Roseate terns are rare and restricted range breeders in NW Europe, with a population size of below 1,100 pairs and 95% of these within four colonies. The apparent survival and breeding dispersal rates of adults was estimated from live resighting data collected at three of the largest colonies between 1995 and 2007. These parameters were estimated using a multi-state model implemented in program MARK, with transition rates equating to breeding dispersal. The program U-CARE identified transience and trap-dependence in the data and so these effects were specified in the model. Time-averaged apparent survival rates were 0.735 yr-1 for birds during the interval following their first encounter and 0.855 yr-1 for “older” birds. Survival varied among years but not colonies. Breeding dispersal rates varied among colonies, being highest towards the largest and most productive colony, intermediate in the reverse direction and lowest between the two smaller colonies. Since the largest colony is not en route to either of the smaller colonies this observation cannot be explained by the order in which birds encounter sites during spring migration, so we propose that birds either follow the bulk of birds migrating north in spring or choose to breed there after prospecting during the staging period of the previous year. The differences in numbers breeding at each colony, however, mean that the numbers moving in different directions among colonies broadly balance out, such that net movements in terms of the number of individuals involved are relatively small.
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Vol. 31 • No. 3