Marine Ornithology

Seabirds are declining worldwide, and predation by introduced and endemic species is one threat that affects many of them. Leach's Storm Petrel Hydrobates leucorhous (hereafter LHSP) populations are declining in a number of locations for reasons that are likely multicausal, and perhaps local or regional, but relative contributions of individual causes are poorly understood. To help understand causes, we quantified predation at two Nova Scotian LHSP breeding colonies that differ in predator management: no management occurs on Bon Portage (BP) Island, whereas management is used on Country Island (CI). Transects were monitored for evidence of predation approximately biweekly on BP and CI in 2018 and 2019, during May-October on BP and May-July on CI. We did not have data on predator densities, but in both colonies, predatory density would have been very location-specific. Evidence such as LHSP carcasses, feather piles, dug up nest burrows, and gull and owl pellets was removed or destroyed during each visit. Predation of eggs and nestlings was scarcely detected. Similarly, adult predation on CI was seldom detected. We estimated that 4 000 individuals were depredated on BP in each year of this study. Approximately 42% of predation events were ascribed to gulls and 10% to owls; 48% of remains could not confidently be ascribed to a particular predator. We do not know how general these rates will be because of differences among colonies in predator communities. In any case, LHSP are listed as “Vulnerable” globally, and although up to a 5% annual loss of adults may be sustainable for a healthy population, such losses may exacerbate population declines if other factors are additive.