RACCOON PREDATION ON ANCIENT MURRELETS ON EAST LIMESTONE ISLAND, BRITISH-COLUMBIA

We evaluated the threat posed by introduced raccoons (Procyon lotor) t o native, burrow-nesting seabirds of the Queen Charlotte Islands, Brit ish Columbia. In 1991, eve described and quantified raccoon predation within a 14.9-ha colony of about 1,200 pairs of ancient murrelets (Syn thliboramphus antiquus). An estimated 3-4 raccoons were present on the island. We obtained radiolocations and sightings for one female and o ne male throughout the seabird breeding season, and investigated these locations for evidence of predatory activity. Eleven of 15 such inves tigations led to fresh evidence of predation on ancient murrelets, suc h as dug burrows, broken eggs, and one or more decapitated adults, sug gesting that raccoons were responsible for these types of predation si gn. The estimated predation from all sources within the seabird colony by conducting transect surveys. These covered about 17% of the colony area, and were carried out every third afternoon from 8 April to 8 Ju ne. Twenty-one surveys yielded 6 headless carcasses, 77 feather piles, 30 broken eggs, 2 dead chicks, and 20 dug burrows, representing an es timated loss of 488 +/- 110 adult and subadult birds, and 188 +/- 102 eggs and chicks within the colony (95% CL). Estimates of raccoon preda tion based on the number of headless carcasses and diggings found on t ransects were 35 +/- 35 and 71 +/- 42, however, these are considered u nderestimates because they do not account for the rapid conversion of seabird carcasses by avian scavengers, or the capture of birds abovegr ound. Monitoring of seabird remains indicated that about 92% (n = 38) of headless carcasses were consumed or converted to feather piles by a vian scavengers within <3 days, indicating that the number of carcasse s found on predation transect surveys could underestimate raccoon pred ation by as much as an order of magnitude. The suggestion that raccoon s were responsible for most seabird losses is supported by changes in the level of predation recorded between years. Predation levels were s imilar in 1990 and 1991, in the presence of raccoons, and declined by 80% in 1992, following the removal of 3 raccoons. Raccoon predation re presents a conservation threat of international significance, because the Queen Charlotte Islands contain the majority of the world's ancien t murrelet population, as well as substantial populations of several o ther burrow-nesting species that are also susceptible to raccoon preda tion.