Northern Prairie Wildlife Research Center
We surveyed fields for breeding birds according to the procedures of Stewart and Kantrud (1972), which allow a fairly rapid assessment of the breeding bird community of a field. Each field was searched once each year by one or two observers on foot. All indicated breeding pairs were tallied, based on vocalizing males, females (for Brown-headed Cowbirds), pairs, or a nest. Surveys were made between dawn and midafternoon, avoiding precipitation or strong winds. Surveys were conducted during 22 May to 2 July 1990 and 28 May to 29 June 1991.
We obtained population trends for birds from Breeding Bird Survey (Robbins et al. 1986) results during 1966-1990 for the Central Region, roughly between the Rocky Mountains and the Mississippi River, and for the continent north of Mexico. Trends were based on methods described by Geissler and Sauer (1990).
In 1990, we counted birds on 240 fields in the nine counties, for a total area of 4654 ha. In 1991 we surveyed 335 fields totaling 6181 ha; 190 of these fields, involving 3662 ha, had also been included in 1990. For the two years, field size ranged from 0.5 to 72.7 ha, with a median of 14.4 ha. Grasses and legumes were the most common plants on Conservation Reserve Program fields, but variation was considerable from county to county and among fields.