Modelling Annual Changes in Numbers of Breeding Fulmars, Fulmarus glacialis, at a Colony in Orkney

(1) During a long-term study of breeding fulmars in Orkney, the number of birds breeding each year has increased on average, but with some considerable variations between years. (2) In a species with deferred maturity, low fidelity to natal colony and which is faithful to both mate and nest site, the factors causing changes in breeding numbers are likely to be annual survival of established breeders and annual recruitment of new breeders. (3) Recruitment could not be measured directly, but an index was determined, based upon the number of unringed breeding birds. However, in the observed data there were some occasions in which not all of the unringed birds breeding in one year necessarily bred the following year. There is some evidence to suggest that this may be due to intermittent breeding and/or emigration, probably involving inexperienced breeders. (4) A new index of 'recruitment', which could be negative, was calculated from the observed data. A model was developed to simulate breeding population size over a series of years, using observed annual values of survival and 'recruitment' as input parameters. (5) Simulations from the model were realistic in terms of variation between years and correlations between variables. (6) The model was therefore run using particular observed values for both 'recruitment' and survival. It was found that 'recruitment' had a much greater effect upon the population size than did survival of breeding adults. (7) The model was then used to predict the size of the breeding population one year in advance, with reasonable accuracy. (8) The simulations highlight the need for further information about the behaviour and movements of potential breeding recruits.