1. Set-aside is arable land rested from normal intensive operations, usually providing, in summer, a relatively sparse, weedy or grass-dominated sward without pesticide or fertilizer inputs. Set-aside is therefore potentially attractive to breeding and foraging birds and is predicted to contribute to increased avian biodiversity on arable or mixed farmland. Set-aside mimics low intensity farmland within the heart of the industrial farm landscape, thereby allowing direct comparison with intensive crops regarding their respective values for the bird fauna.
2. In this study, bird abundance was compared between set-aside and nearby crops or grassland. A particular focus of the study was to identify the breadth or generality of any preferences across a suite of farmland species, using data from a broad representation of English farms. Thus, an extensive survey of birds utilizing fields, including set-aside, was conducted on 92 arable farms in England during 1996 and 1997. Each farm was visited four times in each summer, habitat details were recorded, and all birds seen or heard were mapped using a standard technique.
3. Field type preferences were examined across bird functional groups representing gamebirds, pigeons, crows, skylark Alauda arvensis, thrushes (Turdidae) and granivorous passerines (Passeridae, Fringillidae and Emberizidae). The relationship between bird abundance and field type was analysed using log-linear Poisson regression and compositional analysis.
4. Both analyses revealed that bird abundances were significantly higher on set-aside than on winter cereals for all six functional groups, and were highest on rotational set-aside for all functional groups except crows (which preferred grassland). Winter cereals or grassland were generally the least preferred habitat.
5. On farms where both rotational and non-rotational set-aside was present, preferences were strongest for rotational set-aside for all functional groups except crows (which preferred non-rotational set-aside). This underlines the differences between set-aside sward composition in influencing bird numbers.
6. The results show broad-scale preferences for set-aside over crops or grassland for species representing non-passerines, passerines, insectivores and granivores, for a wide representation of farms in England. For the majority of species, this preference implies that set-aside is utilized as a source of food, and the scale of this preference is impressive given that most set-aside was not managed specifically for bird conservation. However, not all types of set-aside were equally exploited by birds, as the strongest preferences were for natural regeneration rotational set-aside rather than the more structurally uniform non-rotational set-aside.
7. The results are important in the context of the potential loss of set-aside from the arable countryside, but also for the development of future agri-environmental schemes. We suggest that, to reverse population declines of many farmland birds in Britain, such schemes will need to be introduced on a wide geographical scale, like set-aside, but will also need to be carefully tailored, through advice to farmers, to maximize their potential to support bird species.