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 selection 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 lowest population size (or first year of extinction). Crop diversity, hedgerow length, field size and the areas of all crops except wheat and barley did not differ significantly between years of highest and lowest Corn Bunting population. Barley was more extensive in years of highest Corn Bunting population and wheat move extensive in years of lowest population. Variation in Corn Bunting population density in a sample of 18 farms censused in 1974 could not be explained by variations in habitat diversity, hedgerow length, field size, altitude or the area of different crop types. This variation was also unrelated to either of the first 2 components of an ordination of structural and crop data. Population size was also examined in relation to cropping changes over periods 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 permanent pasture. We suggest that Corn Bunting numbers are unlikely to have declined as a result of decreased crop diversity, of changes in the relative areas of tillage or grassland or of the loss of hedgerows, and that more likely causes are reduced winter food supplies resulting particularly from the loss of spring tillage, increased pesticide usage and improved harvesting and storage techniques.