Dietary segregation between two cohabiting species of sparrows revealed with stable isotope analysis

Fox Sparrows (Passerella iliaca (Merrem, 1786)) and Song Sparrows (Melospiza melodia (A. Wilson, 1810)) cohabit on many islands along the Pacific coast of North America, and previous studies suggest that they rely on similar prey types. We used delta C-13 and delta N-15 stable isotope analysis on blood collected from breeding adults of both species in each of two habitats on Triangle Island, British Columbia, Canada, to test the hypothesis that the two species exhibit a consistent pattern (direction) of dietary segregation in different habitat types. Both delta C-13 and especially delta N-15 values differed between habitats, indicating that the two habitats were isotopically distinct. As predicted, delta N-15 values differed consistently between the two species in the two habitats, averaging similar to 1.5% higher in the smaller Song Sparrow than in the larger Fox Sparrow in both. We infer that Song Sparrows included more animal matter and less plant matter in their diets than Fox Sparrows, and suggest that fixed traits related to body size might underlie the dietary (trophic) differences. In contrast, delta C-13 values did not differ between species. We conclude that dietary segregation could help to facilitate the widespread cohabitation of these two species of sparrows.