Web-sourced photographic methods reveal dietary composition and shifts in resident and migrant Coracias rollers in southern Africa

Closely related species coexisting in the same niche often have dietary overlap and interspecific competition can result in resource partitioning e.g., dietary shifts, migration or phenological changes. Traditional methods to study avian diets are typically costly, difficult, and invasive. Using web-sourced photographs is an emerging, non-invasive method to study avian foraging ecology. We used this approach, along with photographic road surveys, to study prey composition of migratory European Rollers Coracias garrulus and resident Lilac-breasted Rollers Coracias caudatus where they co-occur in southern Africa. These insectivorous sister species are frequently photographed with prey in their bills because of their conspicuous perch-and-wait foraging behaviour and bright plumage coloration. We collated over 250 photographs of rollers holding prey and identified prey items representing 15 animal orders. Lilac-breasted Roller diet contained six unique orders of prey during the Austral summer (when the European Roller is present), and only one during winter (when the European Roller has migrated north). This suggests that Lilac-breasted Rollers may broaden their diet seasonally to alleviate competition with European Rollers during summer. Additionally, Lilac-breasted and European Rollers had an 84% overlap in dietary composition, potentially representing high interspecific competition - even during wet savanna summers when resource abundance is high - that might have originally driven the evolution of migratory behaviour in the European Roller. Our findings expand the current knowledge of roller diets and highlighting the growing role of social media in ecological studies.