Feather nitrogen and carbon stable isotope (d15N and d13C) values for Golden-crowned Sparrows

Differential foraging by sex can have important implications for understanding the ecology of a species. This can be especially difficult to study through observations alone in sexually monomorphic species, such as the Golden-crowned Sparrow (Zonotrichia atricapilla), and for species in remote areas. We used nitrogen and carbon stable isotope analysis to determine the relative trophic position between the sexes for 73 individuals for this migrant songbird with little known diet information while breeding in remote locations of Alaska and northwestern Canada. 

We found that females had significantly higher values of feather δ15N (mean 5.44 ‰; mean for males 4.54 ‰) after accounting for year and feather collection location, and in a subset of GPS-tagged birds with known breeding locations, after accounting for year, breeding latitude, elevation, or distance to shoreline. We infer that females may be foraging more items from a higher trophic level than males on breeding grounds, which may reflect a physiological need to replace lost nutrients from nesting. 

Methods
We collected feathers in the months of October-March from individuals at five study sites across the Golden-crowned Sparrow wintering range (western regions of Canada and the United States) from 2019-2021. We collected 8-10 contour feathers for genetic sexing and two outer tail feathers (rectrices) for stable isotope analysis. Due to funding limitations, we were only able to genetically determine sex for a subset of the birds with stable isotope information, leaving some birds with stable isotope values but unknown sex. We used wing chord values to sex an additional 13 birds (see paper for additional details), for a total of 73 individuals with stable isotope values and an assigned sex. Feathers were processed at the Cornell Isotope Laboratory at Cornell University, Ithaca, NY, USA.