Abstract
SUMMARY (1) The proportion of juveniles in a rook population in North-east Scotland declined in late summer, increased temporarily in autumn and rose again in spring. (2) The use of individually wing-tagged birds showed that these seasonal changes were caused by a combination of mortality, dispersal and return of locally-bred young, with a probable influx of migrant young in autumn. (3) The proportion of young among rooks in rookeries changed seasonally in parallel with changes among birds seen in feeding flocks, but the proportion in rookeries was always lower than that in fields. (4) Locally-bred young occurred earlier in their natal rookeries and appeared to stay there longer than did non-local (untagged) young. There was evidence that two rookeries differed in their attractiveness to young between years. (5) Young rooks which continued to visit a rookery until the end of the breeding season (May) and (especially) those which roosted there at night, were more likely to return there to breed in their second year than were those which were seen only in March and April. The factors influencing the choice of rookery in which to settle are not known.
Published Version
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