Simple Summary
Hen Harriers hunt small mammals in open farmland during winter, when food is often scarce and weather conditions can be harsh. To understand how biological and environmental factors shape their hunting performance, we studied birds in Austria over five winters and analyzed more than 1800 hunting attempts. We compared males, territorial females, and non-territorial females. Males, which are smaller and more agile, achieved the highest success per hunting attempt. Territorial females, which defend vole-rich patches, made more hunting attempts per unit time but did not differ from males in the probability of a successful strike. Non-territorial females, lacking both the agility of males and the habitat access of territorial females, showed the lowest hunting success. Adult males tended to be more successful than other age–sex groups, although the effect was not statistically significant. Among weather factors, success increased under cloud cover, decreased with stronger wind, and was unaffected by temperature. These results show that both social status and weather conditions determine hunting success in wintering Hen Harriers. Males succeed through agility, territorial females through resource control, while non-territorial females remain most limited. Conserving open farmland together with small-mammal-rich core areas is crucial to support the different winter strategies of this raptor.
Abstract
Raptors wintering in temperate regions face strong energetic constraints due to low temperatures and limited prey availability. Understanding how intrinsic traits and environmental conditions shape hunting performance helps to clarify the mechanisms underlying winter foraging efficiency. We studied wintering Hen Harriers (Circus cyaneus) in Austria over five consecutive winters (2020/21–2024/25) to quantify touchdown attempt rate and outcome in relation to sex, age, territorial status, and weather. Using generalized linear mixed models, we analyzed 1829 recorded touchdowns with individual identity as a random effect. Territorial females showed slightly higher attempt rates than males, whereas non-territorial females exhibited a tendency toward lower touchdown success. Adult males achieved the highest per-attempt efficiency, but age alone had no significant effect. Touchdown outcomes improved under cloud cover and declined with wind speed, while temperature showed no influence. Attempt rate was unaffected by any environmental variable. These results demonstrate that both intrinsic and extrinsic factors jointly determine winter foraging performance: males, with their smaller body size and agility, can persist even in vole-poor habitats by compensating through efficient flight and prey capture; territorial females benefit from stable access to vole-rich patches; and non-territorial females remain constrained by competition and limited access to prey. The findings highlight the need to maintain open farmland and vole-rich habitat to support diverse overwintering strategies.