Does best-practice crow Corvus corone and magpie Pica pica control on UK farmland improve nest success in hedgerow-nesting songbirds? A field experiment

Abstract

The role of predation by corvids on the breeding output of songbirds is unclear. Using a randomised-pair design, we measured how nest success of hedgerow-nesting passerines responded to the experimental removal of carrion crows and magpies. We worked in southern England at 32 paired sites around 4 km2 each, one with and one without best-practice corvid control, studying four different pairs per year for four years 2011-2014. We counted corvids, and using songbird territory mapping and fledged brood counts without finding nests along transects, we estimated nest success as a brood/ territory ratio for the community of songbirds in 4 km of hedgerow at each site. Crows and magpies were still present at most removal sites but numbers were half as high as at paired non-removal sites. Eighteen songbird species were frequently encountered at most sites with on average (±1 SD) 102 ± 30 territories per site. Using a generalised linear mixed model analysis the songbird community as a whole bred less well in treatment sites without corvid removal and in years with more rainfall. Nest success was down by 10% in non-removal sites on average relative to removal sites over the four years. Excluding 2012 data because of exceptionally high spring rainfall that year, nest success was down 16% in the non-removal sites on average in the other three years. For open-cup nesting species as a group there was no difference in nest success between site types. Our data on hole nesters suggest that they were affected by treatment and contributed to our overall result. For species whose numbers are regulated through territoriality, nest-site or habitat availability, spring abundance is unlikely to be affected by a 15% increase in breeding output. For species limited by nest success, it may be more important.