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The population ecology of buzzards Buteo buteo in the New Forest, southern England, was studied from 1961 to 1982. Until 1973 the population was relatively constant (32–37 pairs), though density and productivity were low compared with elsewhere in Britain, probably as a result of low densities of mammalian prey resulting from intensive grazing by free-ranging cattle and ponies. In 1973 there was a dramatic fall in productivity: only two pairs fledged young. Comparing 1962–1971 with 1973–1982, the average number of breeding attempts declined by 35%, the average number of succesful attempts by 44%, and the average number of young fledged by 40%. The number of occupied territories fell from 33 in 1974 to 21 in 1982 (a 36% decline). It is hypothesised that this was the result of further declines in numbers of small rodents caused by greatly increased grazing pressure by large domestic herbivores, and their penetration into hitherto ungrazed areas. The number of successful breeding attempts varied widely from year to year after 1973. From 1968 the number of successful breeding pairs of buzzards fluctuated in relation to the amount of tree and shrub seed produced the previous autumn, and we suggest that from the early 1970s buzzards responded positively to early spring numbers of small rodents, which were in turn related to seed abundance in the previous autumn. It seems likely that before 1973 population levels of small rodents were sufficiently high even after a poor seed year, to form the basis of the buzzards' food in early spring, which is believed to be a critical period, and that only when population levels were reduced further by habitat deterioration caused by increased grazing pressure did fluctuations become critical to buzzards. The implications of intensive grazing for other predators of small rodents, and the implications for the management of the New Forest are briefly reviewed. |
