Measurement of the breeding success of birds is essential in studies of demographic processes but even if breeding success is reduced by predation this does not mean that breeding population size will be affected ( Newton 1998, Thompson 2007). There is however little direct evidence that predation by corvids alone can cause a significant reduction in passerine breeding success or that this might reduce recruitment and hence lower population size ( Gibbons et al. 2002, Stoate and Szczur 2006, White et al. 2010) and that targeting generalist nest predators locally (including mammals) can have benefits for non-game lowland farmland bird species ( Donald et al. There is also direct evidence from monitoring studies that crows and magpies can be predators of bird species that nest off the ground ( Stoate and Szczur 2005, White et al. ( 2000) showed that spatial variations in nest failure rates for blackbird and song thrush were explained most effectively by variation in corvid abundance. prey increases alongside increasing predators), three negative associations for species with magpie and no negative associations for crows. They found several positive associations (i.e. ( 2010) looked at long-term national monitoring datasets for correlations between spring abundance of 28 lowland farmland songbird species and crows and magpies. ( 2000) looked for relationships between NRS data and population trends in 12 farmland passerines and found that for only one, linnet Carduelis cannabina, was fledgling production supressed by increased nest predation. However they describe the limitations of a study based on ‘associations’. 2003) from 1966 to 1986 to show no decline in nest success for 15 songbird species, when magpies increased at a rate of 5% per year. ( 1991) used data from the UK Nest Record scheme (NRS, Crick et al. ( 2015) refer only to correlative studies. ( 2015) all report that there is a lack of good experimental studies in relation to corvids and passerines. Côté and Sutherland ( 1997), Gibbons et al. There have been several reviews and metaanalyses of previous studies of predator impacts on birds in the last 10 or 20 years. Protecting breeding farmland songbirds is also sometimes cited by game and other land managers as a reason to undertake crow and magpie control but the evidence supporting this practice is mixed. Carrion crow Corvus corone and magpie Pica pica control is often undertaken to reduce predation of ground-nesting game birds or waders because there is solid evidence of a predation impact on these groups ( Tapper et al. For species limited by nest success, it may be more important.Īssessing the potential for predator reduction to have a biologically significant impact on prey species is important for practical and ethical reasons. 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. Our data on hole nesters suggest that they were affected by treatment and contributed to our overall result. For open-cup nesting species as a group there was no difference in nest success between site types. 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. Nest success was down by 10% in non-removal sites on average relative to removal sites over the four years. 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. Eighteen songbird species were frequently encountered at most sites with on average (☑ SD) 102 ± 30 territories per site. Crows and magpies were still present at most removal sites but numbers were half as high as at paired non-removal sites. 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. We worked in southern England at 32 paired sites around 4 km 2 each, one with and one without best-practice corvid control, studying four different pairs per year for four years 2011–2014. Using a randomised-pair design, we measured how nest success of hedgerow-nesting passerines responded to the experimental removal of carrion crows and magpies. The role of predation by corvids on the breeding output of songbirds is unclear.
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