Data from: 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.