Strong cascading effect of weather conditions on prey availability and annual breeding performance in European bee-eaters Merops apiaster

Abstract

Aerial insectivorous birds depend highly on favourable weather conditions for successful foraging because flight activity of insects is constrained by daily weather. Thus, the variation in weather conditions during reproduction, mediated by prey limitations, should be mirrored in annual reproduction performance, and finally in annual breeding success. We analysed the effect of local weather conditions on the availability of airborne insects and on the variation in brood size and nestling condition of European bee-eaters Merops apiaster at the northern edge of their range where years with adverse weather frequently occur. The availability of large flying insects, the common prey of bee-eaters, increased with air temperatures and duration of daily sunshine. As predicted, local weather conditions affected reproductive performance with annual breeding success (mean 3.7 nestlings per breeding pair, range 1.7–4.9 nestlings) being up to 32 % higher in extraordinary dry and hot summers. Additionally, a nestling's body condition (residual mass) was also affected by sunshine duration during their growth period and internally was co-affected by the number of siblings and the individual rank within the sibling hierarchy. Thus, a prolonged duration of daily sunshine causes a cascade from higher insect flight activity, and, thus, higher food availability for chick-rearing bee-eaters, which finally translates into better chick body conditions and higher annual breeding success. Consequently reproduction and population development of European bee-eaters might be especially susceptible to regional changes in weather and climatic conditions.