body condition; colony size; density dependence; food availability; intertidal habitats; hinterland model; population regulation; reproductive success; wading birds
It has been suggested that in most colony-breeding birds, food availability in thefeeding areas surrounding the colonies limits, and thereby regulates, populationsize. However, population size is also determined by adult survival, which willadditionally be influenced by circumstances outside the breeding season. MostEurasian Spoonbills Platalea leucorodia leucorodia in The Netherlands breed onthe Wadden Sea barrier islands. After 30 years of exponential growth, thebreeding population in the Dutch Wadden Sea area is now levelling off towards amaximum of nearly 2000 nests. For these Spoonbills, density-dependent effectson survival by the different age-classes and in the different seasons havealready been demonstrated. However, the mechanisms underlying the densitydependentsurvival of juveniles before and after fledging remain unclear. Toexamine whether these density-dependent effects reflect limitations at thecolony level, we compared colony growth, chick condition and reproductivesuccess among the Wadden Sea colonies. Population growth rates from 1988 to2015 varied widely between the 10 existing colonies, and so did the statisticallypredicted maximum colony sizes. Chick condition, measured for 781 chicks insix different colonies between 2011 and 2015, was lower in stable colonies thanin growing colonies, although not for the very late chicks, and reproductivesuccess tended to be lower as well. Over the longer period of 1991 to 2011,reproductive success showed a strong negative relationship with colony size.We propose that the levelling off of colony sizes in the Wadden Sea is caused bylocal food limitations, and suggest further research in this direction. The continuinggrowth of the Dutch population is now being fuelled by exponentiallyincreasing numbers of Spoonbills breeding in the Delta area.