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, 21-28.


Despite legal protection, the hen harrier Circus cyaneus is a scarce bird of prey in the UK. At the start of the 20th Century, the hen harrier was virtually extirpated as a breeding species on mainland Britain due to human persecution. Although legally protected, hen harriers continue to be persecuted mainly because they have the potential to reduce populations of a highly valued gamebird, the red grouse Lagopus lagopus scoticus.

During the period of virtual extinction on mainland Britain, the Orkney Islands acted as an important refuge, as here they were not and continue not, to be persecuted. However, despite this, since the 1970s hen harriers have declined by around 70%. The causes of the decline are not known with certainty, but it is thought that they may be linked to increased predation of eggs and young, or food shortage during the early breeding season. Hooded crows (Corvus cornix) are thought to be the primary predators responsible, as harriers are able to defend their nests against potential mammalian predators. Here the results of a study investigated the effects of hooded crow removal on breeding success are described.

Study site: The study was conducted on the west of Orkney Mainland island, where over 80% of the Orkney hen harrier Circus cyaneus population breeds.

Experimental design: The experiment was conducted during 1999 and 2000 during the hen harrier breeding season of early April to May. Two treatments were compared:

1) a removal group where hooded crows Corvus cornix were removed from nine harrier territories

2) a control group (comprising 26 male harriers) in which crows were not removed

Additional data from unmanipulated nests in 1998 were also used to increase the sample size of the control group.

Crow removal: Crows were captured and removed from territories using Larsen traps placed within 500 m of the centre of each territory, baited with dead day-old chicken Gallus domesticus chicks and captured live decoy crows. The effectiveness of crow removal was established by assessing predation rates on three artificial clutches of three chicken eggs placed within the territories.

Monitoring breeding success: All hen harrier territories were watched throughout the pre-lay and laying period to count numbers of females associated with males (hen harriers are polygynous) and to find nests. Females were individually marked and males were counted by synchronous observations in the presumed territory and adjacent areas, with multiple observers communicating with short-wave radios. The lay date of the first egg, clutch size and hatching success were determined by nest visits. In some instances, lay dates were inferred from hatching dates, from chick ages estimated by wing-length measurements or from the ratio of egg weight to egg dimensions.

Crow numbers & removal: In total 56 hooded crows were removed from nine hen harrier territories. Artificial chicken clutches survived significantly better in territories in which crows were removed (12 out of 18) than on territories in which no removal was undertaken (two out of 18).

Effects on harrier breeding success: Crow removal had no effect on the average number of breeding female harriers associated with each male. Although the mean clutch size was slightly higher in territories in which crows had been removed, when compared to control territories the difference was not significant. Similarly, laying dates and hatching success were unaffected by crow removal.

Conclusions: The removal of hooded crows had little effect on hen harrier breeding productivity as no impact on degrees of polygynous breeding, lay dates, clutch sizes or hatching success were noted. It appears more likely that other factors, such as food availability (see Case 312) are the determinant of hen harrier breeding success on the Orkney Islands.

Note: If using or referring to this published study, please read and quote the original paper. Please do not quote as a case as this is for previously unpublished work only.

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