Population and Breeding Biology of Marsh Harriers in Britain Since 1900

Abstract

Following recolonization in 1911, the number of breeding marsh harriers C. aeruginosus (L) in Britain was low with between 1 and 5 known nests/annum from 1911 to 1944, slowly increasing to 15 known nests by 1958, but declining to a single pair by 1971. Subsequently there was a rapid increase to 25 nests in 1982. There was a highly significant correlation between changes in the number of breeding adults and summering (i.e., non-breeding) birds in the previous year, but no correlation with the number of fledged young 2 or 3 years earlier. Of all nesting attempts, 76% produced fledged young, with no significant differences between the mean number of young produced from first nests and renests, but a significant trend towards larger fledged brood sizes from nests of polygynous males. The 2 most important reasons for nest failure were desertions and human egg cellectors, but percentage nest failure has shown a downward trend over the last 45 years. Larger clutches produced more fledged young on average, and were less likely to fail, but the expectation of sucess per egg in successful clutches was higher for smaller clutch sizes. sizes. Possible causes of the rapid population decline during the late 1950''s and 1960''s were examined, including the effects of coypu, habitat loss, and organochlorine pesticides. Only the latter were shown to have had an effect on breeding harriers, with a significant reduction of 10% in eggshell thickness since 1948. The most likely cause of the initial rapid increase in numbers during 1972-1982 was recruitment from abroad, particularly from the large population in the Netherlands.