Environmental Conservation 34 (1): 1–11 © 2007 Foundation for Environmental Conservation doi:10.1017/S0376892907003554 Poor evidence-base for assessment of windfarm impacts on birds GAVIN B. STEWART, ANDREW S. PULLIN ∗ AND CHRISTOPHER F. COLES Centre for Evidence-Based Conservation, School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Date submitted: 3 May 2006 Date accepted: 5 December 2006 First published online: 14 February 2007 SUMMARY Concerns about anthropogenic climate change have resulted in promotion of renewable energy sources, especially wind energy. A concern raised against widespread windfarm development is that it may negatively impact bird populations as a result of bird collision with turbines, habitat loss and disturbance. Using systematic review methodology bird abundance data were synthesized from 19 globally-distributed windfarms using meta-analysis. The effects of bird taxon, turbine number, power, location, latitude, habitat type, size of area, time since operation, migratory status of the species and quality of evidence were analysed using meta-regression. Although the synthesized data suggest a significant negative impact of windfarms on bird abundance, there is considerable variation in the impact of individual windfarm sites on individual bird species, and it is unclear if the negative impact is a decline in population abundance or a decline in use owing to avoidance. Anseriformes experienced greater declines in abundance than other taxa, followed by Charadriiformes, Falconiformes and Accipitriformes, and Passeriformes. Time since windfarms commenced operation also had a significant impact on bird abundance, with longer operating times resulting in greater declines in abundance than short operating times. Other variables, including turbine number and turbine power either had very weak but statistically significant effects or did not have a significant effect on bird abundance. Windfarms may have significant biological impacts, especially over longer time scales, but the evidence-base is poor, with many studies being methodologically weak, and more long-term impact assessments are required. There is clear evidence that Anseriformes (wildfowl) and Charadriiformes (waders) experience declines in abundance, suggesting that a precautionary approach should be adopted to windfarm development near aggregations of these taxa in offshore and coastal locations. The impact of windfarm developments on bird populations must also be viewed in the context of the possible impact of climate change in the absence of windfarms. * Correspondence: Dr Andrew Pullin Tel: +44 121 414 7147 Fax: +44 121 41 45925 e-mail: A.S.Pullin@bham.ac.uk Keywords: birds, environmental impact assessment, environ- mental policy, meta-analysis, renewable energy, systematic review INTRODUCTION The United Nations Framework Convention on Climate Change calls for stabilization of greenhouse-gas concentra- tions in the atmosphere to prevent dangerous anthropogenic interference with the climate (United Nations 1992). Cost- effective, carbon-emission-free technologies are required if this is to be achieved (Hoffert et al. 1998). Wind energy is a key element of the shift to carbon-emission-free energy, with a yearly growth rate of 30%, making it the fastest growing energy technology in the world (American Wind Energy Association 2003). However, despite the clean image of wind energy, windfarm developments may have deleterious environmental impacts (Coles & Taylor 1993; Woods 2003). In particular, attention has been brought to possible impacts on bird populations (Gill et al. 1996; Percival 2001; Langston & Pullan 2003; Garthe & H¨ uppop 2004, Barrios & Rodriguez 2004). Reported instances of habitat loss, collision mortality, displacement, disturbance and impeded movement between feeding, roosting, breeding and moulting areas all have potentially adverse impacts (Gill et al. 1996; Percival 2001; Langston & Pullan 2003; Garthe & H¨ uppop 2004; Langston et al. 2006), but it is unclear if these factors lead to reductions in survival or breeding productivity and ultimately to declines in the long-term abundance of bird populations. Here we formally synthesize evidence to test the hypothesis that windfarms reduce the abundance of birds in their vicinity either by displacement or population decline using a systematic review methodology, established in medicine (Khan et al. 2001) but rarely applied to ecological phenomena (Pullin & Knight 2001; Sutherland et al. 2004; Stewart et al. 2005) despite the widespread use of meta-analysis (Hedges & Olkin 1985; Arnqvist & Wooster 1995; Osenberg et al. 1999; Gurevitch & Hedges 1999, 2001; Gates 2002). METHODS Systematic reviews locate data from published and unpublished sources, critically appraise methodology and synthesize evidence to provide empirical answers to scientific research questions. They differ from conventional reviews in that they follow a strict methodological and statistical protocol making them more comprehensive, minimizing the