Using web-sourced photographs to examine temporal patterns in sex-specific diet of a highly sexually dimorphic raptor

Traditional methods to study raptor diet are usually limited temporally, e.g., prey remains at nesting sites, and are unsuitable to examine dietary changes throughout the year. Using web-sourced photography, we explore temporal patterns in prey size and key prey species between sexes of the sexually dimorphic Eurasian Sparrowhawk (Accipiter nisus) throughout the United Kingdom. We examined 666 photographs of sparrowhawk on prey and identified the prey species involved, together with sparrowhawk sex. Changes in prey size and proportions of key prey species over time (seasonally and monthly) were explored for each sex. Prey weight was substantially higher for females than males. However, on average, prey size for both sexes declined during the summer period (May-June) being the lowest in June which is the main nestling-rearing month for both sparrowhawks and their prey. Compared to summer,  Rock Doves (Columba livia) were more important prey for female sparrowhawk in winter. Whereas, for males, Eurasian Blackbirds (Turdus merula) were more important in spring compared to autumn. Web-sourced photography can overcome several limitations of previous methods used to study raptor diet including the ability to quantify diet between the sexes throughout the entire year, however, may also introduce a prey-size bias towards larger prey items.