This study was initiated to examine the methods and biology of recruitment to the breeding group in the Kittiwake. Long-term changes in the population biology are presented for the history of the colony (1949 to 1984).Mortality rates of adults were particularly high in the early 1970's; concurrently recruitment rates increased, age of first breeding declined, and the body weights of recruits decreased in order to compensate. A model of colony growth is presented which shows that small colonies are proportionately more attractive to recruits in that they have higher rates of increase and recruitment. The North Shields colony followed the expected pattern of growth for 17 years. Evidence presented shows that nest sites at North Shields were then socially limited; there were physically available sites which were not used. In consequence of the social structure of the colony, there was a pool of potential recruits which were temporarily restricted from breeding. Recruits (Kittiwakes breeding for the first time) were at least three years old, were present at the colony as prospectors in at least one year prior to breeding, and arrived by early May in the year of first breeding. The attendance of recruits at the colony in May was more than three times that of prospectors, and recruits had higher body weights. These prerequisites separated recruits from prospectors, but there was further selection at the time of recruitment. Kittiwakes preferred to nest as close as possible to other nesting pairs, but aggression forced a compromise. High quality birds chose sites close to other birds and did better reproductively. Thus individual quality segregated recruits on the basis of their ability to compete for sites in dense areas; some Kittiwakes deliberately waited for aggression from established breeders to decline in order to take up preferred sites. The results are discussed in terms of the importance of the buffering effect of the pool of non-breeders on the size of the breeding group, and individual differences in quality
