Grasshopper Warbler Locustella naevia breeding habitat in Britain

Abstract

Capsule Structural aspects were common across a range of different habitat types and there was evidence that this breeding habitat was limited in extent. Aims To characterize Grasshopper Warbler breeding habitat in Britain and to use this assessment to test whether it was limiting the breeding population. Methods In 2005 singing Grasshopper Warblers were surveyed at four groups of sites in southwest Scotland, during four night-time periods through the season, to quantify which period would afford best detection. Breeding habitat characteristics were measured at 210 Grasshopper Warbler singing positions and 210 systematically located comparison sites positioned across 22 sites in England and Scotland. In 2006 tests were conducted using two different procedures to determine whether the preferred breeding habitat was limited. First, by using the 2005 data to map predicted suitable habitat at 15 sites in southwest Scotland before Grasshopper Warblers arrived, and then assessing the extent of actual occupation. Second, by visiting 30 BTO Common Birds Census (CBC) plots mainly in southern England that had lost their breeding Grasshopper Warblers to assess the current suitability of the habitat. Results Grasshopper Warblers sang more in the first four weeks of the survey (in May) and during the dawn and pre-dawn hours, 05:00–08:00 hours. The breeding habitat across a variety of landscape types was characterized by four key attributes: the presence of more dense, dead vegetation and tussock-forming species at ground level; less dense vegetation at or above 2 m; softer soil; and potential song posts. Not all of the apparently suitable habitat was filled by Grasshopper Warblers, but almost all of the old CBC plots were no longer suitable as breeding habitat as a result of maturation, succession or management. Conclusions It should be possible to detect about 85% of singing Grasshopper Warblers by surveying during the pre-dawn/dawn period in the month after their main arrival. It seems likely that the breeding habitat of Grasshopper Warblers is limited at least in some parts of Britain and there was good evidence for the importance of the structural aspects of their preferred habitat, including heterogeneity. The area of preferred Grasshopper Warbler habitat could be increased through strategic and active management in a number of settings.