Mark–recapture and playback surveys reveal a steep decline of European Storm Petrels Hydrobates pelagicus at the largest colony in western Scotland

Abstract

None of the indices of population change for seabirds in the northeast Atlantic have included European Storm Petrel Hydrobates pelagicus, because their cryptic breeding behaviour makes monitoring difficult. The aim of this paper is to present the results of a long-term project to monitor this species on Priest Island, one of the largest breeding colonies in Scotland. Mark–recapture and playback-response methods both showed a steep decline in the population size between 2001 and 2012. The population estimate at the main study site used for mark–recapture was 8,472 birds ± 1,466 (standard error) in 2001 and has declined by about 460 birds per year to 3,854 ± 437 in 2012. The adult annual survival rate was high in 2001/02 and 2002/03 but has since fluctuated between 0.73 and 0.83; these survival estimates are lower than those for the whole of Britain & Ireland estimated from ring recoveries (0.86 from 1967 to 1996, and above 0.9 since 2007/08). Recaptures of birds at three sites at varying distances from the main site used for this study indicated that only birds at the nearest site (c 90 m away) could be considered to be part of the same breeding population. Thus, the minimum distance between study sites for mark–recapture population estimates should be in excess of 100 m. The decline in population revealed by this study may be due to insufficient recruitment resulting from poor productivity, low survival to breeding age, or emigration, and emphasises the importance of future studies to monitor productivity.