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

Abstract

The hen harrier, Circus cyaneus, is a migratory raptor that relies on open grasslands as key winter habitats in Central Europe, where it predominantly preys on common voles, Microtus arvalis. 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.

Keywords: Circus cyaneus; behavior-based management; behavioral indicators; conservation behavior; hen harrier; land use change; territoriality; winter ecology.

Figure 1

Locations of the main study areas (black triangles) in the Austrian provinces of Carinthia (Glantal and Krappfeld) and Burgenland (Parndorf Plain).

Figure 2

Proportional duration of the behavioral category sitting and the pooled flying categories during daylight hours: (a) Territorial females allocated a significantly greater proportion of time to sitting and (b) exhibited reduced flight activity compared to males and non-territorial females, despite the necessity of territory defense against intruders. This underscores the effect of vole-rich territories, where reduced hunting requirements allow for a shift in time budgets. The box indicates the standard deviation, the vertical line represents the median, and the red dot marks the mean.

Figure 3

The establishment of hen harrier territories in Central Europe was strongly driven by vole availability, measured as an abundance index per ha. The difference between areas with and without territorial hen harriers was statistically significant.

Figure 4

Change in forage crop proportion (%) on two inner-alpine study plots. Within just three and five years, respectively, the huntable areas for hen harriers (shown in green) on both study plots declined by approximately two-thirds. In the early years, hen harrier territories were established; however, in the final year, only non-territorial individuals remained, correlating with the low proportion of meadows.

Figure 4

Change in forage crop proportion (%) on two inner-alpine study plots. Within just three and five years, respectively, the huntable areas for hen harriers (shown in green) on both study plots declined by approximately two-thirds. In the early years, hen harrier territories were established; however, in the final year, only non-territorial individuals remained, correlating with the low proportion of meadows.