Rooftop Breeding Does Not Consistently Explain Variation in Yolk Testosterone in Herring Gulls

In oviparous species, females shape offspring phenotype during early development by varying the resources they deposit in the egg prior to laying. Yolk androgen levels, for example, vary within and among females and their deposition is influenced by maternal state, as well as environmental conditions, including the social environment. By providing artificial breeding sites, such as rooftops, urbanisation has created novel breeding contexts for Herring gulls, which, however, differ from a traditional ground-breeding context in several ecological characteristics that may influence yolk androgen deposition. Here, we compared yolk testosterone concentrations in the last-laid egg of rooftop- and ground-breeding Herring gulls across two breeding seasons and tested whether variation was associated with social environment (approximated by nest density) or maternal quality differences (reflected by egg volume, laying date of the last egg and diet composition). Rooftop-breeding pairs showed higher yolk testosterone concentrations than ground-breeding pairs in 2023, while no difference was observed in 2024. Ground-breeding pairs also exhibited significant interannual variation, whereas rooftop pairs did not. Neither nest density nor any proxy of maternal quality explained variation in yolk testosterone levels. These results indicate that yolk testosterone allocation in Herring gulls is variable but not consistently explained by breeding context, social environment or maternal state.