Contributions of the Conservation Reserve Program to Populations of Breeding Birds in North Dakota
Study Areas and Methods
Statewide study.--The statewide study was a repeat of a survey originally conducted in 1967 of 130 legal quarter- sections (64.7 ha each), constituting a stratified random sample throughout the state of North Dakota (Stewart and Kantrud 1972). We did not receive permission to survey birds in two of the original quarter-sections, one in 1992 and another in 1993. The total area surveyed was 8335 ha in 1992 and 8332 ha in 1993.
We surveyed breeding birds using methods of Stewart and Kantrud (1972), which allow a fairly rapid assessment of breeding-bird communities in a field or quarter-section. Surveys were conducted by two observers on foot. (Sometimes only a single observer was available; he then covered the two halves in succession.) Each observer surveyed breeding birds on a rectangular half (805 X 402 m; 32.4 ha) of a quarter-section by following a standardized survey route. This route was 100 m inside of and parallel to the sides of the rectangle. We deviated up to 100 m from the route if necessary to adequately survey all habitats. We searched each quarter-section once or twice each year between late April and mid-July.
Surveys began about dawn and ended by midafternoon. Surveys extended beyond the time of most active bird vocalization, but Stewart and Kantrud (1972) concluded that activities of open-country birds were not appreciably affected by time of day other than early morning and late evening. We tallied all indicated breeding pairs, based on singing or calling males, females (for Brown-headed Cowbirds), observed pairs, or presence of an active nest. For individuals observed on the boundary of a plot, we credited the plot with 0.5 pair. We assigned breeding pairs to one of nine major habitat types, including cropland and CRP. We took care to avoid duplication in the counts and surveys under adverse weather conditions (precipitation or strong winds).
We estimated statewide breeding-bird populations, all habitats combined, by multiplying average densities by biogeographical stratum by the area each stratum made up, and summing across strata (Stewart and Kantrud 1972).
CRP study.--The CRP study is a continuing investigation that examines breeding-bird communities in CRP fields in nine counties in Minnesota, Montana, North Dakota, and South Dakota (Johnson and Schwartz 1993a,b). We restrict attention here to the three North Dakota counties: Eddy, Kidder, and Hettinger, one in each of the major landforms of North Dakota that contain appreciable areas of CRP habitat. Johnson and Schwartz (1993a,b). We surveyed 132 fields totaling 2734 ha in 1992 and 145 fields totaling 3001 ha in 1993. In addition, several quarter-sections in the statewide study included CRP fields, which we used to augment our samples of that habitat type. By doing so we added 11 CRP fields totaling 548 ha in both years.
In the northern Great Plains, most CRP land is planted to a mixture of grasses and legumes. Johnson and Schwartz (1993b) reported that most common species in North Dakota CRP fields included alfalfa (Medicago sativa), crested and other wheatgrasses (Agropyron cristatum and A. spp.), and smooth brome (Bromus inermis). Most CRP fields have remained relatively idle during the contract period, although some fields were released for emergency haying and grazing due to drought in several years and flooding in 1993. These disturbances took place after birds were surveyed each year.
We surveyed birds in CRP fields using the same methods as in the statewide study, except that a single observer surveyed small fields. We surveyed each field once each year between late May and early July.
We computed densities of indicated breeding pairs for each species in CRP habitat. We projected these densities to the state by multiplying them by the total area of CRP in the state (1,287,133 ha). Implicit in this computation is the assumption that the fields we surveyed were representative of CRP fields throughout the state.
We also compared densities of breeding birds in CRP and in cropland. Cropland estimates were based on fields encountered in our statewide survey involving 4115 ha in 1992 and 4146 ha in 1993. To predict the consequences of the termination of the CRP and conversion of CRP fields to cropland, we substituted cropland densities for CRP densities into the values that made up statewide totals. This procedure assumes that all CRP fields would return to cultivation; about 15% of CRP contract-holders in North Dakota surveyed indicated that they would retain permanent cover on their fields (Mortensen et al. 1989). Actual retention would depend on commodity prices and other dynamics of the agricultural marketplace.
Trends.--We obtained trends in abundance of grassland bird species from the Breeding Bird Survey (BBS) (Robbins et al. 1986) during 1967-90 for the Central Region of North America between the Rocky Mountains and the Mississippi River (J. R. Sauer, National Biological Service, and B. G. Peterjohn, pers. commun.). Trends are based on statistical methods described by Geissler and Sauer (1990).
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