Controls on the winter distribution of the Hen Harrier (Circus cyaneus) in southern Britain

Report to the National Birds of Prey Trust

2007

ANDREW D.M. DOBSON
School of Geography, University of Nottingham, University Park, Nottingham NG7 2RD, UK. 
Hawk & Owl Trust.
E-mail: lgxad2@nottingham.ac.uk.
Tel: 07740 457493

FIELDWORK

Introduction Last year I reported my intention to fit platform transmitter terminals (PTTs – satellite tags) to hen harriers at Wicken Fen NNR in order to ascertain their spring migratory destinations.  As has been apt to occur during this project, this plan was unsuccessful.  Habitat management at Wicken Fen meant that the main roosting area for hen harriers was mown too short for the birds to use, despite verbal assurances that this would not occur.  At very short notice, I arranged for a trapping attempt at an alternative site, Roydon Common in Norfolk, a nature reserve managed by Norfolk Wildlife Trust.  Up to seven hen harriers had been seen throughout the winter at this reserve, and they mainly settled in a relatively small patch of heather (Calluna vulgaris).

Methods
An adapted version of the successful method developed at Wicken Fen in the previous winter was applied.  Four observers sat in two different hides overlooking the main roosting area and watched the harriers as they arrived.  The locations of the roosting birds were estimated as accurately as possible by reference to compass bearings and local landmarks.  After dark, two large mist-nests suspended between carbon-fibre poles were erected close to the roost.  Five fieldworkers then circled around the back of the roost before moving forward to flush the birds towards the nets.  No birds were captured. 

Conclusion
This abortive trapping attempt, which had incurred considerable logistical problems due to the large number of qualified helpers required, suggested that the method held little potential.  Without the benefit of the huge tower hide that overlooks the roost at Wicken Fen, it had been impossible to accurately pinpoint the location of the roosting birds, even when two sets of observers had been located at respective right angles to the roost.  Given the apparent unlikelihood of success, it was deemed that further outlay in terms of time and travel expenses were not justified.  No further trapping attempts were made. 
Two solar PTTs were purchased and are still in the possession of the Hawk and Owl Trust.  It is still my intention to mount them on hen harriers at Wicken Fen.  The feasibility of this plan depends mainly upon the behaviour of the harriers at the roost this winter, and I have not yet made the necessary arrangements in view of this uncertainty. 

OFFICE-BASED ANALYSES

This year I have finished the following aspects of research:

• An assessment of the migratory origins of the southern British winter population of hen harriers.  This involved comparative analysis of temporal trends in roost numbers and migration census counts from Scandinavia, and also included a comprehensive analysis of Europe-wide ringing recovery data.  The results refute the accepted wisdom that the majority of these individuals derive from Scandinavian breeding populations, and instead proposes, with supporting evidence, that most are inter-UK migrants from Scotland. 
• A case study of prey selection near Wicken Fen, from which oral pellets were collected and analysed. Proportions of prey species in pellets were compared with relative abundance of the same species in the surrounding landscape.  This study has established that in this roost catchment hen harriers preferentially selected skylarks (Alauda arvensis) over other apparently similar prey species. 
• A spatial model of the distribution of communal winter roosts in southern Britain. Explanatory variables were derived from national climate, topography and habitat datasets.  This analysis has demonstrated that aspect, elevation, minimum winter temperature and the presence of certain land cover types are the most influential variables for dictating hen harrier winter roost site selection.  Detailed maps of habitat suitability across southern Britain were produced, identifying areas most likely to be occupied in winter.   

The following research papers are in preparation:

Dobson, A. & Clarke, M. Montagu’s harrier: a misnomer? Confusion and complications in early taxonomy of the genus Circus. In prep.

Dobson, A., Clarke, M., Kjellen, N. & Clarke, R. Migratory origins of the British wintering population of the hen harrier. In prep. 

Dobson, A., Clarke, M. & Clarke, R. A model of hen harrier winter roost distribution. In prep. 

FINANCIAL SUMMARY

NOTE

Dr Roger Clarke, the instigator of this project, and my mentor and supervisor, died on the 28th January this year.