Variation of parental and chick diet in opportunistic insectivorous European Bee-eaters

Insectivorous Palaearctic bird species associated with open habitats rely on high prey abundances, which are currently declining due to habitat loss and intensive agricultural practices. The European Bee-eater (Merops apiaster) is an opportunistic insectivore of open habitats, preying mainly on medium to large-sized flying insects. Its diet composition received some attention in the past, but the current variation in diet composition of birds breeding across different habitats, and between adults and chicks remains poorly known. In this study, we determine variation in bee-eaters’ diet in colonies located in five common habitats at the Iberian Peninsula. We also assess differences in the diet composition of chicks and adults and investigate seasonal diet selectivity of adults. Finally, we explore the variability in the size of prey provided to chicks throughout their growth period. Hymenoptera and Coleoptera were the most important groups for bee-eaters, with adults and chicks consuming 58.8% and 64.1% of hymenopterans and 37.6% and 28.6% of coleopterans, respectively. The proportion of Hymenoptera (42.3–55.7%) and Coleoptera (43.3–53.5%) in the diet was similar in colonies in pasture and oak habitats, but Hymenoptera dominated (83.8% and 95.7%) in meadow and mixed forest colonies. Despite being a generally opportunistic predator, adult bee-eaters provide their progeny with an increasing proportion of larger insects through chick development. Moreover, they equally take Hymenoptera and Coleoptera for themselves and their chicks, even when the abundance of these insects decreases seasonally. Overall, these results suggest that local prey availability associated with specific habitats influences diet composition and that regional declines in certain groups may, therefore, affect insectivore species differently according to their dietary and habitat preferences.