or food hoarding and in only few species, as in the black-legged kittiwake (Rissa tridactyla) direct aggression between the offspring is present. It has been proposed that the degree of asymmetries between the members of a brood influence the severity and outcome of this conflict. Several inequalities between the offspring have been identified (e.g. age, size, egg quality). First- hatched chicks (A) are older, larger and hatch from a different quality egg and size than second- hatched chicks (B). These inequalities provide different advantages to the chicks within a brood but their influence in sibling rivalry have not been widely tested. In the present thesis egg components of A and B eggs were analysed, comparisons of behaviour, survival and growth of chicks in two different years were made and egg size and quality were experimentally manipulated in order to test their importance in the sibling rivarly outcome of the black-legged kittiwake. In chapter II egg composition analyses of A- and B- eggs within a clutch were made. It was found that mothers allocated more carotenoids and less testosterone to A- eggs, while corticosterone, lipid and protein content did not differ, although A- eggs were 4% larger than B- eggs. In chapter III comparisons of behaviour, growth and survival between two environmentally different years were made. 2004 was a year with poor quality food whilst 2005 was a year with good food quality. The majority of second-hatched chicks in 2004 died before reaching 10 days of age and were all dead before 15 days while in 2005, more than 80% of second-hatched chicks fledged. Same behaviours were performed by A and B chicks in both years but the behavioural patterns did: aggression increased with age in 2004 while begging and feeding decreased in A and B chicks; these behaviours did not change with age in 2005. In chapter IV the influence of egg size on sibling rivalry was experimentally tested by eliminating age and egg quality differences within a brood and manipulating only the egg size differences. Comparisons of survival, growth and behaviour were made between experimental and control broods in which all the natural asymmetries were present. Differences in egg size determined which chick became dominant but these differences did not change the brood behaviour. Furthermore, experimental broods showed very different behaviour from control broods and the frequency of aggression was different between experimental broods formed by A- or B- eggs. In order to test if eggs are adapted for their hatching position, in chapter V an experimental manipulation of the brood composition was carried out and one of the clutch eggs’ was swapped to make it hatch in a different position from the one it was made to hatch. The natural asymmetries in age and egg size were maintained. Broods with two first-laid eggs were less aggressive than control broods and than broods with two second-laid eggs. If A- and B- chicks are specifically made for their hatching position, their stress response should differ when they were artificially made to hatch on the same position and exposed to a stressor. This was tested on chapter VI and it was found that second hatched chicks from A- and B eggs did not differ on their stress response when facing a handling-stress protocol. On each year, natural broods fledged in higher proportion than experimental ones, which indicates that asymmetries within a brood are adaptive. It seems that eliminating within brood asymmetries is costly for the parents and perhaps these differences are optimal for maintaining a high efficiency index for the parents in terms of the amount of investment and the number of fledged chicks. Thus apparently, the main asymmetry influencing sibling rivalry is the difference in age of the offspring caused by hatching asynchronously.