Weather conditions determine reproductive success of a ground-nesting bird of prey in natural dune grasslands

Abstract

The Short-eared Owl (Asioflammeus) inhabits open grasslands and belongs to the ground-breeding birds that have experienced the most severe declines during recent decades. Here, we studied nest and fledgling survival of the owl species in relation to (i) habitat composition, (ii) vegetation structure, (iii) weather conditions and (iv) vole abundance. The study was conducted on the East Frisian Island of Spiekeroog (southern North Sea, Lower Saxony, Germany), which harbours one of the last remaining permanent populations of the species in Central Europe. With a mean hatching success of 5.6 young per nest (N = 34) and an average probability of nest survival of 0.9 (N = 28), values ascertained in this study exceeded those reported in previous research. We attribute this to the special environmental conditions on the island, i.e. (i) the absence of mammalian mesopredators such as the Red Fox (Vulpesvulpes), (ii) nearly no disturbance through agricultural measures and (iii) low level of human disturbance due to legal regulations of the National Park. By contrast, the survival of fledglings was lower than survival of nests and varied considerably between the investigated years. Weather conditions were the key driver of fledgling survival. Maximum wind speed and sunshine duration had a negative effect on the probability that chicks successfully fledged. Both lead to reduced hunting success, and the former is also associated with increased costs for thermoregulation. Consequently, increasing frequency of extreme weather events caused by climate change could negatively impact the breeding success of vole-dependent raptors, such as the Short-eared Owl.