Breeding biology of the White-faced Storm-petrel on Selvagem Grande Island, North-east Atlantic

The White-faced Storm-petrel (Pelagodroma marina) breeds on several tropica l, sub-tropical and temperate islands in both hemispheres, but some aspects of its breeding biology are still poorly known. The European subspecies hy poleuca is almost confined to a small archipelago, the Salvage Islands,:abo ut 300 km south of Madeira Island, North-east Atlantic. Because of its very localized distribution, this subspecies is relatively vulnerable to extinc tion and its population dynamics need to be understood and monitored. We st udied the breeding biology of the White-faced Storm-petrel at Selvagem Gran de Island in 1996. Birds first arrived at the colony in mid-December, eggs were laid from mid-March to early June, and the last chicks fledged in mid- August. Mean incubation period was 53.7 days, but was highly variable due t o frequent egg-neglect that affected at least two thirds of the clutches. M ean nestling period was 60.3 days. Chick growth (body mass and wing length) is described. Hatching success was 60.7% (N = 89) and fledging success was 88.9% (N = 54). Breeding success (53.9%) was not low, in spite of frequent predation by house mice (Mus musculus), the main cause of nesting failures . Rabbits (Oryctolagus cunniculus) and Yellow-legged Gulls (Larus cachinnan s), two species thought to pose threats to other seabirds of the region, ha d no measurable negative effect on breeding performance at our study nests. However, we had some evidence that gulls prey upon chicks and adult birds. Breeding success did not correlate with egg size. There was a highly signi ficant seasonal decline in both hatching and fledging success. Using data o n burrow density and occupancy obtained in the main colony, we estimated th e breeding population on Selvagem Grande at around 36,000 pairs.