Northern Prairie Wildlife Research Center
The weather of the northern plains in spring can be harsh, and bird numbers and species increased only moderately over winter populations. The principal birds present were horned larks and transient species migrating to northern breeding grounds.
Minimum-tillage fallow fields may have been more attractive to birds in spring because these fields were usually unplowed and provided enhanced resources in the form of food and cover. Organic and conventional fallow and wheat fields in general had reduced value to birds in spring because seeds and plant cover was reduced the previous fall by tillage.
In summer, a wide variety of species occurred in crop fields, including species nesting in fields and species nesting elsewhere but foraging in fields. Although we used different survey techniques, the 70 species and 6 - 23 birds/10 ha that we observed in cropland in summer were similar to the 73 species and 12 pairs/10 ha counted in CRP fields in the northern Great Plains (Johnson and Schwartz 1993). Bird densities we observed in cropland were only one-third the density counted in nearby CRP fields in a study using similar survey techniques (R. R. Koford, Natl. Biol. Serv., Ames, Ia, unpubl. data).
Birds were probably attracted to fallow fields in summer because the fields contained the best plant cover and the greatest plant variety, primarily from crop residue of the previous year plus new plant growth. Fallow fields in our study area were treated with tillage, mowing, and chemicals to reduce weed populations and store moisture and nutrients. However, fallow field alterations often were not initiated until late May or June, after the peak period for nesting. In contrast, wheat fields and sunflower fields usually had little plant residue in early summer when birds began nesting due to prior fall and spring tillage to control weeds and to prepare the ground for the new crop. In Texas, Flickinger and Pendleton (1994) noted greater bird species diversity in summer in fields with reduced tillage.
Populations were highest in the cropland in fall, bolstered mainly by migrant blackbirds feeding in sunflower fields. We could not determine field and crop preferences in fall because birds were concentrated in large flocks. A few species, such as horned larks, snow buntings, and upland game birds were present in winters when there was little snow. However, when snow accumulated in fields, small passerines departed and resident game birds either perished or moved to farm yards and wooded areas.
Four of the 9 most numerous nesting species (killdeer, mourning dove, horned lark, and vesper sparrow) in cropland were responsible for 69% of all nests (Table 4). All of these 4 species are either abundant in North America or have increasing populations. These species are probably doing well because they are adapted to nesting on bare or nearly bare soil that is abundant in many crop fields. Another 3 of the 9 most numerous nesting species (northern pintail [Anas acuta], grasshopper sparrow, and lark bunting) initiated 16% of all nests are breeding species of special concern to Midwestern managers due to declining populations (Peterjohn and Sauer 1993). These 3 species generally require moderate cover for nesting and are not attracted to bare crop fields. Nests of these 3 species occurred primarily in organic fallow, secondarily in minimum-tillage wheat stubble, and were nearly absent in other fields. The other 2 of the 9 most numerous nesting species (upland sandpiper and red-winged blackbird) were responsible for 8% of all nests. Upland sandpipers nested mainly in growing wheat when its growth form was similar to open grassland and red-winged blackbirds nested only in weeds in unsprayed organic wheat.
A density of about 8 nests/10 ha was found in CRP fields, in the same general area during the same period as our study (R. R. Koford, unpubl. data). This density is 6 times larger than minimum-tillage stubble and organic fallow in our study and 11 times larger than densities in the other field types and crops. In Iowa, Basore et al. (1986) found nest densities of 36 nests/10 ha in no-tillage crop fields and observed that adjacent grass strip cover had 6 - 100 times more nests than cropland. Other authors concluded that birds in cropland benefitted from permanent edge covers, and populations were higher at field perimeters (Rodenhouse and Best 1983; Best et al. 1990). Our bird population densities may have been low as we did not survey birds in field-borders, and our fields were large with a small ratio of edge/area. Also, drought conditions present during our study most likely reduced bird populations, particularly waterfowl and shorebirds.
Although alternative cropping practices provided more plant cover on cropland and enhanced breeding populations, hatching and fledging success was low and not different among field types or crops. Both eggs and young suffered high losses due primarily to predation, secondarily to farming activities, and lastly to other causes such as nest abandonment or weather. There is concern that more birds might be attracted to nest in fields with reduced tillage and enhanced cover, but many nests would be lost during delayed seeding and tillage operations (Best 1986). However, low success for passerine nests seems to be a common problem in a variety of study areas and an assortment of cover types in cropland habitats. In nearby CRP fields, the nest success rate of 18% (R. R. Koford, unpubl. data) was similar to the 14% we found in cropland. The 22% fledging success reported by Basore et al. (1986) for nests in cropland was similar to 17% fledging success for nests in adjacent grassy strips. If the nest success rates in grassland are really higher than cropland, the small difference would be important to overall population success.
By changing the frequency and timing of the tillage operations it may be possible to moderately enhance hatching success. When we examined hatching success without predator losses in the equation, nests were more successful in wheat and minimum-tillage fields where there were fewer tillage treatments. Hatching success might be improved in cropland if tillage operations can be delayed. In winter wheat in North Dakota, Duebbert (1987) found relatively "good" nest success of 26 and 29% for ducks. He indicated that nesting birds in winter wheat had adequate time to lay eggs and complete incubation without being disturbed by farming activities. Winter wheat probably provides better nesting cover for birds than spring wheat because winter wheat growth starts several weeks earlier.
Vegetative values of crop fields in spring are difficult to characterize accurately because plants are growing rapidly and are being altered by mechanical treatments. Generally, there was increased bird and nest densities in fields with increased cover, particularly litter cover. Other studies of wildlife in cropland found similar preferences for plant cover, particularly plant residue and inter-row cover (Warburton and Klimstra 1984; Castrale 1985). Work in Iowa found birds selecting nest sites in fields with more cover residue, but not necessarily increased cover height (Basore et al. 1986). Plant litter may be critical to birds in crop fields because litter provides cover, food for birds, and food for insects eaten by birds. Litter is continually diminished in crop fields by herbicides, cultivation, and harvest operations.