Analysis of Prey Remains Excavated from an Historic Bald Eagle Nest Site on San Miguel Island, California

This paper reconstructs the food habits of bald eagles (Haliaeetus leucocephalus) that nested historically on San Miguel Island by analyzing prey remains recovered from an historic nest site. Of the 9,946 faunal elements recovered, 9,743 (98%) were from vertebrates and 203 (2.0%) were from invertebrates. Based on minimum number of individuals, the percent diet composition for this sample was 61.2% birds, 18.6% fish, 13.6% invertebrates, 5.2% mammals, and 1.4% reptiles. Remains of 297 birds (45–48 species), 90 fish (four species, two genera and seven families), seven reptiles (one species), 25 mammals (seven species), and 66 invertebrates (23 species, three genera and three families) were identified. Domestic sheep (Ovis aries) comprised most (43.8%) of the other vertebrate remains with small numbers of three species of newborn pinnipeds (California sea lion [Zalophus californianus], northern fur seal [Callorhinus ursinus], and northern elephant seal [Mirounga angustirostris]) and three species of terrestrial mammals (island fox [Urocyon littoralis], deer mouse [Peromyscus maniculatus], and black rat [Rattus rattus]) also recorded. Marine gastropods (39 individuals of 14 species) and bivalves (15 individuals of seven species) composed 81.8% of the invertebrate prey remains but only represented 11.1% by MNI and 5.6% by weight of the eagle’s nesting season diet. Three sensitive wildlife species (island fox, ashy storm-petrel [Oceanodroma homochroa], and Xantus’s murrelet [Synthliboramphus hypoleucus]) were identified in the sample; all were incidental to the eagle’s overall diet. Data obtained from this study clearly indicate that bald eagles nesting historically at the northwest end of San Miguel Island exploited a wide variety of marine and terrestrial foods during the nesting season with marine birds and near-shore marine fish being the most important prey categories. This study shows that it is possible to examine the food habits of an extirpated population of eagles by analyzing prey remains excavated from an historic nest site.