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Hen Harrier Winter Roost Survey

Joint research project by the Hawk and Owl Trust and the British Trust for Ornithology yields results

Hen Harrier 101Female Hen Harrier. Photo by Luke Delve

The Hen Harrier Winter Roost Survey 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 Hen Harrier Winter Roost Survey provides an invaluable tool for keeping track of changes in Hen Harrier numbers in the UK.

The Hawk and Owl Trust and British Trust for Ornithology (BTO) would like to see this survey grow to cover already known roosts which are not, at present, counted as part of the survey.

Want to get involved?

The Hen Harrier Winter Roost Survey would be delighted to welcome more volunteers. Contact Nigel Middleton for more details by email at This email address is being protected from spambots. You need JavaScript enabled to view it. or Anne Cotton This email address is being protected from spambots. You need JavaScript enabled to view it.

As well as Hen Harriers, lucky surveyors frequently encounter other raptor and owl species that either use the same roost or the habitat around it. For instance, Marsh Harriers are frequent in the south and counts of these are encouraged and passed on to those monitoring these birds. Short-eared Owl and Merlin are frequently recorded near the roost site as well as Barn Owl and Buzzard also. 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. Counts from the north of England and Wales were too low to interpret, so more data from these regions would be particularly helpful in establishing if this is a countrywide trend.

Hen Harrier 10Results of the project so far have shown that there is a difference in gender ratios in different parts of the country. Males are more likely to be encountered at winter roosts in southern and western Britain. Photo by Luke Delve.

Reasons for such intersexual segregation are currently unclear, although 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, although 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 data from this survey can help to illuminate in the future.

To take part, all you need to do is find a suitable vantage point near roosting harriers and wait and see…contact us for more details!