The importance of arable habitat for farmland birds in grassland landscapes.
Abstract
Over the last 25 years, populations of seed-eating birds have declined severely over most of western Europe. Local extinctions have occurred in grassland-dominated areas in western Britain, which may be influenced by loss in habitat diversity and a decline in the amount of arable cultivation. We used the large-scale British Breeding Bird Survey of 1998 to investigate the importance of arable habitat within grassland landscapes for 11 common seed-eating birds and four similar sized insectivores. Generalized linear models were used to model the number of birds recorded in agricultural habitat within survey squares as a function of the amount of arable habitat present. Numbers of grey partridge Perdix perdix, skylark Alauda arvensis, tree sparrow Paser montanus, corn Miliaria calandra and reed buntings Emberiza schoeniclus, yellow-hammer Emberiza citrinella and whitethroat Sylvia communis increased with the amount of arable habitat present in a survey square; the numbers of house sparrow Paser domesticus, four finch species, dunnock Prunella modularis, robin Erithacus rubecula and blackcap Sylvia atricapilla did not. The positive association between numbers of some species and arable habitat within 1-km squares was strongest where arable habitat was rare in the surrounding area, and weakest or even reversed when arable habitat was common. These results demonstrate the scale-dependence of bird-habitat associations in agricultural landscapes, only demonstrable where data are available at fine grain over large geographical areas. These results support the hypothesis that range contractions (i.e. local extinctions) of some granivorous species have occurred because of contraction in arable cultivation. The loss of arable habitat where it is scarce may be causing declines in some areas, even though intensification of arable management is thought to be the main cause of declines elsewhere. Agri-environment schemes may need to vary between regions, for example to encourage arable cultivation in pastoral areas.