Action: Control predatory mammals and birds (foxes, crows, stoats and weasels)
- A total of nine individual studies from France and the UK (including five replicated controlled studies and a systematic review) looked at the effects of removing predators on birds. Three studies found controlling predatory mammals or birds (sometimes alongside other interventions) increased the abundance or population size of some birds. One of these studies from the UK found numbers of nationally declining songbirds increased on a site where predators were controlled, but there was no overall difference in bird abundance, species richness or diversity between predator control and no-control sites.
- Five studies (two replicated and controlled, two before-and-after trials) from the UK found some evidence for increased productivity, nest or reproductive success or survival of birds following bird or mammal predator control (sometimes alongside other interventions). A randomized, replicated, controlled study found hen harrier breeding success was no different between areas with and without hooded crow removal.
- A global systematic review including evidence from European farmland found that reproductive success of birds increased with predator removal.
This intervention involves controlling predatory mammals and birds, which may provide benefits to birds nesting on farmland. Predators commonly controlled include carrion crow Corvus corone and red fox Vulpes vulpes.
See also ‘Control mink’ for studies looking at the results of controlling non-native American mink Mustela vison.
Supporting evidence from individual studies
A replicated, controlled study in the spring of 1974 on a cereal farm in France (Westerskov 1977) found that grey partridge Perdix perdix were significantly more abundant in areas provided with ‘partridge cafeterias’ which included stoat Mustela erminea box-traps, than in control areas. A total of 48 pairs (1 pair/4.7 ha) and four single birds were recorded in the southern section of the farm (224 ha), where 27 partridge cafeterias had been constructed. The northern section (200 ha), with no cafeterias, had 24 pairs (1 pair/8.3 ha). As well as the stoat traps, cafeterias comprised a barrel with a feed mixture (grain and weed seeds), mouse Mus spp. traps, a mini-midden to provide maggots and insects, and a sand-bath, sheltered by a leaning roof that collected rainwater in a drinking trough.
A replicated, controlled study at two farmland and woodland sites in southern England between 1985 and 1990 (Tapper et al. 1996) found that grey partridge Perdix perdix breeding success and brood sizes were significantly higher when predators were controlled, compared to years without removal. This led to August partridge numbers being 75% higher and breeding numbers the next year being 36% higher. Over three years this led to breeding densities being 2.6 times greater when predators were removed. Predators removed through trapping and shooting were predominantly red foxes Vulpes vulpes, carrion crows Corvus corone and black-billed magpies Pica pica.
A study at three farmland sites in central England in 1992-1998 (Stoate & Szczur 2001) (partly the same study as (Stoate 2002) and extended in (White et al. 2008)) found that nest survival rates of four songbird species were negatively related to the breeding density of carrion crows Corvus corone following the control of nest predators. These species were Eurasian blackbird Turdus merula, song thrush T. philomelos, dunnock Prunella modularis and yellowhammer Emberiza citrinella. Non-significant negative relationships were also found for whitethroat Sylvia communis and chaffinch Fringilla coelebs nesting success and predator densities. Brown rats Rattus norvegicus, red foxes Vulpes vulpes, stoats Mustela erminea, weasels M. nivalis, carrion crows and black-billed magpies Pica pica were controlled through trapping and shooting. Between 151 and 951 nests of each species were studied.
A randomized, replicated, controlled study in 1999-2000 on Orkney Mainland, Scotland (Amar & Redpath 2002) found that the breeding success of hen harriers Circus cyaneus was no different in nine territories where hooded crows Corvus (Corone) cornix were removed, compared to territories without crow removal. The number of clutches/male, clutch size, hatching success and laying date were not affected, although experiments with artificial nests containing chicken eggs showed that predation had been reduced by crow removal (12 of 18 clutches surviving vs two of 18). A total of 113 crows were removed from the nine territories.
A before-and-after study between 1996 and 1998 at a farmland site in eastern England (Donald et al. 2002) found that daily survival rates of Eurasian skylark Alauda arvensis nests in non-rotational set-aside were significantly higher (96% daily survival for 168 nests) following the introduction of intensive control of mammalian predators than when predator control was either ‘light’ (95.6% survival for 51 nests) or absent (91% survival for 192 nests). There was no significant difference between light control and no control. These differences resulted in average overall survival rates of 41%, 23% and 12% for heavy, light and no control, respectively. The main species targeted were mustelids (Mustelidae), hedgehogs Erinaceus europaeus and red foxes Vulpes vulpes.
A small replicated, controlled study from May-June 1992-1998 in Leicestershire, England (Stoate 2002) (partly the same study as (Stoate & Szczur 2001) and extended in (White et al. 2008)) found that the abundance of nationally declining songbird species and species of conservation concern significantly increased over time on a 3 km2 site where predators were controlled. However there was no overall difference in bird abundance, species richness or diversity between the experimental and control sites. Numbers of nationally declining species rose by 102% (except for Eurasian skylark Alauda arvensis and yellowhammer Emberiza citrinella). Nationally stable species rose (insignificantly) by 47% (eight species increased, four decreased). The other interventions employed at the same site were: managing hedges, wild bird cover strips, beetle banks, supplementary feeding, and reducing chemical inputs generally.
A before-and-after study on a mixed farmland-woodland site in central England (Stoate & Szczur 2006) found that the fledging success of spotted flycatcher Muscicapa striata nests was significantly higher when predators (grey squirrels Sciurus carolinensis, brown rats Rattus norvegicus, red foxes Vulpes vulpes, black-billed magpies Pica pica and carrion crows Corvus corone) were controlled (77% for 11 nests in 1997-2001) than when there was no control (16% for 28 in 2002-2004).
A replicated, controlled trial at 13 lowland wet grassland sites in England and Wales between 1996 and 2003 (Bolton et al. 2007) found no overall increase in the success of 3,139 northern lapwing Vanellus vanellus nests during four years with predator control, compared to four years without. However, when differences in initial predator densities were accounted for, control did improve survival, having a greater impact at sites with higher predator densities. At two sites where predators were controlled for all eight years, nesting success was not significantly different from the 11 other sites. Predators were red fox Vulpes vulpes and carrion crow Corvus corone, with average declines of 40% for foxes and 56% for crows.
A before-and-after study on a mixed farm in central England (White et al. 2008) between 1992 and 2007 (a continuation of the data series used in (Stoate & Szczur 2001, Stoate 2002)), found that controlling predator populations (carrion crow Corvus corone, black-billed magpie Pica pica, red fox Vulpes vulpes and other mammals) appeared to increase blackbird Turdus merula breeding population. However, the authors caution that the study is not experimental and that other explanations for the trends seen cannot be eliminated.
A controlled study in 2002-2009 on mixed farmland in Hertfordshire, England (Aebischer & Ewald 2010) found that the number of grey partridge Perdix perdix increased significantly on an experimental site, where predators were controlled (along with several other interventions), but only slightly on a control site without predator control. This increase was apparent in spring (from fewer than 3 pairs/km2 in 2002 to 12 in 2009, with a high of 18 pairs/km2 on the experimental site, compared to approximately 1 pair/km2 on the control site in 2002, increasing to approximately 4 in 2009) and autumn (from fewer than 10 birds/km2 in 2002 to approximately 65 in 2009, with a high of 85 birds/km2, compared to approximately 4 birds/km2 on the control site in 2002, increasing to approximately 15 birds/km2 in 2009). Predators controlled were red fox Vulpes vulpes, stoat Mustela erminea, brown rat Rattus norvegicus, carrion crow Corvus corone and black-billed magpie Pica pica. The experimental site also had supplementary food provided and habitat creation.
A 2010 global systematic review covering habitats including European farmland (Smith et al. 2010) found that removing predators tended to lead to increased reproductive (hatching and fledging success) success and breeding populations in birds. On mainlands, but not islands, predator removal also tended to increase post-breeding population size. Whether predators were native or not, the population trend of the bird population and whether the species was migratory or a game species did not affect responses to predator removal.
- Westerskov K.E. (1977) Covey-oriented partridge management in France. Biological Conservation, 11, 185-191
- Tapper S.C., Potts G.R. & Brockless M.H. (1996) The effect of an experimental reduction in predation pressure on the breeding success and population density of grey partridges Perdix perdix. Journal of Applied Ecology, 33, 965-978
- Stoate C. & Szczur J. (2001) Could game management have a role in the conservation of farmland passerines? A case study from a Leicestershire farm. Bird Study, 48, 279-292
- Amar A. & Redpath S.M. (2002) Determining the cause of hen harrier decline on the Orkney Islands: an experimental test of two hypothesis. Animal Conservation, 5, 21-28
- Donald P.F., Evans A.D., Muirhead L.B., Buckingham D.L., Kirby W.B. & Schmitt S.I.A. (2002) Survival rates, causes of failure and productivity of skylark Alauda arvensis nests on lowland farmland. Ibis, 144, 652-664
- Stoate C. (2002) Multifunctional use of a natural resource on farmland: wild pheasant (Phasianus colchicus) management and the conservation of farmland birds. Biodiversity and Conservation, 11, 561-573
- Stoate C. & Szczur J. (2006) Potential influence of habitat and predation on local breeding success and population in spotted flycatchers Muscicapa striata. Bird Study, 53, 328-330
- Bolton M., Tyler G., Smith K. & Bamford R. (2007) The impact of predator control on lapwing Vanellus vanellus breeding success on wet grassland nature reserves. Journal of Applied Ecology, 44, 534-544
- White P.J.C., Stoate C., Szczur J. & Norris K. (2008) Investigating the effects of predator removal and habitat management on nest success and breeding population size of a farmland passerine: a case study. Ibis, 150, 178-190
- Aebischer N.J. & Ewald J.A. (2010) Grey Partridge Perdix perdix in the UK: recovery status, set-aside and shooting. Ibis, 152, 530-542
- Smith R.K., Pullin A.S., Stewart G.B. & Sutherland W.J. (2010) Effectiveness of predator removal for enhancing bird populations. Conservation Biology, 24, 820-829