10 Oct 2012
Determining the cause of the Hen Harrier decline on Orkney

BRANTA — Arjun Amar

Determining the cause of the Hen Harrier decline on Orkney

Institution: University of Aberdeen, UK
Supervisors: SM Redpath, X Lambin & RW Summers (RSPB)
Details: PhD. 2001 (Completed)

Current Address: RSPB, The Lodge, Sandy, Beds, SG19 2DL (Oct 2005) Email

Subject Keywords: conservation, polygyny, Orkney vole
Species Keywords: Hen Harrier Circus cyaneus



The decline of the hen harrier population on Orkney began around the start of the 1980s. The number of adults in the population has declined and so has the breeding success of males. This lower breeding success was due to a decrease in the amount of polygyny, an increase in the failure rate of secondary females and an increase in the number of males which failed to breed at all. I tested the hypothesis that the decline of this population was due to a decrease in food supply during the pre-lay and incubation period.
Data on provisioning rates in Orkney supported this hypothesis. Males harriers in Orkney provisioned their females with food at a far lower rate, during both the pre-lay and incubation period, than males from another population with a higher breeding performance. Within Orkney, the likelihood of a male having a breeding female was positively associated with the rate at which he supplied food. The hatching success of incubating females was also positively associated with the rate at which they received food.
The experimental provision of supplementary food, proved that male breeding was limited by food supply during the early stages of breeding. Males provided with extra food had more breeding females than controls, which resulted from the combination of an increase in polygyny and a decrease in the number of non-breeding males. The sex ratio of the adult population was very similar to that found during the late 1970s when polygyny levels peaked. Thus, together with the results of the experiment, it appears that polygyny in this population is most heavily influenced by food, rather than by skewed sex ratios.
I also examined the evidence for an additional hypothesis that the decline in the population was due to a increase in predation. Hooded crows are the main egg predator of hen harriers in Orkney and although there was evidence to suggest that their numbers may have increased, experimental removal of crows had no detectable effect on harrier breeding success.
The strongest evidence therefore, was that the decline in the population had been caused by a decrease in the amount of food that males were able to supply to their females in spring. During the spring, the amount of time males spent hunting in an area was positively associated with the amount of unmanaged rough grass. This preference for areas with unmanaged grass was most likely attributable to the positive relationships that existed between this habitat variable and many of the important prey species in spring. Decreases in the amount of land used for rough grazing and an increase in the amount of land under pasture has probably decreased this habitat variable. Additionally, a doubling in the sheep density in Orkney over the last 20 years may have also decreased the amount of unmanaged grass.
Predicting the future of this population using demographic stochastic models suggested that if it were a closed population, then extinction of the breeding population might occur within the next 10-15 years. However, the evidence suggests that it is not an isolated population and that there may be more immigration than emigration. The lack of data on this aspect, however, makes it difficult to predict with any accuracy the future rate of the decline.
Management for this population should focus on increasing the abundance of food supplies for the males in the spring. Creation of areas with large amounts of unmanaged rough grass would appear to be the best way of achieving the objective and could potentially reverse the decline of this population.


Published Papers

Amar, A. & Redpath, S. 2005. Habitat use by hen harriers Circus cyaneus on Orkney: implications of land use change on this declining population. Ibis 147: 37-47.
Amar, A., Picozzi, N., Meek, E.R., Lambin, X. & Redpath, S.M. 2005. Decline of the Orkney Hen Harrier Circus cyaneus population: do changes to demographic parameters and mating system fit a declining food hypothesis? Bird Study 52: 18-24.
Amar, A., Redpath, S. & Thirgood, S. 2003. Evidence for food limitation in the declining hen harrier population on the Orkney Island, Scotland. Biological Conservation 111: 374-388.
Amar, A., Redpath, S., Lambin, X. & Meek, E. 2003. Could the hen harrier decline on Orkney be due to a shortage of food? In: Birds of Prey in a Changing Environment. (eds D. B. Thompson, S. M. Redpath, A. Fielding, M. Marquiss & C. A. Galbraith). The Stationery Office, London.
Amar, A. & Redpath, S. 2002. Determining the cause of the hen harrier decline on the Orkney Islands: an experimental test of two hypotheses. Animal Conservation 5: 21-28

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