Tracing salmon nutrients in riparian food webs: isotopic evidence in a ground-foraging passerine

The predictable annual spawning of anadromous salmon (genus Oncorhynchus Suckley, 1861) provides an important marine resource subsidy to terrestrial species throughout the North Pacific. Using isotopic ratios of nitrogen (delta N-15) and carbon (delta C-13), we examine pathways of salmon nutrient uptake by a ground-foraging passerine, the winter wren (Troglodytes troglodytes (L., 1758)), captured above and below waterfall barriers to salmon migration from two rivers on the central coast of British Columbia. Wren feathers exhibited substantial variation in delta N-15 (range = 18.6%) and delta C-13 (range = 7.9%), approximating the isotopic range of invertebrates sampled at our study sites. Mean delta N-15 values of feathers and feces representing the fall diet of winter wrens were enriched by 4.8 parts per thousand and 8.0 parts per thousand, respectively, below versus above the waterfall barriers. We observed positive relationships between delta N-15 and delta C-13 in fall-grown feathers and fecal samples below the falls but not above the falls, and large positive shifts (as high as 14.3 parts per thousand) in N-15 from summer to fall within individual birds. The large isotopic variation observed in this system likely can be explained by the mobility of wrens, isotopic variation in their prey, and individual variation in the consumption of fly larvae from salmon carcasses. Our results identify both direct and indirect pathways by which salmon-derived nutrients are cycled into top-level consumers.