Does supplementary feeding reduce predation of red grouse by hen harriers?

Abstract

1. Hen harriers Circus cyaneus can reduce the numbers of red grouse Lagopus lagopus scoticus available for shooting. We conducted a supplementary feeding experiment on Langholm Moor, UK, in 1998 and 1999 to determine whether feeding hen harriers could reduce the numbers of red grouse killed. The experiment was done at two distinct stages of the breeding cycle: prior to incubation (spring experiment) and after hatching (summer experiment). In spring, Langholm Moor was divided into two areas, one with food and one without. In summer a number of birds were provided with food in both areas.
2. Providing harriers with food in spring had no significant effect on the breeding density of males or females, although feeding was associated with an increase in density on one area in one year. In addition, over the 2 years of the experiment, there was no evidence that feeding led to more chicks returning to breed in subsequent years. Fed harriers had larger clutches but did not lay earlier than unfed birds.
3. A minimum of 78% of the radio-tagged grouse that were killed during spring were killed by raptors. The mortality rates of adult grouse did not differ between the two areas or between the two years despite the availability of supplementary food and the large differences in harrier breeding density between areas. We infer that other raptors were responsible for much of the predation of adult grouse.
4. During the nestling period, female harriers took supplementary food at a higher rate than males. Females that were fed during the spring took more supplementary food in summer than those fed only during summer. Fed birds did not deliver more food overall to nests than those not provided with food.
5. Both male and female harriers at nests where supplementary food was available caught grouse chicks at a lower rate than harriers at nests not provided with food. For both years combined, fed harriers delivered on average 0.5 grouse chicks to their nests per 100 h, compared with 3.7 grouse chicks delivered to nests without supplementary food.
6. We estimated that feeding all harriers at Langholm would cost approximately £11,000 per annum. In both 1998 and 1999, the numbers of grouse chicks lost were 10 times higher than expected from harrier predation rates. Some other, unknown, factor had a strong influence on grouse chick survival in these years. Feeding some of the breeding harriers did not lead to an increase in grouse density at Langholm.
7. The results suggest that supplementary feeding may provide a useful tool in reducing the number of grouse chicks taken by harriers. Further experiments are now necessary to see under what conditions this reduced predation will lead to increases in grouse density.