Studies of recruitment in the great skua

Abstract

This thesis examines different variables affecting recruitment, and provides evidence of recent events affecting a seabird colony. Fieldwork was conducted during 2003-05 in the island of Foula, Shetland. Foula holds the largest colony of great skuas Stercorarius skua in the world; although numbers of breeding pairs in the colony increased rapidly from 1900-70, recently numbers have been decreasing. Data were collected by marking non-breeders and taking individual measures, individuals were followed in subsequent seasons to record their behaviour. An extended database was used to determine how long-term effects of variables such as hatching date, food availability and climate change affect the process of recruitment. The results show that food availability is related to breeding success and early hatching, as well as the probability of returning to the colony to breed. The variable used to quantify climate change (NAO winter index) was not related to recruitment, however it is suspected to influence food abundance. Contrary to expectation, individual quality did not have an effect on the probability of breeding for the first time, and there was no difference in body condition between potential recruits and established breeders; however historic data suggest a difference. The current situation faced by great skuas in the Foula colony may be a determined for the changes in recruitment rates as well as for the parameters that determine the recruitment process. Compared to two decades ago, numbers of pre-breeders have decreased substantially which may give evidence of density dependent effects preventing the addition of new recruits to the colony