Predominantly terrestrial foraging and reproductive gains from a high trophic level diet in roof-nesting Herring Gulls (<em>Larus argentatus</em>)

Wild animal species often use human-modified environments for foraging and reproduction, but this may require dietary diversification with fitness consequences. The extent to which colonising species successfully exploit such habitats is poorly understood. We used stable isotope analysis of egg yolk to investigate the association between foraging choices and reproductive success in 102 female Herring Gulls (Larus argentatus) over 3 years in a roof-nesting, pericoastal breeding colony. Stable isotopes of egg yolk predominantly reflect maternal diet during egg production. We measured δ13C as an indicator of foraging habitat, and δ15N as an indicator of trophic level. We predicted diverse foraging choices across marine, terrestrial, and urban environments due to gulls’ generalist foraging strategy and the variety of nearby foraging opportunities. We also predicted higher reproductive success associated with marine feeding compared to terrestrial feeding or feeding on human food and refuse, because marine food has historically been gulls’ natural food type and has been previously associated with greater reproductive success. Surprisingly, δ13C values indicated predominantly terrestrial foraging for egg production. Egg mass increased significantly with lower δ13C, indicative of more terrestrial feeding. These findings may reflect the availability of habitats and foods nearby or indicate adaptive dietary choices. Fledging success increased significantly with elevated δ15N, indicating that mothers feeding at higher trophic levels before laying produced higher quality eggs and/or had superior offspring-rearing capacity. A high trophic level maternal diet may nutritionally benefit offspring or improve parental condition. Egg stable isotope ratios of δ13C and δ15N were highly repeatable within clutches, enabling us to predict stable isotope values of unsampled eggs from sampled sibling eggs. Our results highlight high usage of terrestrial foods for egg production, whereas marine and anthropogenic feeding were rare. The reasons for this preference warrant further investigation to advance understanding of species that use human-modified environments.