Stable-carbon and nitrogen isotope ratios of songbird feathers grown in two terrestrial biomes: Implications for evaluating trophic relationships and breeding origins

The measurement of stable-carbon (C-13/C-12) and nitrogen (N-15/N-14) isotope ratios in bird feathers has potential to reveal important information on trophic position and source of feeding during feather formation. This study investigated how stable-nitrogen (delta(15)N) and carbon (delta(13)C) isotope ratios in feathers of insectivorous songbirds and in their supporting foodwebs differed between boreal forest sites (50 feathers representing 5 species) and upland sites associated with southern prairie agro-wetland complexes in Saskatchewan and Manitoba, Canada (108 feathers representing 12 species). No difference in feather delta(13)C values between these biomes was found, but feathers from boreal forest songbirds were depleted in N-15 compared to feathers from birds in southern agricultural areas. This suggests that general trophic models cannot be constructed based on delta(15)N values of feathers of birds from these different biomes alone unless they are related first to local foodweb isotopic signatures. However, foodweb delta(15)N analysis in each biome indicated that the stable isotope approach could be applied successfully to delineate trophic relationships involving birds. In addition, this technique will be a useful tool for investigating the degree of frugivory or nectarivory in birds.