Protocol - Monitoring avian prey abundance within occupied merlin breeding territories and unoccupied moorland areas

Background

The causes of recent declines in breeding merlin are poorly understood. One hypothesis is that recent changes in the intensity of heather burning and cutting on grouse moors may be increasing heather dominated habitat and thus reducing the availability of key avian prey, particularly meadow pipits and skylarks.

To investigate merlin nesting requirements and habitat use on heather moorland we have been liaising with raptor workers and gamekeepers in three geographically separate study areas – North Pennines, North York Moors and the Yorkshire Dales to identify areas occupied by breeding merlin, and also unoccupied areas of moorland. The aim being to enable us to take comparative measures of moorland habitat and avian prey abundance between both occupied and unoccupied areas, and also potentially between successful and failed nests.

Avian prey surveys

Within breeding territories (n=66) and unoccupied areas (n=61) all birds will be surveyed using a modified Breeding Bird Survey technique. Surveys will be conducted along two parallel 1-km transects placed in the centre of the breeding territory situated 500m apart, which correspond with the two central transects where vegetation measures were taken. In addition, birds will be surveyed along the two line transects which join each end.

Thus, within each breeding territory birds will be surveyed along 3-km of transect in total. Two visits will be made to each site during the merlin breeding season. One in mid- April to mid- May to coincide both with merlin establishing breeding territories in April-May, and the second visit in June timed to coincide with merlin nesting and brood rearing. These periods also coincide with the return of meadow pipits to moorland habitats in April. The second survey in June coincides with the peak brood fledging estimated from a peak first egg laying date of 21 May at sites >300m in the north of Great Britain and assuming 12 days incubation and 15 days to fledging (Coulson 1956).

Methods

1. Box transects 3km in length will be surveyed within 2021 merlin territories, made up of 1km horizontal lines 250m above and below the nest site, and 500m vertical lines adjoining each of the horizontal transects.
2. Two visits will be made to each merlin territory and equivalent unoccupied sites during the breeding season to survey the abundance of avian prey. Once between April 11th and May 15th, and a second visit between June 1st and June 30th.
3. Surveys will only be conducted in good visibility with no precipitation or strong winds.
4. Surveys will commence from 30 minutes after sunrise and no later than three hours after sunrise.
5. Upon arrival at the transect start, note the start time, weather conditions and observer name (which should be incorporated into analyses to test and adjust for any potential observer biases).
6. Walk the transect route and record all birds of every species seen or heard along the length of the transect. It should take approximately one hour to walk the full 3 km transect and record observations.
7. Birds will be noted in three distance categories (within 25 m, 25–100 m, or more than 100 m on either side of the line, measured at right angles to the transect line), or as in flight.
8. Before starting each survey, observer estimates of distance bands will be calibrated using a tape measure at 25 m to increase accuracy when estimating bird distance as recommended by Buchanan et al. (2006).
9. At the end of each survey, note the end time.

Citations

Buchanan, G.M., Pearce-Higgins, J.W., Grant, M.C., (2006). Observer variation in estimates of Meadow Pipit Anthus pratensis and Skylark Alauda arvensis abundance on moorland. Bird Study 53: 92–95.
Coulson, J. C. (1956) Mortality and Egg Production of the Meadow Pipit with Special Reference to Altitude, Bird Study, 3:2, 119-132.