# Nitrogen Isotopes Suggest Sex‐Based Diet Differences on the Breeding Grounds for a Sexually Monomorphic Migratory Passerine

## TL;DR

This study shows that female Golden-crowned Sparrows eat higher trophic level food than males during breeding, possibly to replace nutrients lost from nesting.

## Contribution

The study reveals sex-based diet differences in a sexually monomorphic migratory bird using nitrogen isotope analysis.

## Key findings

- Females had significantly higher feather δ15N values than males, suggesting higher trophic level foraging.
- No sex-based differences were found in feather δ13C, indicating similar habitat use.
- Feather δ15N values increased by 0.3‰ annually, possibly reflecting increasing water stress.

## Abstract

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 individual Golden‐crowned Sparrows, a migrant songbird species with little known diet information from remote breeding locations of Alaska and northwestern Canada. We found no evidence of differences in feather δ13C between the sexes suggesting similar habitat use, but we found an average 0.3‰ increase each year that may indicate increasingly water stressed habitats. We found that females had significantly higher values of feather δ15N (mean 5.4‰; mean for males 4.5‰) 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, and distance to shoreline. We infer that females may be foraging on more food items from a higher trophic level than males on breeding grounds, which may reflect a physiological need to replace lost nutrients from nesting. If females rely on insects during the breeding season, then their success will be tied to insect populations which are generally experiencing large declines. Additionally, we provide mass and wing chord measurements from genetically sexed individuals to add to currently low published sample sizes for this monomorphic species.

Differential foraging by sex can have important implications for understanding the ecology of a species. Using stable isotope values from feathers, we found evidence that female Golden‐crowned Sparrows are eating at a higher trophic level than males during the breeding season molt, which may reflect a physiological need to replace lost nutrients from nesting.