A survey of Leach’s Oceanodroma leucorhoa and European Storm-petrel Hydrobates pelagicus populations on North Rona and Sula Sgeir, Western Isles, Scotland, in 2009

In 2001, surveys of both Leach’s Oceanodroma leucorhoa and European Storm-petrel Hydrobates pelagicus were made on North Rona and Sula Sgeir, Western Isles, Scotland, using tape playback for the first time. North Rona held 1,133 apparently occupied sites (AOS) of Leach’s Storm-petrel (1,084 AOS after re-analysis of the 2001 data in 2009) and 371 AOS of European Storm-petrel; numbers on Sula Sgeir were five and eight AOS respectively. A repeat survey in 2009 using the same methods found 713 AOS of Leach’s Storm-petrel on North Rona, a decline of about 34% since 2001, and none on Sula Sgeir. A partial survey of European Storm-petrel on North Rona found 313 AOS, suggesting no significant population change since 2001; limited time on Sula Sgeir prevented a survey there for this species. Great Skuas Stercorarius skua and Great Black-backed Gulls Larus marinus breed on North Rona and take an unknown number of storm-petrels each year, but it has yet to demonstrated whether either or both together have caused the decline in Leach’s Storm-petrel. The timing of the decline in Leach’s Storm- petrels can only be dated from 2001, because earlier population estimates are not comparable with the tape playback surveys of 2001 and 2009.