Declining Hen Harrier populations in the UK
The hen harrier (Circus cyaneus) was once widespread across Britain, but it has endured decades of persecution, which first forced this bird of prey out of mainland Britain by 1900. From remnant populations in the Orkneys and the Western Isles, changing land uses and decline of persecution allowed them to spread south once more, reaching England shortly after the Second World War.
The 2010 national survey of Hen Harrier populations in the UK and Isle of Man showed a marked decline, but the reasons for this are not clear. The Hen Harrier is a ‘Red Listed Bird of Conservation Concern’ in the UK and hence, its population is closely observed and has been subject to frequent national surveys.
Previous surveys have shown increasing number of Hen Harrier pairs; in 1988/9, 478-669 pairs, in 1998, 570 pairs and in 2004 a 41% increase since 1988 to 806 pairs. The 2010 survey showed a decline in numbers by 18% from 806 to 662 pairs.
The survey was conducted in 5 regions, Scotland, England, Isle of Man, Wales and Northern Ireland between April and July. Within each region breeding territories were divided into 10K grids and data was gathered from a number of sources such as the RSPB, Natural England, BTO, Birdwatch Ireland, SOC, Raptor Study Group, volunteers and paid fieldworkers.
The Isle of Man had 29 pairs, a 49% drop since 2004. There were no obvious reasons visible for this as persecution is at a very low level and climate and environment remain unchanged in this area. In the UK there were 633 pairs (749 in 2004). Of that Scotland held 76% of the total UK and Isle of Man at 505 pairs, a 10% drop on 2004. In England there were 12 pairs, a drop from 19 in 1988, Northern Ireland 59 pairs, little change since 2004. Finally, Welsh populations increased by 33%, possibly due to better temperatures, increased breeding productivity and less human interference.
Historical research has shown that illegal persecution has been the main cause of declining numbers, especially were moorland is managed for Red Grouse shooting. Habitat change, climate change and abundance of prey also contribute to decreases in Hen Harrier population. The authors in part found that their survey backed up the notion that intensely managed grouse moorland resulted in fewer breeding pairs. They also found changes in chosen habitats of breeding pairs but overall concluded that more research is needed to determine causes for the declining population.
Watch a hen harrier in flight here.