Long-term Changes in Recruitment to the Breeding Group, and the Quality of Recruits at a Kittiwake Rissa tridactyla Colony

Abstract

The number of recruits to the North Shields kittiwake colony in N.E. England (U.K.) (Coulson and Thomas 1985) increased from 1954 to 1967, but the proportion of first breeders decreased during that time. Increased mortality of the breeding birds or the provision of more sites increased recruitment. This implied that there was a pool of potential recruits from which the new recruits needed could be drawn. In most years there appeared to be a non-breeding reservoir. About 11% of each cohort returned to breed in their natal colony. Immatures did not suffer the same variations in mortality as did adults: this finding is consistent with the premise of Coulson and Thomas (1985) that increased adult mortality associated with the decline in numbers in the late 1960s occurred prior to the breeding season. The age of first breeding fell from 5 to 4 years in response to high adult mortality in the early 1970s; body weights and wing-lengths of male and female recruits diminished, but the body weights of male and female prospectors fluctuated little. A model of colony growth is presented, based on the relationship between colony size and rate of increase in expanding colonies of British kittiwakes. Small colonies grow faster than large ones, and are proportionately more attractive to recruits. Small colonies cannot produce enough young during the first 70 years and are sustained by immigration. The North Shields colony is not a closed system and is not currently growing; it is supported by immigration. There is a positive relationship between colony size and nesting density; colony growth appeared to be limited by the number of attractive sites in the dense center of the colony. There were sites available on the periphery which were not used. The importance of breeding of the buffering effect of the pool of potential recruits is discussed.