Targeting efforts in rescue programmes mitigating light-induced seabird mortality: First the fat,then the skinny

Oceanic conditions determine food availability to seabirds and affect seabird reproductive parameters, such as breeding success, chick growth, and survival rates. In seabirds, juvenile survival at sea is positively correlated with body condition at fledging. In addition, in several seabird species, especially petrels and shearwaters (Order Procellariiformes), fledglings are disoriented by artificial lights during their maiden flights from their nests to the sea, and many of them fall on the ground and are rescued by volunteers to mitigate light-induced mortality. We studied variations in body condition and body mass in Cory’s Shearwater (Calonectris borealis) fledglings on Faial Island (Azores), using data from rescue campaigns conducted over 15 consecutive years. We checked if body condition was related to oceanic conditions. Late fledglings were in poorer body condition than early ones. Significant inter-annual variations in fledging body condition were observed. These were not related to North Atlantic Oscillation fluctuations. However, annual mean fledgling body condition was positively correlated with sea surface temperatures measured in the autumn of the previous year in a northern feeding area used by adults throughout the breeding season. This study broadens our knowledge of the factors affecting fledgling body condition in Procellariiformes and provides advice to better manage the rescue campaigns. Optimal management of rescue campaigns is essential given the limited economic or human resources allocated to such an aim.