Supporting Hen Harriers In Novel Environments

Hen Harrier Silhouette

Partners: University College Cork, BirdWatch Ireland, Coillte, IRD Duhallow
Funding Body: Department of Agricutlure, Food & the Marine
Duration: December 2017 - December 2019


Project Background
Though widely distributed across the island of Ireland, hen harriers are scarce in this country with a current estimated breeding population of just 108-157 pairs in 2015 (Ruddock et al. 2016). Hen Harriers are Amber-listed in the Birds of Conservation Concern in Ireland, due to historical declines and continued vulnerability to habitat loss and persecution. They are also listed in Annex I of the EU Birds Directive, which is a legislative framework designed to protect birds. Six Hen Harrier SPAs were designated in Ireland in 2007 and these must be managed to maintain suitable habitat for hen harriers. Despite conservation efforts, there ihas been an estimated overall decline of 33.5% between 1998 – 2000 and 2015 (Ruddock et al. 2016).

An intenstive study of breeding Hen Harriers undertaken at UCC bewteen 2007 and 2011 at three study sites in Ireland revealed that the number of breeding pairs in each area declined over the study period (Irwin et al. 2011) During this time, successive research projects at University College Cork have investigated the ecological requirements of Hen Harriers to inform their conservation management in an Irish context. This research has focussed on their breeding ecology in Ireland including aspects such as habitat selection, breeding biology and nesting ecology and the impact of land use change including afforestation and wind energy development.

Research at UCC has shown that in the absence of their traditional heather moorland and bog habitats, Hen Harriers in Ireland nest in young forest plantations with open canopies and strongly avoid grassland areas for nesting (Wilson et al. 2009). Young forests have been recorded as the most important nesting habitat in the most recent national hen harrier surveys (Ruddock et al., 2016). However, research has identified a conflict with maturing forests and areas of canopy closure in upland areas in Ireland within the hen harriers distribution (Wilson et al., 2006). At one study area in Ireland a negative association between second rotation pre-thicket forest and breeding success was subsequently identified, which suggests that this species may be subject to an ecological trap (Wilson et al., 2012).

Given the large numbers of hen harriers now using open forests, finding an appropriate balance between forest management, industry priorities and conservation is key to the success of hen harrier populations. 


This project set out to exploit large, archived datasets on hen harrier breeding ecology in Ireland, a large proportion of which has been collected by UCC as well as the latest 2015 survey data. The relationship between hen harrier breeding success and forest management practises in the face of climate change was investigated.


Research Methods
Existing and new data were used to examine whether variation in hen harrier breeding success was related to forest management practises. Two field studies generated new data on previously identified knowledge gaps – the vulnerability of breeding hen harrier in different habitats to predators and the impact of clear felling debris (brash) within second rotation forests on habitat quality. The findings of this study were used to derive recommendations for the management of forest expansion in Ireland in relation Hen Harrier conservation.


Research Findings
This research revealed that Hen Harriers in Ireland select for a narrow range of habitat features and that their breeding success is influenced by both climatic instability and high rainfall during the early breeding season (Caravaggi et al., 2019a). A strong influence of agriculture, forestry and related activities, predator activity and recreational activities on their territories was found, with considerable scope for cumulative impacts of independent pressures. Furthermore, areas of suitable habitat for Hen Harriers in the Irish uplands were found to overlap with areas that experience anthropogenic pressures known to negatively impact on this vulnerable bird species. Combined with clear evidence for the co-occurrence of multiple pressures at a regional scale, this demonstrates a clear need for statutory agencies to consider the potential cumulative impacts of individual pressures when developing conservation strategies for Hen Harriers (Caravaggi et al., 2019b).

Recent afforestation of upland habitats in Ireland has contributed to a decline in suitable nesting and foraging habitat for breeding Hen Harriers. Using data from The 2015 National Survey of Breeding Hen Harrier in Ireland, forest data sources and CORINE land class maps we found a concerning lack of heath and bog habitat at landscape-scale surrounding typical Hen Harrier breeding territories in Ireland (Caravaggi et al., 2020). The high proportion of habitat of poor suitability in the landscape may reduce the reproductive potential of breeding Hen Harrier pairs, ultimately impacting at the population level.  

Archived nest camera data were used to describe patterns of adult attendance, incubation, brooding and prey delivery rates (Fernández Bellon et al., 2017). Nest camera footage also yielded new information on predation events (Fernández Bellon et al., 2017). Archived data were used to assess the potential disturbance impacts of fieldwork, and no negative effect on nesting success was observed (Wilson et al., 2017).

The consequences of afforestation of upland habitats for Hen Harrier prey (birds and small mammals) were examined by comparing open moorland, early pre-thicket conifer forests (2–4 years post-replanting) and late pre-thicket conifer forests (6–8 years post-replanting) across 24 upland study sites. This study revealed that afforestation can alter the abundance and community composition of Hen harrier prey with bird and small mammal prey showed contrasting patterns of abundance between moorland and pre-thicket forests (McCarthy et al., 2020). Bird density was higher in moorland, while small mammal abundance was higher in pre-thicket forests. Bird diversity, by contrast, was lowest in moorland and highest in late pre-thicket forests, while small mammal diversity was highest in moorland and lowest in late pre-thicket forests. 

The influence of forest interior edge effects on Hen Harrier nest predators was investigated using remote cameras in young forests, which were home to a diverse predator community. Patterns in detections of mammalian and avian predators in the Irish uplands were also described. Hen Harrier prey groups in pre-thicket forests and in open moorland showed differing trends, with birds occurring at higher densities in open moorlands and small mammals occurring at highest abundances in young forests.

Consultation with European Hen Harrier experts revealed that the situation for Hen Harriers in Ireland is similar to that of European populations that have recently become extinct (Fernández-Bellon et al., 2020). This research indicated that protected areas are considered to be largely ineffective for Hen Harrier conservation, and are only potentially effective when undertaken concurrently with pro-active conservation actions aimed at achieving specific goals.

At a time when upland habitats in Ireland are under more anthropogenic pressure than ever before, these research findings provide support for practical planning and management measures to increase the habitat value of forests for Hen Harriers and to reduce the influence of pressures on Hen Harriers within forested landscapes. 



  • McCarthy A, Caravaggi A, Fernández-Bellon D, Irwin S, Lusby J, O’Halloran J. 2021. Landscape and temporal influences on the winter diet of a threatened diurnal raptor, the Hen Harrier Circus cyaneus. Bird Study 68(3):408-421. View paper here.
  • McCarthy A, Caravaggi A, Fernández-Bellon D, Irwin S, Lusby J & O'Halloran J. 2021. Bird and small mammal community composition and abundance in upland open habitats and early conifer forests. European Journal of Wildlife Research 67, 26 (2021). View paper here
  • Fernández-Bellon D, Lusby J, Bos J, Schaub T, McCarthy A, Caravaggi A, Irwin S & O'Halloran J. 2020. Expert knowledge assessment of threats and conservation strategies for breeding Hen Harrier and Short-eared Owl across Europe. Bird Conservation International. View paper here
  • Caravaggi, A, Irwin, S, Lusby J, McCarthy A, Mee A, Nagle T & O'Halloran J. 2020. Forest management and Hen Harrier Circus cyaneus conservation in Ireland. Irish Birds 42:1-12. Caravagietal2020IB
  • Caravaggi A, Irwin S, Lusby J, Ruddock M, O’Toole L, Mee A, Nagle T, O’Neill S, Tierney D, McCarthy A & O'Halloran J. 2019a. Factors influencing Hen Harrier Circus cyaneus territory site selection and breeding success. Bird Study 66(3):366-377. Caravaggietal2019aBS
  • Caravaggi A, Irwin S, Lusby J, Ruddock M, Mee A, Nagle T, O'Toole L, O'Neill S & O’Halloran J. 2019b. Anthropogenic pressures within the breeding range of the Hen Harrier Circus cyaneus in Ireland. Bird Study 66(4): 461-471.  Caravaggietal2019bBS
  • Fernández-Bellon D, Wilson MW, Irwin S, Kelly TC, O'Mahony B & O'Halloran J. 2018. Video evidence of siblicide and cannibalism, movement of nestlings by adults and interactions with predators in nesting Hen Harriers. Journal of Raptor Research 52: 393-399. FernandezBellonetalJRR2018
  • Fernández-Bellon D, Wilson MW, Irwin S, Kelly TC, O’Mahony B & O’Halloran J. 2017. Activity patterns of breeding Hen Harriers Circus cyaneus assessed using nest cameras. Bird Study 64: 557-561. Fernandez Bellon et al 2017
  • Wilson MW, Irwin S, O'Mahony B, Kelly TC, O'Halloran J. 2017. The effects of fieldwork on Hen Harrier Circus cyaneus breeding success. Irish Birds 10: 457-462. Wilson et al 2017b


Project Team

University College Cork:
Principal Investigator: John O'Halloran
Project Manager: Sandra Irwin
Post-Doctoral Researcher: Anthony Caravaggi 
Post-Doctoral Research: Darío Fernández-Bellon
MSc Student: Alan McCarthy

BirdWatch Ireland:
John Lusby

Pat Neville

Forest Ecology

Dept. of Zoology, Ecology & Plant Science, University College Cork, Distillery Fields, North Mall, Cork