Assessing temporal and spatial variation in the diet of the Common Kestrel (Falco tinnunculus) using web-sourced photography

Food serves as the fundamental source of energy for animal survival and reproduction, directly shaping individual fitness and population dynamics, and has therefore long been a central focus in animal ecology. Traditional approaches to studying wildlife diet, including direct observation, pellet analysis, and camera monitoring, demand intensive field sampling and specialized expertise, and are generally confined to the breeding or wintering seasons. In recent years, emerging techniques such as stable isotope analysis and DNA barcoding have expanded dietary research. However, large-scale spatiotemporal investigations remain scarce. With the rise of citizen science, analyses based on web-sourced photographic data now offer novel opportunities for advancing dietary studies. Here, we aimed to: (1) quantify seasonal and latitudinal variation in diet composition; and (2) examine seasonal and latitudinal variation in prey size in the Common Kestrel (Falco tinnunculus). To address these aims, we systematically collected 80, 158 photographs of the Common Kestrel worldwide from 1973 to April 2025 via the Macaulay Library. We identified 1708 independent predation events, of which 1693 were retained for detailed analysis. Kestrels primarily preyed on mammals (36.4%) but also consumed invertebrates (14.9%), reptiles (14.6%), birds (11.9%), and, rarely, amphibians (0.2%), with 21.9% of prey remaining unidentified. Dietary composition exhibited significant variation across both seasons and latitudes: mammals contributed a higher proportion of the diet in winter and at higher latitudes, whereas predation on invertebrates increased in autumn and at lower latitudes. We found no significant seasonal variation in prey size, and no overall latitudinal trend. However, mammal prey size decreased significantly with increasing latitude. These findings provide the first large-scale quantification of spatiotemporal variation in the diet of the Common Kestrel, offering new insights into its feeding ecology and adaptive foraging strategies. Furthermore, this work highlights the value and applicability of citizen science image data for advancing raptor dietary research at continental scales.