Data from: Dietary studies in birds: testing a non-invasive method using digital photography in seabirds

Dietary studies give vital insights into foraging behaviour, with
implications for understanding changing environmental conditions and the
anthropogenic impacts on natural resources. Traditional diet sampling
methods may be invasive or subject to biases, so developing non-invasive
and unbiased methods applicable to a diversity of species is essential. We
used digital photography to investigate the diet fed to chicks of a
prey-carrying seabird and compared our approach (photo-sampling) to a
traditional method (regurgitations) for the greater crested tern
Thalasseus bergii. Over three breeding seasons, we identified >24
000 prey items of at least 48 different species, more than doubling the
known diversity of prey taken by this population of terns. We present a
method to estimate the length of the main prey species (anchovy Engraulis
encrasicolus) from photographs, with an accuracy <1 mm and
precision ~ 0·5 mm. Compared to regurgitations at two colonies,
photo-sampling produced similar estimates of prey composition and size, at
a faster species accumulation rate. The prey compositions collected by two
researchers photo-sampling concurrently were also similar. Photo-sampling
offers a non-invasive tool to accurately and efficiently investigate the
diet composition and prey size of prey-carrying birds. It reduces biases
associated with observer-based studies and is simple to use. This
methodology provides a novel tool to aid conservation and management
decision-making in the light of the growing need to assess environmental
and anthropogenic change in natural ecosystems.