Importance of bumblebees (Hymenoptera: Apidae: Bombus spp.) in the diet of European Bee-eaters (Merops apiaster) breeding in oceanic climate

Abstract

Breeding attempts of European Bee-eater (Merops apiaster) pairs have occasionally been documented far north of the species’ normal range, exceptionally as far as the 17 °C July isotherm. These attempts often failed—in many cases in association with adverse and fluctuating weather conditions, which are thought to lead to, amongst other effects, starvation of the nestlings. This lead to the question: what dietary conditions must be fulfilled for the adults to stay at sites characterized by a cool-wet sea climate and raise chicks successfully? Information regarding this question is very sparse in the literature, but an exceptional opportunity to investigate it arose in 2015 when ten pairs of the species arrived as classic ‘overshooters’ at the Frisian North Sea coast in northwest Germany (53°70′N, 7°88′E) and started to breed. By means of photographic documentation, 1205 prey items were identified; bumblebees (Bombus spp.) were the most abundant prey group representing 38.0% of the items and 60.2% of the total prey mass. A review of 32 European studies (north of the 21 °C July isotherm) on the species’ diet composition revealed that the proportion of bumblebees in the prey increases the more the climate approaches oceanic in character (median of the proportion for adults = 38.6%, for nestlings = 55.0%). As the climate becomes increasingly continental, bumblebees decline in their abundance and importance behind other airborne insect groups (median adults = 16.1%, nestlings = 29.4%). All in all, the following factors may well have been instrumental in successful chick rearing at the Frisian North Sea coast: (1) a manifestly high abundance of bumblebees as a basic food supply, (2) considerable insect diversity (and abundance) in the main feeding habitat, and (3) the presence of beehives to provide an emergency food supply during cold and rainy weather when no other aerial insects were flying.