Terns in trouble: demographic consequences of low breeding success and recruitment on a common tern population in the German Wadden Sea

Abstract

The nationally endangered breeding population of Common Terns Sterna hirundo in the German Wadden Sea has been rapidly declining in recent years. In an attempt to identify the causes for this decline we analysed the demography of the Jade Bay population based on the “Banter See” colony. This colony has been intensively studied for over 18 years and has been declining in recent years. We estimated apparent survival rates of breeders and non-breeders of different ages using multi-state capture–recapture models based on individual life histories. The population growth rates of the Banter See colony and Lower Saxony population were estimated, and possible future trajectories were projected using a matrix model approach. Adult survival rates (0.9) have remained constant, but since 2002 subadult survival has decreased significantly (0.36–0.28). We found significantly lower breeding success as well as lower and delayed recruitment. These changes in vital rates probably caused the shift in the Banter See colony growth rate from an increasing phase before 2002 to a declining phase after and most likely occurred due to low pelagic prey fish stocks in the North Sea, the most important food source for terns in the Wadden Sea. Should the population decline continue at the current rates, the Banter See colony could decrease by 11% and the Lower Saxony Wadden Sea population by an alarming 57% between 2009 and 2015.