Census of European Storm-petrels Hydrobates pelagicus on Skomer Island

Comprehensive tape playback surveys were carried out in 2003 and 2004 in order to provide repeatable estimates of the number of European Storm-petrels Hydrobates pelagicus breeding on Skomer Island. This included the measurement of response-rates by repeated visits to two sub-colonies, one each year. In 2003, the cumulative number of Apparently Occupied Sites was still increasing on the 11th and final visit, and so the data were extrapolated to predict an asymptote. The 2004 calibration plot levelled out after the 7th visit (probably due to the consistent use of a more effective tape) but the discovery of a new site on the 13th visit resulted in a slightly higher predicted asymptote than that was found. The respective response-rates of 0.27 and 0.44 were significantly lower than previously thought, and thus give higher population estimates when responses were adjusted by the reciprocal correction factors. The Skomer European Storm-petrel population is now thought to be in excess of 300 AOS, and is becoming a significant component of the Skomer and Skokholm SPA (now the population on the latter is apparently declining). Tape quality has a significant effect on the number of responses elicited, which has important implications for future surveys.