Specialisation reduces foraging effort and improves breeding performance in a generalist bird

While competition is generally presumed to promote intraspecific niche diversification, populations of many apparent generalist species still exhibit considerable individual variation in foraging specialisation. This suggests that different cost-benefit trade-offs may underlie individual variation in foraging specialisation. Indeed, while specialisation may improve foraging efficiency by a better knowledge of the spatio-temporal availability of resources, individuals may also become more vulnerable to fluctuations in these resources. In this study, we used multi-year GPS tracking data of 19 Herring Gulls (Larus argentatus) breeding along the Belgian coast to assess whether foraging effort and reproductive success varied among different levels of foraging specialisation. First, we quantified spatial and habitat specialisation during incubation and chick-rearing for 31 individual breeding cycles during which birds raised young until the age of 21 days. Next, we tested whether spatial and habitat specialisation were related to the daily distance covered (as a proxy for foraging effort), and to chick growth (as a proxy for reproductive success). We found that birds primarily varied in their extent of habitat specialisation. Habitat specialisation was associated with reduced daily distances covered and increased offspring growth rates, in particular the growth rate of the youngest chicks. Yet, positive effects of habitat specialisation on chick growth decreased at high levels of spatial specialisation. Our results thus demonstrate fitness benefits of foraging specialisation during our five-year study period, but also highlight the need for longer-term studies as environmental changes may cause benefits to vary throughout a lifetime.