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.

DATA ARCHIVE ANCHOVY STANDARD LENGTHData include: 1) The measurements of the anchovy used for the allometric regressions (training set) 2) The measurements of the anchovy in the test set. 3) The measurements of the anchovy measured from photos and regurgitates at Robben Island and from Seal Island.DATA ARCHIVE PHOTO-SAMPLE & REGURGITATIONSThis file include: The ordered list of species in the photo samples from Observer A and B and Seal Island and the ordered list of species in the regurgitation in both Robben Island and Seal Island.List of species recordedList of species recorded using photo-sampling method in Robben Island (2013-2015)