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.

'id_code' = unique photograph identifier, 'age' = sparrowhawk age (categorical; 1 level), 'sex' = sparrowhawk sex (categorical; 2 levels), 'prey_spp' = prey species identified by authors and voluntary ornithologists (see Methods; categorical; multiple levels), 'prey_mass' = mean prey weight (g) for adult prey species derived from Robinson (2005; https://www.bto.org/birdfacts accessed on May 25 2021), 'prey size' = prey size classes (categorical; 3 levels), 'month' = month of the year (categorical; 12 levels) and 'season' = season of the year (categorical; 4 levels).