Effects of agricultural intensification on the breeding success of corn buntings Miliaria calandra

1. Corn buntings Miliaria calandra have declined steeply in Britain and nor th-western Europe since the mid-1970s; changes in farming practice are beli eved to have been partly responsible. 2. We studied nesting corn buntings on the South Downs in west Sussex betwe en 1995 and 1997 to examine the possible effects of agricultural intensific ation on breeding success. The abundance of invertebrates around individual nests was sampled by sweep-netting in July. 3. Corn buntings provisioning nestlings foraged in grassy margins more than any other habitat relative to their availability within the maximum foragi ng range. The other habitats used more than expected were spring-sown barle y, unintensified grass and set-aside. Those used less than expected include d winter-sown wheat and intensively managed grassland. The invertebrates mo st commonly fed to chicks were more abundant in foraging areas than elsewhe re. Their density was negatively correlated with the number of insecticide applications both when cereal fields only were considered and when all fora ging habitats were included. 4. The lower the abundance of chick-food invertebrates close to nests, the greater the distance from the nest at which parents foraged, and the longer such trips were in duration. The weights of nestlings, corrected for age u sing tarsus length, were positively correlated with the abundance of chick- food invertebrates. 5. The probability of nest survival was negatively correlated with the abun dance of chick-food invertebrates close to the nest, apparently as a result of an increased risk of predation. 6. Agricultural intensification in Britain, including the increased use of pesticides, has led to a widespread decrease in the availability of chick-f ood invertebrates on lowland farmland. If our results are typical of corn b untings in an arable environment, this decrease correlates with reduced bre eding success. Depending on the mortality rates for fledged chicks and olde r birds, this reduction may have contributed to the corn buntings' decline and may hamper recovery. 7. Farming practices that increase invertebrate availability ought to benef it breeding corn buntings. Large-scale measures such as set-aside and the s pring-sowing of cereals (especially if undersown with grass) depend heavily on overall agricultural policy. Small-scale initiatives might therefore be more feasible; these include the provision of grassy margins or beetle ban ks and selective spraying of headlands.