Playback re-survey and demographic modelling indicate a substantial increase in breeding European Storm-petrels Hydrobates pelagicus at the largest UK colony, Mousa, Shetland

Abstract

The island of Mousa, Shetland, is designated as part of the European Natura 2000 reserve network as a Special Protection Area (SPA) for breeding European Storm- petrels Hydrobates pelagicus. In 1996 the population was censused using playback methods and reported as 6,800 (95% CI 4,800–8,800) apparently occupied sites (AOS), c. 26% of the UK breeding population. Re-examination of the 1996 data highlighted some calculation inaccuracies, resulting in a reduced population estimate for 1996 of 5,410 AOS (95% CI 3,932–7,022). Resurvey of the colony in 2008 using identical methods revealed a 118% increase to 11,781 AOS (95% CI 8,100–17,728). A sample of c. 90 nest sites in natural crevices has been monitored annually since 1990 to determine breeding productivity. A deterministic population model, incorporating observed annual productivity estimates, annual survival rates and age of first breeding, also indicates a population increase of similar magnitude (95%) between 1996 and 2008. The similarity in trend estimates from the two independent approaches is perhaps surprising, given the large potential sources of error associated with each, and suggests that both the playback survey method and productivity monitoring from a single annual visit provide reliable demographic estimates. The large increase in population size of European Storm- petrels contrasts starkly with the trends of many other seabird species in the region, and likely results from aspects of Procellariiform foraging and reproductive strategies that confer resilience to unpredictable and scarce food supplies.