Using fledged brood counts of hedgerow birds to assess the effect of summer agri-environment scheme options

Abstract

A key aim of many European agri-environment schemes (AES) is to improve biodiversity on farmland. In recent years, several countries have been looking at long term trends in the spring adult population size of a target group of farmland birds as an indicator of this. The overall trend in these indicators is however not upwards. While this might suggest we need to look at the design and deployment of habitat management options within schemes, there is an increasing view that we also need to refine and improve our indicators, or the way we monitor them.Relating spring adult bird population size to AES options is problematic not least because of the time lag between the deployment of those designed to enhance bird breeding success in summer, and the spring surveys the following year. At the other end of the scale detailed studies of breeding success in farmland birds have practical/cost restraints. We argue that to understand the impact on farmland birds of particular summer options within AESs we need to be able to estimate the breeding success of local bird populations quickly and cheaply. This would enable us to relate particular AES options to the breeding performance of the birds actually using them.Complementing a previous study of woodland birds, we assess the likelihood of encountering fledged broods of hedgerow nesting bird species during transect surveys without finding nests, and then apply a simple mark-recapture analysis technique to provide an index of breeding success for those species. Following spring adult assessments, counts of fledged broods were undertaken four times a week during April, May, June and July, in four 2.5km hedgerow transects, at four sites in southern England in 2010. Mean daily detection probabilities of fledged broods of 16 common hedgerow birds were calculated from these counts using the software Presence. For 15 out of these 16 species these detection probabilities were sufficiently high for a programme of fledged brood surveys, involving just two or three visits per week from mid-May to mid-July, to provide a useful estimate of breeding success.The survey technique and associated analyses make certain assumptions when providing estimates of breeding success and these are discussed. Little is known about initial dispersal in passerine fledglings and a study in hedgerows may be useful here. However our pilot study suggests that the method could have application as a relatively easily derived productivity index for hedgerow birds, and hence an additional method available to study the impact of certain AES options on indicator species, or for research studies.