Seasonal ecology and the conservation of hen harriers (Circus cyaneus) in Ireland

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McCarthy, Alan.
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University College Cork
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The Hen Harrier (Circus cyaneus) is an Annex-I listed species under the EU Birds Directive (Directive 2009/147/EC) whose breeding populations have declined in recent decades across its Western European range. The species is of conservation concern in Ireland, where it is on the Amber list of Birds of Conservation Concern. The population of the Republic of Ireland was most recently surveyed in 2015 and 108-157 breeding pairs were recorded, representing a 15.6% decline over the preceding five-year period. Previous research and conservation efforts have primarily focussed on their breeding ecology, where they experience clear human-induced pressures on the breeding grounds. However, we have a poor understanding of Hen Harrier ecology and the pressures that they face outside of the breeding season. A year-round understanding of their ecology is crucial to develop effective conservation strategies for Hen Harriers. By gathering new information on the movement, survival, and habitat use of birds throughout the annual cycle, we can better understand the pressures they face and whether their ecological requirements are being met. The aim of this PhD was to address knowledge gaps in both the breeding and non-breeding ecology of Hen Harriers in Ireland, and to develop a suite of conservation recommendations to inform the development of appropriate future conservation management actions. This PhD research was conducted in Ireland between October 2017 and March 2022 using a range of methods, including satellite tracking, vantage point roost watches, pellet analysis, camera trapping, bird point counts and small mammal trapping. Analysis of Hen Harrier satellite tracking data revealed new insights into the juvenile dispersal period, during which they experienced high levels of mortality. Juvenile dispersal typically involved a sudden, long-distance (>25km) initial movement away from the natal area, followed by exploratory movements around Temporary Settlement Areas (TSAs). Arable and lowland bogs were important habitats for juveniles at that time. The use of protected areas throughout the juvenile dispersal period and into winter was low as birds dispersed away from the breeding Special Protection Area (SPA) network. This shows that the current network of protected areas for Hen Harriers are insufficient for their year-round protection. Winter roosts represent an important focal point of Hen Harrier distribution and behaviour during the non-breeding season. The habitat of landscapes surrounding these roosts has an important influence on the number (size) and age/sex (composition) of Hen Harriers using the roosts. Variation in the size and composition of winter roosts is driven by several factors, including habitat/prey associations in the surrounding landscape and Hen Harrier sexual dimorphism. Winter roosts are at high risk from destruction or degradation as they occur mostly outside of protected areas. Conservation measures are therefore required to provide protection for roost sites and surrounding foraging areas. This thesis includes the first detailed analysis of Hen Harrier winter diet in Ireland. During the winter months, the Hen Harrier’s diet is dominated by small birds, though the importance of small birds, medium-sized birds and small mammals in the diet varies across habitats, regions, and time. Specifically, small birds are more abundant in the diet in lowland coastal and arable areas, while medium-sized birds are more abundant in the diet in upland and peat bog areas. This variation in diet has important implications for Hen Harrier winter distribution and the identification of appropriate targeted conservation measures in their wintering grounds. In some areas, non-native small mammals can constitute a significant proportion of the winter diet, and their continued spread may have an important influence on Hen Harrier ecology into the future. On the breeding grounds, in landscapes now dominated by conifer plantations, many Hen Harriers nest in young conifer forests. Although these habitats are used by Hen Harriers for nesting and foraging, our remote camera study identified a diverse predator community in young plantation forests which may be contributing to the high failure rates through predation of nests in Ireland. In addition, prey groups in pre-thicket forests and open moorland were markedly different, with highest bird densities in moorlands and highest small mammal abundances in young forests. As Hen Harriers’ preferred breeding season prey is small birds, this suggests young conifer forest is a suboptimal foraging habitat for Hen Harriers compared with traditional open habitats. Changes to policy focus and conservation strategies that address the pressures experienced by Hen Harriers across the full annual cycle must be implemented urgently in order to halt the continued decline of the species in Ireland. The findings of this thesis provide evidence-based information for policy makers and practitioners to further the development of appropriate Hen Harrier conservation management plans. Twenty-five recommendations for year-round management of Hen Harriers in Ireland were identified and refined during an external stakeholder review, encompassing habitat management, further research, and conservation policy. Effective conservation measures are required to fulfil the habitat requirements of Hen Harriers at their nesting, winter roosting and foraging sites. The future for Hen Harrier populations depends on the actions taken now, and filling research and conservation priority gaps has important implications for Hen Harriers in Ireland and beyond. While this study provides a detailed understanding of their ecology across the full annual cycle allowing us to better understand the pressures they face and improve conservation actions, further research is required on their responses to human activities and habitat loss, to inform conservation management and policy development. Continuous assessment of the effectiveness of Hen Harrier conservation measures should form part of ongoing conservation efforts, including the measures proposed in this study.
Seasonal ecology , Conservation , Diet , Winter roosts , Satellite tracking , Afforestation , Hen harrier
McCarthy, A. 2022. Seasonal ecology and the conservation of hen harriers (Circus cyaneus) in Ireland. PhD Thesis, University College Cork.
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