Published in:
Journal of Animal Ecology, 55 (1986) 2, Seite 463-480
Language:
English
ISSN:
1365-2656;
0021-8790
Origination:
Footnote:
Description:
<p>(1) In Eskdale, in south Scotland, the nesting population of sparrowhawks remained fairly stable during 1972-84; in nearby Annandale the population declined by 45% during 1971-80. In Eskdale land-use remained stable during this period, but in Annandale tree-felling and agricultural changes reduced nesting and hunting habitat. So breeding populations were ultimately limited by resources. (2) Annual survival of established breeders was the same in both areas, but recruitment of incoming breeders was lower in the declining population than in the stable one. This was the main proximate cause of decline; the two populations were similar in other respects. (3) In Eskdale, density-dependent processes prevailed, acting to stabilize the population around a mean of thirty-five pairs. Birds probably competed for prime home ranges, so that entry to the breeding population depended largely on gaps created by deaths or movements of previous breeders. This led to a density-dependent recruitment of new breeders, and hence a density-dependent loss of pre-breeders. Early spring weather modified the density dependence: cold wet conditions in February-April led to lower populations than expected from nest numbers the previous year. In these months, prey numbers reached their lowest level, and hunting was further impaired by rain. (4) Annual losses of established breeders were not density dependent, nor was production of young. Organochlorine pesticides had no obvious influence on the year-to-year changes in breeding numbers or performance during the period concerned. (5) Breeding performance was similar in the two areas and tended to fluctuate in parallel from year to year, apparently influenced by weather. When it was warm and dry during March-April, egg-laying in both areas was early, clutches were large, and overall production was high; but when it was cold and wet during this period, laying in both areas was late, clutches were small, and production was low.</p>