Determining the cause of hen harrier decline on the Orkney Islands: an experimental test of two hypothesis

Published source details 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.
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This study is summarised as evidence for the following.

	Action	Category
	Control predatory mammals and birds (foxes, crows, stoats and weasels) Action Link
	Provide supplementary food for birds or mammals Action Link
	Control avian predators on islands Action Link
	Provide supplementary food for raptors to increase reproductive success Action Link

Control predatory mammals and birds (foxes, crows, stoats and weasels)

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.
Provide supplementary food for birds or mammals

A randomized, replicated and controlled study in heathland on Orkney Mainland, Scotland, in 1999-2000 (Amar & Redpath 2002) found that male hen harriers Circus cyaneus provided with supplementary food (chicken Gallus domesticus chicks and quarter pieces of European rabbit Oryctolagus cuniculus or brown hare Lepus europaeus) bred with significantly more females than control (unfed) males (100% of 11 fed males mated and 36% mated with more than one female vs 80% of nine unfed males mated, 11% mated with more than one female). There was no effect of feeding on clutch size or hatching success (average of approximately 5.1 eggs/clutch for 13 fed clutches vs 4.7 eggs/clutch for four unfed clutches), but productivity still increased. Hooded crows Corvus cornix were also removed from all territories.
Control avian predators on islands

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’.
Provide supplementary food for raptors to increase reproductive success

A randomised, replicated and controlled study in heathland on Orkney Mainland, Scotland, in 1999-2000 (Amar & Redpath 2002) found that male hen harriers Circus cyaneus provided with supplementary food (chicken Gallus domesticus chicks and quarter pieces of European rabbit Oryctolagus cuniculus or brown hare Lepus europaeus) bred with significantly more females than control (unfed) males (100% of 11 fed males mated and 36% mated with more than one female vs. 80% of nine unfed males mated, 11% mated with more than one female). There was no effect of feeding on clutch size or hatching success (average of approximately 5.1 eggs/clutch for 13 fed clutches vs. 4.7 eggs/clutch for four unfed clutches), but productivity still increased. Hooded crows Corvus cornix were also removed from all territories, discussed in ‘Control avian predators on islands’.