Within-colony predation of Leach's Storm-petrels (Hydrobates leucorhous)
LE3 .A278 2021
Master of Science
Predation at breeding colony islands threatens half of all seabird species. Although islands provide some refuge from predators, their remote nature may foster high population densities and fluctuating food availability for rodents, possibly leading to deviations from herbivory to meet energy demands. Leach’s Storm-petrel Hydrobates leucorhous (LHSP) populations are declining for reasons that are likely multicausal, and perhaps local or regional, but relative contributions of individual causes are poorly understood. To help understand causes, I used transect surveys for LHSP remains to quantify predation at two NovaScotian LHSP breeding colonies that differ in predator management: Bon Portage (BP) Island (no management), and Country Island (CI; management used). Predation of LHSP eggs and nestlings on each island and adult predation on CI were scarcely detected. However, I estimated 4000 adult-plumaged LHSP were depredated on BP annually. We do not know how general these rates are because of differences among colonies in predator communities. Meadow voles (Microtus pennsylvanicus) have been suspected of depredating LHSP nests on CI. I used stable isotope analysis of vole faeces and fur, vegetation, and LHSP tissues from each island to examine whether vole diet deviated from herbivory. Stable isotope values in vole fur and faeces may indicate omnivory, especially on CI; however, high δ15N values may also be reflective of the influence of guano on vegetation and invertebrates in vole diets. Sampling of more food sources and mainland controls are needed to get a better picture of vole diets and determine whether they are consuming LHSP eggs or nestlings. LHSP are listed as “Vulnerable” globally, and although up to a 5% annual loss of adults and occasional nest depredation by voles may be sustainable for a healthy population, such losses may exacerbate population declines if other factors are additive.
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