Winter Ecology of the Hen Harrier, Circus cyaneus: Bridging Behavioral Insights and Conservation Requirements

The hen harrier, <i>Circus cyaneus</i>, is a migratory raptor that relies on open grasslands as key winter habitats in Central Europe, where it predominantly preys on common voles, <i>Microtus arvalis</i>. However, habitat loss due to agricultural intensification, land use change, and human disturbances increasingly threatens these landscapes. Using the framework of conservation behavior, this study examines how hen harriers respond to human-induced environmental changes, how behavior-based habitat management can support conservation, and how behavioral indicators such as territoriality inform conservation planning. Based on 278 h of field observations, we analyzed space use, foraging ecology, time and energy budgets, and territoriality of wintering hen harriers. Our results indicate that non-territorial females require ~20% more energy intake than territorial females, reflecting the advantage of defending vole-rich habitats. This corresponds to an estimated daily intake of up to 8 voles for non-territorial females, compared to 4-6 voles for territorial ones. Winter territory establishment was primarily driven by vole availability, and habitat degradation due to grassland loss impaired territory formation, influencing energy allocation and conservation prospects. Our findings highlight the role of territory establishment as a quantifiable behavioral indicator for habitat quality and conservation planning. At a landscape scale, habitat alteration and loss, anthropogenic disturbances and top-down effects further shape hen harrier distribution, emphasizing the need to account for these factors in conservation strategies. Finally, we identify open research questions, especially regarding large-scale environmental pressures and cross-seasonal effects.