1. Dietary studies give vital insights into foraging behaviour, with implications for understanding changes in environmental conditions and the impacts of human activities 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.
2. 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.
3. Over three breeding seasons, we identified >24,000 prey items of at least 47 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, and resulted in a faster species accumulation rate. The prey composition collected by two researchers photo-sampling concurrently were also similar.
4. 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 diet studies and is simple to use. This methodology provides a novel tool to aid conservation and management decision-making in light of the growing need to assess environmental change and human impacts on natural ecosystems.
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