Bird Census Data Do Not Indicate a Lack of Impact on Songbirds From the Growth of Avian Predator Populations in Britain in the Late 20th Century

Christopher Paul Bell

doi:10.3389/fevo.2020.00277

Abstract

The possible role of avian predators in limiting songbird populations has been largely discounted since the publication of findings showing a lack of statistical association in UK bird census data between changes in prey species populations and those of a range of predatory species, including raptors and corvids. I re-applied the methodology behind these findings, covering a wider range of prey species and using site-level modelling to estimate predator abundance instead of a mixture of spatial modelling and raw count data. A significant aggregate predator effect was found in 33 out of 40 prey species, compared to only 10 out of 27 in the original study, as well as a higher rate of significant individual predator effects, with 41 significantly negative and 84 significantly positive effects out of a total of 320. The greater explanatory power of predator variables estimated using site-level modelling suggests that this has significant advantages over the use of predator variables derived from spatial modelling, which may not capture variation in predator abundance at a local scale, or from raw count data, which may lead to attenuation of effect estimates. The prevalence of positive associations between predators and prey is consistent with a common response to local habitat variation, which may absorb negative covariance resulting from the impact of predators on prey populations. Both positive and negative predator-prey associations may also occur as a result of independent demographic processes that manifest as sequential habitat occupation or withdrawal. Analyses of census data cannot discriminate among these possible scenarios and may therefore have limited value in determining whether predators have been limiting prey populations. Inference to a lack of impact of avian predators on prey populations from such analyses may therefore be unsafe, and a role for increased predator numbers remains a viable hypothesis with respect to bird population declines. The recent neglect of this possibility should therefore be urgently reversed, with a particular need for field experiments that can support strong inference regarding population limitation of songbirds by avian predators.

Highlights

Conservation management of wild populations takes place against a background of controversy regarding the basis of their persistence and stability
Support for this position has emerged from investigations that use data from the British Common Birds Census (CBC) to test for the presence of relationships between local population trends and changes in the local status of predators during the main period of decline in the late 20th century (Thomson et al, 1998; Newson et al, 2010), which found very little to suggest a significant impact of a range of increasingly abundant predator species
Studies focusing on data from the Garden Bird Feeding Survey (GBFS) have found more evidence of impact (Chamberlain et al, 2009; Bell et al, 2010; Swallow et al, 2016a,b, 2019; Jones-Todd et al, 2018), but since this refers to numbers at winter feeding stations it permits less certain inference regarding impacts on breeding populations

Summary

Introduction

Conservation management of wild populations takes place against a background of controversy regarding the basis of their persistence and stability. Bottom–up regulation forms the usual context for explanation of such declines, which cites changes in farming practices and consequent decline in food supply as the ultimate cause, while discounting the possible role of increasing predator populations (Gibbons et al, 2007) Support for this position has emerged from investigations that use data from the British Common Birds Census (CBC) to test for the presence of relationships between local population trends and changes in the local status of predators during the main period of decline in the late 20th century (Thomson et al, 1998; Newson et al, 2010), which found very little to suggest a significant impact of a range of increasingly abundant predator species. Studies focusing on data from the Garden Bird Feeding Survey (GBFS) have found more evidence of impact (Chamberlain et al, 2009; Bell et al, 2010; Swallow et al, 2016a,b, 2019; Jones-Todd et al, 2018), but since this refers to numbers at winter feeding stations it permits less certain inference regarding impacts on breeding populations

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Conclusion