Northern Prairie Wildlife Research Center
Among habitat types, the greatest diversity occurred in lowland forest (55 species), and native prairie (51 species). Wheat and corn had the lowest species diversity, 18 and 3 species respectively. Those habitat types made up over 36% of the total habitat availability in the study area.
The mean density of all breeding birds within the study area was 110 per km2. The projected breeding bird population in the valley was about 3,100,000 breeding pairs. Investigations of total breeding bird populations over large areas have usually considered the population of a state (Stewart and Kantrud 1975; Graber and Graber 1963) or a country (Fisher 1940). Only Merikallio (1958) in Finland reported a lower mean breeding density (96 pairs/km2) than ours determined for the Platte River Valley. Mean breeding bird densities in Great Britain were estimated at 261 per km2 (Fisher 1940). Graber and Graber (1963) reported statewide densities in Illinois ranged from 200 to 227 pairs per km2 over several years. Stewart and Kantrud (1972) reported a statewide density of 143 pairs per km2 in North Dakota.
Distribution and extent of habitats, and probably climatic variability, influenced the population densities we observed. Particularly important was the amount of cropland available. Stewart and Kantrud (1972) censused 130 randomly selected 65-ha census plots in North Dakota. Of these, about 48% consisted mostly or entirely of summer fallow or cereal grain fields. Stewart and Kantrud considered the difference in breeding densities between their study and Illinois was related to the greater degree of habitat variability, particularly woodlands, that existed in Illinois. Our lower densities were also related to differences in habitat variability. Among the important aspects were the large extent of cropland, and the existence of the native grasslands in the Sandhills physiographic region. Native grasslands accounted for nearly 50% of the area, but breeding bird densities were relatively low.
Four orders accounted for 95% of the population. The Passeriformes were by far the most important group, contributing nearly 83% of the total, followed by Apodiformes, Piciformes, and Charadriiformes. In North Dakota, Stewart and Kantrud (1972) also reported that passerines were the dominant order, but that waterfowl and rails were among the two most common groups. In our area, waterfowl and rails combined accounted for 0.6% of the total population. These striking differences are related primarily to differences in habitat availability between the two areas. In North Dakota, wetlands made up 9.5% of the area censused, whereas in the Platte River Valley, wetlands contributed less than 0.3% of the area. Percentages of Charadriiformes in both areas were similar. The Platte River population of Charadriiformes was made up of five species; killdeer and upland sandpiper made up the bulk of the total.
Fifteen families made up 95% of the population (Table 7). The blackbirds and emberizine finches made up over 51% of the total. Five additional families (Passeridae, Tyrannidae, Apodidae, Picidae, and Sturnidae) made up an additional 26% of the population.
Thirty-five species made up about 95% of the total breeding bird population (Table 8). Among these, grasshopper sparrow, western meadowlark, and house sparrow made up over 26% of the total birds, each represented by over 240,000 breeding pairs. The common grackle and red-winged blackbird made up over 13% of the total population, each contributing over 190,000 breeding pairs. The remaining 30 species were represented by breeding populations of less than 150,000 pairs.