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.
Assessing the potential for predator reduction to have a biologically significant impact on prey species is important for practical and ethical reasons. 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. 1996, Summers et al. 2004, Bolton et al. 2007, Fletcher et al. 2010). 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. There have been several reviews and metaanalyses of previous studies of predator impacts on birds in the last 10 or 20 years. Côté and Sutherland (1997), Gibbons et al. (2007, Holt et al. (2008) and Madden et al. (2015) all report that there is a lack of good experimental studies in relation to corvids and passerines. Holt et al. (2008) include none and Madden et al. (2015) refer only to correlative studies.
Gooch et al. (1991) used data from the UK Nest Record scheme (NRS, Crick et al. 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. However they describe the limitations of a study based on ‘associations’.
Siriwardena et al. (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. Newson et al. (2010) looked at long-term national monitoring datasets for correlations between spring abundance of 28 lowland farmland songbird species and crows and magpies. They found several positive associations (i.e. prey increases alongside increasing predators), three negative associations for species with magpie and no negative associations for crows. Paradis 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.
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. 2008, Smith 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. 2002, Stoate and Szczur 2006, White et al. 2008, 2014, Aebischer et al. 2016). 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. 2007). 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).
In their review, Madden et al. (2015) suggested that rigorous corvid-only removal experimental studies were required to assess the biological significance of corvid predation on passerines. Other more general reviews of predators and birds stress the value of experimentally removing predators to see if the prey responds (Paradis et al. 2000, Nicoll and Norris 2010). This has been done for a few sedentary or philopatric ground-nesting birds such as lapwing Vanellus vanellus (Bolton et al. 2007), grey partridge Perdix perdix (Tapper et al. 1996) and moorland waders (Fletcher et al. 2010). For passerines there are essentially no data of this kind.
Here we present the results of a four-year experimental study based on a randomised pair design, with corvid control as the treatment. We measured the effectiveness of the corvid control and compared songbird nest success between the treatment (corvid removal) and an untreated ‘control’ site in each pair. To avoid b