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Yearly roost populations, temporal patterns of crop use, the relative importance of age-classes and sex classes in depredation, foraging dispersion, and individual movements of radio-equipped birds were assessed in a 3-year study of blackbird depredation in Simcoe County, Ontario. The population of roosting birds totalled 107 400 in 1977, 89 300 in 1978, and 118 200 in 1979 and appeared to be relatively stable, although the number of blackbirds using different flyways entering the roost varied. Sightings of colour-tagged birds indicated that the birds foraged over a wide area and moved from crop to crop depending on crop maturity. The movement pattern was not dependent on age or sex. Colour-tagged male red-winged blackbirds were sighted more frequently than females in field corn in 1978 but there was no difference in 1979. The earlier maturing of wheat may contribute to the lack of bird activity in standing wheat crops. Radiotelemetry studies indicated that there may be a considerable turnover of individuals foraging in a field over the term of a crop's susceptibility to depredation. Observations of starlings and monitoring of one radio-equipped starling suggested that this species may be a potential corn depredator. The relevance of results to management strategies for depredating red-winged blackbirds is discussed.

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