THE EFFECTS OF AGRICULTURAL CHANGE ON POPULATION-SIZE OF CORN-BUNTINGS MILIARIA-CALANDRA ON INDIVIDUAL FARMS

This paper uses Common Birds Census (CBC) data to examine habitat sele ction by Corn Buntings and the relationship between population change and land-use change at the scale of the individual farm. On each of 29 farms, structural data and cropping patterns in the year of peak Corn Bunting population size were compared with those in the year of lowes t population size (or first year of extinction). Crop diversity, hedge row length, field size and the areas of all crops except wheat and bar ley did not differ significantly between years of highest and lowest C orn Bunting population. Barley was more extensive in years of highest Corn Bunting population and wheat move extensive in years of lowest po pulation. Variation in Corn Bunting population density in a sample of 18 farms censused in 1974 could not be explained by variations in habi tat diversity, hedgerow length, field size, altitude or the area of di fferent crop types. This variation was also unrelated to either of the first 2 components of an ordination of structural and crop data. Popu lation size was also examined in relation to cropping changes over per iods of between 10 and 30 years on 10 individual farms. Changes in the numbers of Corn Bunting territories on individual farms surveyed over long periods were also poorly correlated with changes in cropping. On these farms, Corn Buntings exhibited no preference between barley and wheat and none between spring- and winter-sown cereals. Cereals were preferred to grassland, and temporary grassland was preferred to perma nent pasture. We suggest that Corn Bunting numbers are unlikely to hav e declined as a result of decreased crop diversity, of changes in the relative areas of tillage or grassland or of the loss of hedgerows, an d that more likely causes are reduced winter food supplies resulting p articularly from the loss of spring tillage, increased pesticide usage and improved harvesting and storage techniques.