Prey deficit for reintroduced Golden Eagles (Aquila chrysaetos) in Ireland

Golden Eagles (Aquila chrysaetos) were reintroduced to Ireland in 2001 following prey availability and habitat feasibility studies undertaken in the 1990s which deemed northwest Ireland as suitable for their ecological requirements. However, they have failed to reach the predicted population growth necessary to sustain a viable population in the long-term. It has been suggested that inadequate prey biomass may have led to the poor reproductive performance. To determine prey availability within the core range of reintroduced Golden Eagles in Ireland, pre-release density estimates for Irish Hare (Lepus timidus) and Red Grouse (Lagopus lagopus) - their main prey - were compared with post-release estimates derived from transect and camera trap surveys. Camera trapping suggested 0.2 hare detections/km2 in upland areas in Cloghernagore Bog and Glenveagh National Park Species Area of Conservation (SAC) indicating that the hare population is very low, corroborated by the local results in the National Hare Survey 2017-2019 which detected zero hares in the vicinity of Glenveagh. Tracks and signs on walked transects suggested 1.6 grouse males/km2 which was comparable to the 1.2 males/ km2 reported during 2006/2008. The availability of hares and grouse as sources of live prey was estimated at 1.5-2.4 kg prey/km2/year, which was 74-83 % lower than the 9.2 kg prey/km2/year estimated as required to maintain Golden Eagle productivity in Scotland. Thus, prey deficit may explain the poor performance of reintroduced Golden Eagles in Ireland. Land management or site interventions could potentially increase hare and grouse numbers to improve eagle productivity. As such, integrated landscape management interventions may be necessary to ensure the success of species reintroduction programmes.