Western Marsh Harrier (Circus aeruginosus) Science Article 4
The level of predation by a pair of Marsh Harriers Circus aeruginosus on ducklings and gamebird chicks was calculated from observations of hunting ranges, feeding rates and diet, and estimates of the availability of prey during the breeding season. The harriers took 22% of the Pheasant chicks 4.2% of the Grey and Red-legged Partridge chicks and 25.3% of the Mallard ducklings, which hatched within their hunting territories. Many of the chicks taken by the harriers would otherwise have died before reaching maturity. It was estimated that an additional 10.2% of Pheasant chicks, 0.7% of Grey and 1.1% Red-legged Partridge chicks and 11.1% of Mallard chicks would have survived to fledging in the absence of harrier predation according to calculations on survival rates of those chicks not taken by harriers. However, mean fledged brood sizes of the four prey species were similar to those recorded elsewhere, where harriers did not breed. The conclusion is that predation by the harriers had little effect on final numbers of gamebirds and ducks.
Underhill-Day J.C., ARDEA 77 (1): 47-56.