Action: Control mammalian predators on islands for gamebirds
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A single replicated and controlled study on two Swedish islands found that four species of gamebirds had larger broods, and more females had chicks, when predators were controlled. Two of the species also showed population-level responses.
The effectiveness assessment of the control of mammalian predators on islands was carried out across all bird species groups.
Supporting evidence from individual studies
A replicated, controlled study on two islands (18 km2 and 23.5 km2) in the Gulf of Bothnia, Sweden between 1976 and 1984 (Marcstrom et al. 1988) found that gamebird brood sizes were significantly larger and a higher proportion of females had chicks over a four year period when predators were controlled, compared to when predators were not removed (with predator control: 5.5 chicks/brood, 77% of 378 hens had chicks; without predator control: 3.3 chicks/brood, 59% of 314 hens had chicks). . Species studied were capercaillie Tetrao urogallus, black grouse Tetrao tetrix, hazel grouse Bonasa bonasia and willow ptarmigan Lagopus lagopus, with adult capercaillie and black grouse counts increasing by 56-80% after predators had been controlled for two years, and counts at leks increasing by 166-174%. Predators (European pine martins Martes martes and red foxes Vulpes vulpes) were trapped and shot.