ABSTRACT
Circus cyaneus hudsonius lives in North America. Direct observation with binoculars or spotting scopes on flyways or on wintering grounds produces useful data on relative numbers, but misses on age and sex composition. The slate gray, or almost white plumage of adult males, is easy to spot. Brown harriers, however, may be adult females or immatures of either sex. The most meaningful census is obtained by trapping and color-marking, especially in the breeding territories. At high densities, harriers are semicolonial, polygynous, and immature males tend to breed. Marking about half the breeders on a study area seems to suffice for individual identification for census. During the nest-brood season (which spreads from late May to mid August in Wisconsin), adult harriers of both sexes defend their nests against live or stuffed great horned owls, Bubo virginianus.
