Where to settle in a rapidly expanding bird colony: A case study on colony expansion in High Arctic breeding geese

As colonies fill up with more individuals, areas of preferred nesting habitat can become scarce. Individuals attracted to the colony by the presence of conspecifics may then occupy nest sites with different habitat characteristics to that of established breeders and, as a result, experience lower nesting success. We studied a rapidly growing colony of Svalbard pink-footed geese Anser brachyrhynchus to determine any such changes in nest site characteristics and nesting success of newly used nest locations. Svalbard pink-footed geese are a long-livedmigratory species that breeds during the short Arctic summer and whose population has doubled since the early 2000s to c. 80,000. From 2003 to 2012, nest numbers increased over fivefold, from 49 to 226, with the majority (range 57-82 %) established within 30m of another nest (total range 1-164 m). Most nests, particularly during the early stages of colony growth, shared common features associated with better protection against predation and closer proximity to food resources; two factors thought key in the evolution of colony formation. As nest numbers within the colony increased, new nests occupied locations where visibility from the nest was restricted and foraging areas were further away. Despite these changes in nest site characteristics, the nesting success of geese using new sites was not lower than that of birds using older nests. Hence, we propose that nesting in dense aggregations may offset any effects of suboptimal nest site characteristics on nesting success via the presence ofmore adults and the resultant increased vigilance towards predators.