Adult and pre-breeding survival estimates of the Whiskered Tern Chlidonias hybrida breeding in southern Poland

We present the first-ever survival estimates of the Whiskered Tern Chlidonias hybrida, based on the analysis of capture-recapture data (403 adults and 1,484 chicks ringed) collected between 1993 and 2011 in an increasing breeding population in southern Poland. Data were modelled using multistate models with an unobservable state, accounting for the period during which young terns remain at their winter quarters. Model-averaged pre-breeding and breeding survival were estimated to be 0.54 [standard error (SE) 0.28] and 0.80 (SE = 0.05), respectively. All models were in agreement that the relative proportion of breeders was nearly zero in the second calendar year, increasing to reach values close to 0.8 in the fifth calendar year, which confirms the observation of a much delayed maturation of the Whiskered Tern. Our data indicate that most Whiskered Tern start to breed about 1 year earlier than members of genera Sterna and Onychoprion. However, the precision of the estimates for the parameter describing the transition probability from the unobservable pre-breeding to the observable breeding state was extremely poor; therefore, these estimates should be treated as tentative until more data are collected. The three best-supported models indicated significant annual variation in recapture probability. The results also suggested that forced exchange of breeding colonies is frequent in the study area; consequently, a large proportion of birds ringed as chicks are breeding in colonies other than their respective natal colony. This exchange is best explained not by the trapping and ringing activity but by human management of the environment, such as water level changes in dam reservoirs and carp Cyprinus carpio farming at fish ponds, both of which result in breeding habitats becoming unstable and periodically unavailable, possibly forcing birds to change breeding sites.