Action: Control avian predators on islands
- Out of 10 studies, six before-and-after studies from North America, Australia and Europe found that controlling avian predators led to increased population sizes, reduced mortality or increased reproductive success in seabirds on islands. The North American studies had several interventions, so increases could not be linked directly to predator control, and one found that increases were only at one of two sites studied.
- Two controlled studies in Europe found little evidence that crow control led to increased reproductive success in gamebirds or raptors on islands. A North American study found that, despite higher reproductive success, very few birds returned to the study site after predator removal. A study from North America found that an Atlantic puffin Fratercula arctica translocation programme, combined with the culling of predatory gulls, appeared to be successful.
- A study from the UK found that the number of common terns Sterna hirundo and black-headed gulls Larus ridibundus declined on gravel islands despite the attempted control of large gulls.
Invasive and introduced avian predators can be problematic: islands with more introduced birds have lost more native bird species (Case 1996), but we have captured no evidence of the effects of their control or eradication. However, eight studies describe the effects of controlling or removing native predators on target bird populations.
The control of invasive and introduced competitor species is discussed in ‘Reduce competition with other species for nests sites’ and ‘Reduce competition with other species for food’.
Case, T.J. (1996) Global patterns in the establishment and distribution of exotic birds. Biological Conservation, 78, 69–96.
Supporting evidence from individual studies
A before-and-after study on an island in Lake Onatario, Canada (Morris et al. 1980) found that the fledging success of common terns Sterna hirundo was significantly higher in May and June 1976 (0.44 chicks/egg laid for 66 eggs) when ring-billed gull Larus delawarensis nests were destroyed and vegetation manually removed from the site, than in May and June 1975, when no gull removal was used (0.18 chicks/egg laid for 217 eggs). Fledging success was still higher if late-laid eggs (laid after 4th June 1976) were included in the analysis, although all of these died. Despite the increases, only three pairs of terns returned to the site in 1977.
A controlled study on Karlsøy Island (7.7 km2), Norway, in 1978-81 (Parker 1984), found that removing hooded crows Corvus cornix and ravens Corvus corax from an experimental area did not decrease predation of black grouse Tetrao tetrix nests, compared to control areas (49 nests studied). Predation of willow ptarmigan Lagopus lagopus nests was lower in the first year of the experiment (21 nests studied) but not in the next three (total of 214 nests). The author suggests that compensatory predation by ermine Mustela erminea may have prevented corvid removal from having an effect. Corvids were removed by poisoning with alpha-chloralose-treated eggs and shooting.
An Atlantic puffin Fratercula arctica translocation programme in 1973-81 (Kress & Nettleship 1988) found that almost all of the 774 puffin nestlings moved from Newfoundland, Canada, to Maine, USA, survived. In 1974-5 herring gulls Larus argentatus and great black-backed gulls L. marinus were culled and their nests destroyed during 1974-5 in an attempt to reduce predation. This study is discussed in ‘Translocate individuals’.
A before-and-after studies during 1977-89 at a common tern Sterna hirundo colony in Lake Ontario, Canada (Morris et al. 1992), found the nesting population increased at one colony but decreased at another following several interventions, including the control of particular ring-billed gulls L. delawarensis that were heavily predating tern eggs. This study is discussed in ‘Replace nesting substrate following severe weather’.
A before-and-after study on Cabbage Tree Island, Australia, between 1992 and 1994 (Priddel & Carlile 1995) found that fewer Gould’s petrels Pterodroma leucoptera leucoptera were killed by pied currawongs Strepera graculina following intensive control at the start of the 1993-4 breeding season. Twenty petrels were killed by currawongs in October-November 1992 and 43 more were found (83% of all adult mortalities recorded) over the 1992-3 breeding season, despite the destruction of seven currawong nests. However, only four petrel mortalities (25% of the total) could be attributed to currawongs in 1993-4 following the killing of 22 currawongs, leaving a population of four to six individuals of which none bred. This study is also discussed in ‘Remove problematic vegetation’.
A randomised, replicated and 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 cornix (previously 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. This study is also discussed in ‘Provide supplementary food to increase reproductive success’.
A before-and-after study on two small islands in the Columbia River Estuary, Oregon, USA (Roby et al. 2002), found that an entire Caspian tern Sterna caspia colony (approximately 8,900 pairs) relocated from Rice Island to East Sand Island between 1999 and 2001. Several interventions were used to encourage movement including the culling of a ‘limited number’ of glaucous-winged-western gull hybrids (Larus glaucensis x L. occidentalis). Reproductive success was also higher on East Island. This study is discussed in detail in ‘Move fish-eating birds to reduce conflict with fishermen’.
A before-and-after study on the Isle of May, southeast Scotland over a 23 year period (1975-1998) (Finney et al. 2003) found that the breeding population of Atlantic puffins Fratercula arctica increased from 3,000 to approximately 19,000 breeding pairs during a period of gull control (1972-89). Adult herring gulls Larus argentatus and lesser black-backed gulls L. fuscus were culled and gull nests destroyed, reducing the population from 17,000 to 2,500 pairs and reducing nesting density by 30%-100%. Following the end of the control in 1989 (and an increase in gull population to 4,100 pairs), the puffin population continued to increase, reaching 42,000 pairs in 1998. Puffin recruitment between 1989 and 1998 was significantly higher in areas with low gull density, or that were maintained as gull-free through the destruction of nests.
A before-and-after study at a former gravel pit in Kent, England (Akers & Allcorn 2006), found that the number of common terns Sterna hirundo and black-headed gulls Larus ridibundus declined on gravel islands, despite attempts to remove the nests and eggs of large gulls (e.g. herring gulls L. argentatus) in the 1990s and early 2000s. This study is described in detail in ‘Reduce inter-specific competition for nest sites by removing or excluding competitor species’ and ‘Provide artificial nesting sites’.
A before-and-after study on Alborán Island (7 ha), southern Spain (Paracuellos & Nevado 2010), found that the population of Audouin’s gulls Larus audouinii increased from an average of 181 pairs in 1997-2000 to 626 pairs in 2009, following the control of yellow-legged gulls Larus michahellis from 2000 to 2009. Reproductive success also increased, from 0.3 chicks/pair in 1997-2000 to 0.25-0.9 chicks/pair in 2000-9. On average 106 adult yellow-legged gulls were culled each year, approximately 25% of the breeding population. Combined with the destruction of all eggs found, this reduced the population of gulls from 320-440 pairs in 1997-2000 to approximately 100 pairs in 2009.
- Morris R.D., Kirkham I.R. & Chardine J.W. (1980) Management of a declining common tern colony. The Journal of Wildlife Management, 44, 241-245
- Parker H. (1984) Effect of Corvid Removal on Reproduction of Willow Ptarmigan and Black Grouse. The Journal of Wildlife Management, 48, 1197-1205
- Kress S.W. & Nettleship D.N. (1988) Re-establishment of Atlantic puffins (Fratercula arctica) at a former breeding site in the Gulf of Maine. Journal of Field Ornithology, 59, 161-170
- Morris R.D., Blokpoel H. & Tessier G.D. (1992) Management efforts for the conservation of common tern Sterna hirundo colonies in the Great Lakes: two case histories. Biological Conservation, 60, 7-14
- Priddel D. & Carlile N. (1995) Mortality of adult Gould's petrels Pterodroma leucoptera leucoptera at the nesting site on Cabbage Tree Island, New South Wales. Emu, 95, 259-264
- 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
- Roby D.D., Collis K., Lyons D.E., Craig D.P., Adkins J.Y. & Myers A.M. (2002) Effects of colony relocation on diet and productivity of Caspian terns. The Journal of Wildlife Management, 66, 662-673
- Finney S.K., Harris M.P., Keller L.F., Elston D.A., Monaghan P. & Wanless S. (2003) Reducing the density of breeding gulls influences the pattern of recruitment of immature Atlantic puffins Fratercula arctica to a breeding colony. Journal of Applied Ecology, 40, 545-552
- Akers P. & Allcorn R.I. (2006) Re-profiling of islands in a gravel pit to improve nesting conditions for terns Sterna and small gulls Larus at Dungeness RSPB reserve, Kent, England. Conservation Evidence, 3, 96-98
- Paracuellos M. & Nevado J.C. (2010) Culling yellow-legged gulls Larus michahellis benefits Audouin's gulls Larus audouinii at a small and remote colony. Bird Study, 57, 26-30