Hen Harrier Winter Roost Survey

Hen Harrier male
The magificent male Hen Harrier © Pete Walkden

Joint research project yields results

Long-running UK research by the Hawk and Owl Trust and the British Trust for Ornithology

The Hen Harrier Winter Roost Survey (HHWRS) has been running continuously since its instigation by the late artist and ornithologist Donald Watson and harrier enthusiast Dr Roger Clarke in the early 1980s.

Dedicated volunteers record harriers at roost sites on six co-ordinated days throughout the winter. Roost counters are encouraged to record sex and, if possible, age of the birds seen as well as the numbers of birds (though typically the results comprise numbers of adult ‘grey’ males and ‘ringtails’, which includes adult females and juveniles of both sex). Recorders are asked also to provide some details of the weather conditions and to report sightings of other raptor and owl species.

In a world of agricultural intensification, climate change and, unfortunately, continued persecution of hen harriers and other raptors, the HHWRS provides an invaluable tool for keeping track of changes in hen harrier numbers in the UK, and indeed, is the only one of its sort for non-breeding raptors in this country.

The Hawk and Owl Trust has joined with the British Trust for Ornithology (BTO) to continue the HHWRS, which the two trusts have helped fund for some years. They would like to see this survey grow to cover already known roosts which are not, at present, counted as part of the survey.

Female Hen Harrier quartering her moorland breeding grounds Pete Walkden
Female Hen Harrier quartering her moorland breeding grounds ©Pete Walkden

The HHWRS would be delighted to welcome more volunteers. Contact Nigel Middleton for more details by email at or Anne Cotton at the BTO  or tel 01786 458 021).

Those lucky enough to carry out hen harrier roost surveys frequently encounter other raptor and owl species that either use the same roost or the habitat around it. Marsh harriers are frequent in the south and counts of these are encouraged and passed on to those monitoring these birds. Short-eared owls and merlins are frequently recorded near the roost site and Barn Owls and Buzzards also. One lucky observer in 2009-2010 found a vagrant pallid harrier near his roost and in 2010-2011 a northern harrier (North American hen harrier, regarded by some as a separate species) was reported.

Data analysed from the 2009-2010 winter showed up apparent differences in winter distribution between grey birds (mainly adult males) and brown birds (females and immature males).

Data highlights distribution puzzle
Proportions of males were higher in the south and southwest of England than in the east of England (south of Lincolnshire), matching the patterns reported from the survey data in the 1980s and 90s (see below for a list of HHWRS-related publications). Counts from the north of England and Wales were too low to interpret, so more data from these regions would be helpful in establishing if this is a countrywide trend.

Reasons for such intersexual segregation are currently unclear, though climate (and in particular temperature) may be involved, the smaller males potentially needing more benign winters in which to survive than the larger females. Prey availability is likely to be the other key environmental factor, though no current research can offer any insight into either hypothesis.

Alternatively, a modest but significant influx of dispersing juveniles from northern Europe into eastern England could tip the ratio in favour of ringtails in this region, meaning that the regional segregation would be one of age-groups, not sex. Demographic patterns and migratory behaviour are just two of the many fundamental biological processes that HHWRS data can help to illuminate in the future.

The survey requires very little in the way of bird ID skills, so is ideal for the tyro birdwatcher who is hesitant at undertaking one of the BTO’s more general volunteer surveys; all you need to do is find a suitable vantage point near roosting harriers and wait and see!

Further reading
Clarke, R. 1988. The diet of hen harriers roosting in Wicken Fen. Cambridgeshire Bird Report 62: 52-58.
Clarke, R. & Watson, D. 1990. The Hen Harrier Circus cyaneus Winter Roost survey in Britain and Ireland. Bird Study 37: 84-100.
Clarke, R. & Watson, D. 1997. The Hen Harrier Winter Roost Survey. The Raptor 24: 41-45.
Dobson, A., Clarke, R. & Clarke, M. 2009. Factors affecting the vulnerability of farmland birds to predation by Hen Harriers Circus cyaneus in winter. Bird Study 56 (1): 132-136.