Extensive Arctic Tern chick mortality in W and NE Iceland: influence of dietary composition and provisioning rates
Many seabirds populations across the North Atlantic have been in decline and have been subjected to breeding failures. Many of these declines have been linked to local or large scale reductions in food availability during the breeding season. Arctic Terns Sterna paradisaea in W-Iceland experienced very low breeding success in 2008-2011, primarily driven by low chick fledging success as a consequence of low growth rates and starvation. In order to explore the factors contributing to these poor conditions for chick growth and survival, we quantify the types and size composition of prey delivered to chicks in colonies across the Snaefellsnes peninsula in W-Iceland, and relate between-colony variation in prey delivery rates to chick growth and survival. However, as the main marine prey resource in W-Iceland is sandeels, which are only present in the waters of S- and W-Iceland, we also quantify prey composition, delivery rates, chick growth rates and survival in colonies in NE-Iceland, in order to compare Arctic Tern success rates in areas with differing prey communities. Prey composition and size varied greatly between regions and colonies. In W-Iceland, higher delivery rates were associated with greater fledging success, but sandeels were delivered at low rates at all colonies. In NE-Iceland, capelin replaced sandeels as the main marine prey, but delivery rates of capelin were either very low or very inconsistent, and fledging success in NE-Iceland was similarly low. Constraints on the availability of food resources for chicks therefore seem to be driving the recent very low breeding success, and understanding the causes of changing food availability will be key to reversing these trends.