Counting birds on farmland habitats in winter

Capsule Perimeter counts underestimate the number of birds using agricultural fields. Aims To determine the degree to which farmland birds may be undercounted on field surveys and the factors influencing this. Methods In a study of 96 fields, birds in the field were counted during a walk of the perimeter. Afterwards transects were walked across the field to determine the number of birds missed. Additionally in a national survey we looked at the birds seen in field transects expressed as a proportion of the total seen in the transect and on perimeter counts. Results To obtain an accurate count of birds on a field requires the use of the whole-area search methodology especially for species such as Grey Partridge Perdix perdix, Snipe Gallinago gallinago, Skylark Alaudo arvensis and Meadow Pipit Anthus pratensis. However, for a large number of species, including thrushes, flocking finches and buntings, over 90% of individuals can be recorded using the much less time-consuming perimeter count method. Conclusions Perimeter counts are likely to be reliable for most species but special efforts may be required for a few cryptic species.