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 in 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 the size and diversity of their associated bird populations. Species diversity was significantly higher on organic farms in the 1994 breeding season, but in no other year or season. Of 18 species, eight showed a significantly higher density on organic field boundaries in at least one season/year, with a greater number of significant results being detected in the autumn. There were very few significant differences in bird density in fields outside the breeding season. The density of breeding skylarks Alauda arvensis, the principal field nesting species, was significantly greater on organic farms in one breeding season. Hedges tended to be higher and wider, field boundaries tended to have more trees and field sizes tended to be smaller on organic farms. Canonical correspondence analysis indicated that components of habitat structure were important in explaining differences in bird density between farm types for certain species. (C) 1999 Elsevier Science Ltd. All rights reserved.