Evidence on the impacts of corvid removal is mixed – crows and magpies are accepted by the RSPB and others as potentially important nest predators of ground-nesting birds, but not of birds nesting off the ground (eg hedgerow nesting songbirds).

To address this evidence gap, between 2011 and 2014 the GWCT carried out a programme of work to assess the effect of corvid removal on farmland songbird post-nesting productivity, involving a cost-effective multi-site experimental study.

This was a four-year, 16-site experimental study with randomised treatments and paired controls. The programme used four sites, each with two experimental plots. Each lowland farmland site consisted of a pair of plots about 200 to 300 hectares each.

Crows and magpies were controlled using best-practice techniques (mainly Larsen trapping) under Natural England licence in one randomly selected treatment plot containing at least four kilometres of hedgerow. Corvid control operators caught crows and magpies in every removal plot but at no sites were crows or magpies completely removed. No corvids were removed from the other pair plot.

Productivity of hedgerow-nesting songbirds in each plot was measured using assessments of adult numbers and fledged brood counts without finding nests.


Summary of Findings

  • At 10 of the 16 sites in our study, overall songbird productivity was, to some extent, better in plots with corvid control

  • Corvid removal did not eliminate crows and magpies from any study plots, so complete removal may lead to a slightly larger effect

  • At none of our sites did uncontrolled crows and magpies eradicate hedgerow songbird nests

  • Our findings support the idea that best-practice corvid control can lead to a measurable overall improvement in the breeding output of farmland hedgerow birds.

The key finding was that overall nest success of the hedgerow-nesting songbird community was down by 10 per cent 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, in the other three years nest success was down 16% in the non-removal sites on average relative to removal sites.


Publications

Does best-practice crow Corvus corone and magpie Pica pica control on UK farmland improve nest success in hedgerow-nesting songbirds? A field experiment
Rufus B. Sage and Nicholas J. Aebischer
2017. Wildlife Biology wlb.00375

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