A Review of the Impacts of Crows, Ravens and Eurasian Magpies on Bird Productivity and Abundance. MADDEN, C.F. , ARROYO, B
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1 Running head: Impacts of crows, ravens and magpies 2 A review of the impacts of crows, ravens and Eurasian Magpies on bird productivity 3 and abundance. 4 MADDEN, C.F.1, ARROYO, B.2,3 and AMAR, A.1* 5 1University of Cape Town, Percy FitzPatrick Institute of African Ornithology, DST/NRF 6 Centre of Excellence, University of Cape Town, Rondebosch 7701, South Africa 7 2Instituto de Investigación en Recursos Cinegéticos, IREC (CSIC-UCLM-JCCM), Ronda de 8 Toledo s/n, 13005 Ciudad Real 9 3 Centre D´Etudes Biologiques de Chizé (CEBC-CNRS), Villiers en Bois 79360. France 10 *Corresponding author. 11 Email: [email protected] 12 13 Corvids are often viewed as efficient predators capable of limiting prey species populations. 14 Despite this widely held belief, a comprehensive review quantifying the effect of corvids is 15 lacking. We examine the impacts of crows, ravens and Magpies (Corvus spp. and Pica pica) 16 on population parameters of other bird species. We summarise results from 42 studies 17 (arising from a systematic search in Web of Science), which included 321 cases (i.e. explicit 18 evaluations of corvid-target species relationships). Population parameters of studied target 19 species were pooled as “abundance-related” (numbers, nest density, etc.) or “productivity- 20 related” (nest success, brood size, etc, as an estimate of post-breeding abundance). 21 Information was examined from both experimental removal studies, and correlative studies 22 assessing relationships between corvid abundance and target species´ parameters. Combining 23 all studies, no negative influence of corvids on either abundance or productivity of target 24 species was found in the vast majority of cases (82%). Negative impacts were significantly 25 more likely in cases examining productivity rather than abundance (49% vs 8%). 26 Experimental studies which removed only corvid species were significantly less likely to 27 show a negative impact on productivity than those removing corvids alongside other 28 predators (14% vs 63%). This suggests that the impact of corvids is smaller than that of other 29 predators, or that compensatory predation occurs. The impact of corvids was similar between 30 different avian groups (such as waders, passerines and gamebirds; or ground-nesting and non- 31 ground nesting species). Crows were found to be significantly more likely to have a negative 32 impact on target species productivity than Magpies (58% vs 20%), but no differences were 33 found in relation to abundance. We conclude that whilst corvids can have a negative impact 34 on bird species, their impact was nearly 6 times more frequent for productivity than for 35 abundance. These results therefore suggest that in the vast majority of cases birds species are 36 unlikely to be limited by corvid predation pressure and therefore that conservation measures 37 may generally be better targeted at other limiting factors. However, negative impacts still 38 existed in certain cases, and those may require further investigation to develop management 39 tools to mitigate them where they are of economic or conservation concern. 40 Keywords: Corvus, Pica pica, predation, systematic review, ground-nesting species, predator 41 removal, correlation, experiment, game management. 42 43 Predator control is often implemented to boost breeding performance and population 44 densities of target species, either for game management or conservation purposes (Butchko & 45 Small 1992, Meckstroth & Miles 2005). This practice is based on the assumption that 46 predation is a limiting factor in prey populations, and that predator control results in 47 population increases (Holt et al. 2008). However, despite the widespread use of predator 48 control, its efficacy in enhancing species’ populations is still contentious (Mcdonald & Harris 49 2002, Park et al.2008). The impacts of predator removal on prey populations may be 50 complicated by compensatory predation, where predation from one predator is replaced by 51 another, or by mesopredator release, when removal of one predator leads to increased 52 abundance of another (Palomares et al. 1995, Courchamp et al.1999). Additionally, the goals 53 and objectives of predator control may differ depending on the management objectives. For 54 example, game management is primarily concerned with post-breeding abundance (i.e. 55 abundance at the start of the hunting season), whereas conservation actions are typically 56 aimed at enhancing breeding population densities (Gibbons et al. 2007). Predator control may 57 have effects on one of these parameters but not the other. Coupled with this, predator control 58 through lethal removal of predators is controversial, potentially socially unacceptable, time- 59 consuming, difficult, expensive and may only have short-term results (Ivan et al.2005, Shwiff 60 et al.2005, Valkama et al.2005, Smith et al.2010a). Given these factors, and the limitation of 61 resources for either management or conservation, it is important to evaluate the efficacy of 62 such management tools and allocate resources to management options that produce the 63 desired results (O’Connor 1991). 64 Several reviews have explored whether predator control is effective in reducing predation or 65 increasing breeding populations of target species (Côté & Sutherland 1997, Newton 1998, 66 Gibbons et al. 2007, Holt et al. 2008, Smith et al. 2010b). Côté and Sutherland (1997) 67 conducted a meta-analysis on the effectiveness of predator removal to enhance bird 68 populations, and concluded that predator removal improved hatching success and increased 69 post-breeding population sizes by 75%. However, they found that predator removal did not 70 generally result in subsequent increases in breeding population sizes. Thus, they concluded 71 that although it might be useful for game-managers, it was unlikely to be effective in the 72 conservation of declining species. Smith et al. (2010b) updated this review using many more 73 recently published studies; they too concluded that predator removal enhanced hatching and 74 fledging success. However, in contrast to Côté and Sutherland (1997), they found that 75 predator removal did not necessarily increase post-breeding population size, but that it did 76 enhance subsequent breeding population size. Nordström (2003) reviewed studies of ground- 77 nesting birds in the Baltic and again found that predator removal resulted in a general 78 increase in nest success, post-breeding population size and breeding population size. Holt et 79 al. (2008) also conducted a meta-analysis on studies in the UK and found an average 1.6-fold 80 increase in prey breeding population size following predator removal. These authors also 81 found that removing multiple predator species had a much stronger effect than removing a 82 single species, an idea previously suggested by Newton (1998), who postulated that removing 83 single predator species was less effective due to compensatory predation. Additionally, 84 Newton (1993) suggested that the species studied tend to be biased toward ground-nesting 85 birds, which may be particularly prone to predation, and therefore may be more likely to 86 respond to predator control. 87 Within the northern hemisphere, and particularly northern and western Europe, management 88 of generalist predators most frequently involves the control of Red Fox Vulpes vulpes and 89 corvids, both of which are common and usually unprotected (Parker 1984, Tapper et al.1996, 90 Draycott et al. 2008, Bodey et al. 2009), but whether the potential efficacy of predator 91 control relates to both carnivores and corvids has not been evaluated. Corvid species are also 92 frequently identified as major nest predators of bird species of conservation concern (Andrén 93 1992, Soderstrom et al.1998, Baláž et al.2007, Klausen et al.2009). Yet, despite a plethora of 94 studies and experiments on corvid predation, their overall impact on other birds remains to be 95 assessed. Because corvids are diurnal and conspicuous nest predators, their importance in 96 prey population regulation is often assumed prior to any assessment of the evidence 97 (Marzluff & Angell 2005). There are several biological reasons why corvids may have a 98 negative impact on birds and could be a conservation or management problem. They are 99 adaptable, opportunistic, generalist predators with high cognitive abilities. Yet, further 100 problems may result if corvid management decisions are based on misconceptions or poorly 101 substantiated conclusions about their ecological impact on prey populations (Amar et al. 102 2010). 103 In this study we therefore undertake a systematic literature review to explore the relationships 104 between corvids and populations of their target species, and investigate if the causes for 105 concern are established within the scientific literature. Within this review we examine the 106 impacts of corvids on various prey population parameters related to both productivity and 107 breeding population size, and use information from both removal experiments and correlative 108 studies. 109 110 METHODS 111 Literature survey 112 The scope of the study included species of the genus Corvus and the Eurasian Magpie Pica 113 pica. The Corvus genus includes a third of all corvid species, and most of them are notorious 114 nest predators (dos Anjos 2009). We also included the Eurasian Magpie (hereafter Magpie) as 115 they are common and widespread nest predators, and they are one of the main targets of legal 116 predator control