Régulation par le recrutemet, la fidélité et la non-reproduction chez un oiseau colonial, la mouette tridactyle (Rissa tridactyla)

Abstract

A long term study of the Kittiwake colonies of Cap Sizun (Brittany, France) shows that, even though Kittiwakes usually recruit at the age of 4, the population growth rate from year to year is highly correlated with the breeding success of the previous year. This is due to a strong recruitment, a large proportion of breeders, and a high adult fidelity after years of high fecondity. Furthermore, for a given individual breeding performance (breeding success or failure), adult fidelity to the colony and the proportion of breeders in the following year increase with the breeding success of the colony in which they bred in the previous year. This leads us to propose a mechanism of population regulation through Attraction, Recruitment, Non-breeding, and Adult Fidelity, in which breeding success and social context — two characteristics which reflect environmental quality — play prominent roles. These two parameters are the main components of the attraction exerted by a breeding site, i.e. its capacity to recruit new breeders (young or adults) and to retain its own adults as breeders. Breeding success and social context are assessed by potential recruits when prospecting different breeding places. If fecondity and social context are good, prospectors may settle there and recruit in the following year. Prospectors are attracted to nests with chicks, on which they squat when they are momentarily left unattended by the parents after the middle of the rearing period. These squatters are mainly prebreeders or failed breeders ; both are actively looking for a breeding site. Males are more prone to squat on chicks. Prebreeding squatters on chicks show a high proportion of recruitment in the following year : 34 % of prospecting males recruit on one of the nests they have squatted during the previous year, and 24 % on the most consistently squatted nest. Thus, squatting is an appropriation behaviour of a breeding site. The density of parasites influences the colony breeding success : breeding groups with high densities of parasites on the chicks have the lowest fecondity. The appearence of parasites is correlated with a drop in fecondity of the kittiwakes and parasites are likely to be the cause of this reduced fecondity. This acts on the numbers through prospectors’ and adults’ behaviour which in turn can explain the positive correlation between the fecondity in a given year and the rate of population increase in the following year.