A comparison of bird populations on organic and conventional farm systems in southern Britain

Field boundaries and fields on 22 pairs of organic and conventional farms i n England and Wales were surveyed over three breeding seasons (April-July) and two autumn (September-November) and winter (December-February) periods in order to ascertain whether organic and conventional farms differed in th e size and diversity of their associated bird populations. Species diversit y was significantly higher on organic farms in the 1994 breeding season, bu t in no other year or season. Of 18 species, eight showed a significantly h igher density on organic field boundaries in at least one season/year, with a greater number of significant results being detected in the autumn. Ther e were very few significant differences in bird density in fields outside t he breeding season. The density of breeding skylarks Alauda arvensis, the p rincipal field nesting species, was significantly greater on organic farms in one breeding season. Hedges tended to be higher and wider, field boundar ies tended to have more trees and field sizes tended to be smaller on organ ic farms. Canonical correspondence analysis indicated that components of ha bitat structure were important in explaining differences in bird density be tween farm types for certain species. (C) 1999 Elsevier Science Ltd. All ri ghts reserved.