Northern Prairie Wildlife Research Center
One of the most reliable methods for estimating numbers of breeding avian predator species is intensive searches for nests (Fuller and Masher 1981). Study personnel recorded locations of occupied nests of American crows, great horned owls, and large hawks (northern harrier or larger) in each study area. Occupied nests contained eggs or young or were tended by adults (Postupalsky 1974). After 1983, this survey was expanded to include a systematic search for occupied nests of these avian predators (except northern harrier) of all wooded habitat in each study area during late May-early June. The search for nests of great horned owls was advanced to early April in 1987 in the small unit management study areas. This resulted in two complete searches of all wooded habitat in those study areas, one for nests of great horned owls and one for nests of other large raptors.
With these procedures we located nearly all occupied nests of American crows and large hawks, except northern harriers. We had little difficulty finding and determining occupancy of the large, open, deep-cupped nests of American crows (Bent 1964) and the bulky, stick-layered nesting platforms used by raptors (Bent 1961). Eggs and young of American crows could often be seen by looking into nests from above (we used mirrors on extendable poles to examine many nests). Incubating females of all species were often seen from the ground, and adults were usually seen defending nests, especially after eggs had hatched. Nests of northern harriers are usually inconspicuous in dense cover on the ground and in wetlands (Scaly 1967; Duebbert and Lokemoen 1977; Evans 1982; Bildstein and Gollop 1988). Thus, we conducted no systematic searches for nests of northern harriers. Instead, reported nests of northern harriers were found opportunistically, primarily during searches for duck nests (see Greenwood et al. 1987 and Johnson et al. 1987 for details about searches for duck nests). Ferruginous hawks also occasionally nest on the ground (Lokemoen and Duebbert 1976; Gilmer and Stewart 1983; Schmutz 1984), but such nests are conspicuous and usually on hills in open grassland areas (Stewart 1975; Gilmer and Stewart 1983).
Great horned owls usually nest in old nests of other species such as red-tailed hawks (Orians and Kuhlman 1956; Dunstan and Harrell 1973; Houston 1975; Petersen 1979; Bohm 1980), we had little difficulty finding the adopted nests because they offer little concealment to adults and young. However, nesting begins in late February (Stewart 1975) and some young have departed nests by early June (Bent 1961). Our systematic searches were conducted in 1985 and 1986.
We conducted no systematic searches for occupied nests of black-billed magpies. Although the large, woven-domed stick-nests of black-billed magpies (Bent 1964) were easy to find, the species sometimes builds false nests (Linsdale 1937) that may last for years. Moreover, whether a nest is occupied is often difficult to determine because eggs and young are deep inside and not easily observed and adults tending nests are secretive. We recorded locations of occupied nests that we found opportunistically.
We could seldom account for nests that failed before surveys began. Also, searches did not account for unmated and paired adults that did not nest. The proportions of birds without nests of some species, especially the great horned owl, in local populations probably varied and could have been substantial (Adamcik et al. 1978; Houston 1987).