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Changes in Breeding Bird Populations in
North Dakota: 1967 to 1992-93

Study Areas and Methods


Study Area
Methods
Species Classification
Calculation of Population Estimates
Breeding Bird Survey Trends
Land Use Changes

Study area

Situated in the geographical center of North America, North Dakota has a climate characterized by warm summers and long, cold winters. The distribution of precipitation is highly seasonal; about 75% of the annual precipitation occurs during the growing season (April-September; Jensen 1972). Precipitation for January through June (1961-1990 average: 22.4 cm) in North Dakota was nearly normal in 1967 (21.8 cm), below normal in 1992 (18.0 cm), and above normal in 1993 (25.7 cm; NOAA 1967, 1992, 1993). Many areas in North Dakota and the northern Great Plains experienced moderate to extreme drought conditions from 1988 to early 1993 (NOAA 1988-1993, Igl and Johnson 1995).

In 1967, Stewart and Kantrud (1972) divided the state into eight major strata based on biogeographical, physiographical, and ecological characteristics (Fig. 1). From these eight strata, Stewart and Kantrud (1972) selected 130 sample units by random selection without replacement. The number of sample units allocated to each stratum was proportional to the area of the stratum. Within each stratum, sample units were proportionately distributed according to the relative size of substrata that were differentiated on the basis of prevalent habitat types. The number of substrata ranged from two to five for each of the eight major strata and totaled 27 for the state. The stratification used by Stewart and Kantrud (1972) was effective in reducing the estimated variance in population estimates by as much as 15% compared with simple random sampling (Nelms et al. 1994).

GIF - Survey Map
Fig. 1. Distribution of 128 quarter-sections in North Dakota where bird surveys were conducted during 1967 and 1992-93. Strata are indicated by dashed lines: (1) Agassiz Lake Plain, (2) Northeastern Drift Plain, (3) Southern Drift Plain, (4) Northwestern Drift Plain, (5) Missouri Coteau, (6) Coteau Slope, (7) Missouri Slope, and (8) Little Missouri Slope.

To facilitate a direct comparison, we surveyed the same sample units used by Stewart and Kantrud (Fig. 1; H. A. Kantrud, unpubl. data). We visited 128 of the 130 quarter-sections (each ca. 64.7 ha) originally surveyed by Stewart and Kantrud (1972) in 1967; landowners denied access at the other two quarter-sections. Comparisons among years are based on the 128 quarter-sections that were surveyed in all three years.


Methods

We surveyed breeding birds using the same methods employed by Stewart and Kantrud (1972; H. A. Kantrud, pers. comm.). Surveys were conducted by two observers on foot. Each observer surveyed breeding birds on a rectangular half (805 × 402 m; 32.37 ha) of a quarter-section by following a standardized survey route. This route was 100 m inside of and parallel to the boundary of the rectangle. Deviations up to 100 m from the route were often necessary to survey all habitats adequately. The rectangular halves were usually surveyed simultaneously, and an interval of 400 m was maintained between observers. Both observers compared field notes at the end of each coverage of a sample unit to prevent duplications in the counts of wide-ranging birds, such as vultures, hawks, and crows. We minimized observer bias in 1992 and 1993 by using the same two observers in both years. In 1993, some of the quarter-sections were surveyed by a single, experienced observer, who censused both rectangular halves on the same day.

Large wetlands required a different type of coverage. Birds on open water were counted with a spotting scope from the shoreline. In large zones of emergent vegetation, one observer attempted to flush large (e.g. ducks and herons) or secretive (e.g. rails and bitterns) species by wading in a zigzag course throughout the wetland while making noise. From a nearby vantage point, the second observer recorded all birds flushed, including conspicuous, colonial marsh birds such as Red-winged Blackbirds (Agelaius phoeniceus).

The phenological advance in seasons during the spring and early summer is about two weeks earlier in southwestern North Dakota than in the northeastern portion of the state. To compensate for these differences, the sequence in which sample units were covered progressed from southwestern to northeastern North Dakota. We matched the date that a quarter-section was surveyed in 1992 and 1993 as closely as feasible to the date that it was surveyed in 1967 (H. A. Kantrud, unpubl. data). The surveys of breeding birds extended from 24 April to 19 July in 1967, from 27 April to 18 July in 1992, and from 24 April to 21 July 1993. The overall absolute difference between the 1967 surveys and the 1992 and 1993 surveys averaged 3.3 days and 1.7 days, respectively.

In each year, sample units were surveyed once or twice; the number of breeding pairs for each species, however, was based on single counts during each species' peak breeding period. All sample units were surveyed for early-nesting species between 24 April and 7 June, for mid-nesting species between 14 May and 10 July, and for late-nesting species between 22 May and 21 July (Table 1). When a survey was conducted during an overlapping portion of the peak breeding periods, counts of early-, mid-, and late-nesting species coincided. Thus, quarter-sections that were visited between 22 May to 7 June were surveyed only once, and those that were surveyed before 22 May were surveyed again after 7 June to include species from all three breeding periods. Peak breeding periods for some species differ from those in Stewart and Kantrud (1972) due to typographical errors in the original publication; these errors did not affect statewide population estimates or variances in Stewart and Kantrud (1972). Stewart and Kantrud (1972, Kantrud 1982) felt justified in estimating bird populations in open habitats using single counts because many species have behavioral adaptations (e.g. elevated perches, flight songs, synchronous displays) that tend to increase their detectability compared with birds inhabiting extensive wooded areas (see Speirs and Orenstein 1967, Cody 1985).

Species were identified by sight or sound. Counts during precipitation and strong winds (&gt24 km/h) were avoided. Surveys of open-country birds were conducted between 0.5 h after sunrise and 0.5 h before sunset. Although some surveys occurred outside the time of peak vocal activity (i.e. early morning or late evening), Stewart and Kantrud (1972) concluded that singing and other activities of open-country birds were not appreciably affected by time of day. Quarter-sections containing extensive woodland habitats were usually covered on relatively calm (&lt8 km/h), sunny days between 0.5 h after sunrise and 1000 CST. These limitations were necessary because song frequencies and other activities of most woodland birds are reduced on cloudy days, in moderate or high winds, and at midday.

Counts of breeding birds were based primarily on the number of indicated breeding pairs on territories or home ranges during peak breeding periods. For most species, nearly all indicated pairs were observed as segregated pairs or as territorial males. For Wilson's Phalarope (Phalaropus tricolor), segregated pairs and lone females were recorded as indicated pairs. Although currently not in vogue, for consistency we based the number of indicated pairs of Brown-headed Cowbird (Molothrus ater) on the total number of males seen per sample unit. In the case of colonial birds that are not sexually dimorphic (e.g. Black Tern [Chlidonias niger] and Cliff Swallow [Hirundo pyrrhonota]), the number of indicated pairs was based either on a count of occupied nests or was derived by halving the total number of individuals counted.

The procedures used to determine the number of pairs of breeding waterfowl followed Hammond (1969) with one exception. Occasionally, the number of lone females on a given quarter-section exceeded the number of males unaccompanied by females. In this case, each excess lone female was considered to represent an indicated pair.

We excluded from our results birds that we considered to be nonbreeders. These included: (1) migrant flocks and individuals of species that are not known to breed in North Dakota (Faanes and Stewart 1982); (2) nonbreeding, vagrant waterbirds in the summer and oversummering shorebirds (i.e. transient shorebirds remaining in North Dakota during the boreal summer); (3) wide-ranging colonial waterbirds passing high overhead (e.g. pelicans and gulls); and (4) other birds passing overhead in high, direct flight. By counting birds only during their peak breeding periods, we maximized the potential for recording breeding pairs and territorial males and, at the same time, minimized the likelihood for confounding territorial birds with migrants.

Vernacular and scientific names follow the American Ornithologists' Union (1983) and subsequent supplements, with one exception. We recorded Red-shafted (Colaptes auratus cafer) and Yellow-shafted (C. auratus auratus) subspecies of the Northern Flicker separately to reflect their separate species status in 1967. One obvious intergrade was recorded as a Red-shafted Flicker in 1967.


Species classification

Based on published accounts, we classified each species into one of three groups according to its migratory behavior (Table 1): permanent resident (present in North Dakota year-round), short-distance migrant (winters primarily north of the U.S./Mexico border), and long-distance migrant (winters primarily south of the U.S./Mexico border; Faanes and Stewart 1982, AOU 1983, Harrison 1983, Rappole et al. 1983, Hayman et al. 1986, Madge and Burn 1988, Thompson et al. 1993). The migratory status of some year-round residents (e.g. Blue Jay [Cyanocitta cristata], American Crow [Corvus brachyrhynchos], European Starling [Sturnus vulgaris]) was difficult to determine because some wintering individuals may have originated from breeding populations north of North Dakota. We considered these species to be short-distance migrants. In addition, we categorized each species into a general breeding habitat based on the literature (Ehrlich et al. 1988, Peterjohn and Sauer 1993) and personal observations. Habitats were described as wetland (including wet meadow), grassland, open habitat with scattered trees, woodland, open or semiopen deciduous woodland and edge, shrubland, residential areas and human-made structures (hereafter human-made structures), and "other" (mostly unvegetated habitats including clay buttes, cliffs, banks, etc.). Waterfowl were included in the wetland class, i.e. their primary foraging and brood-rearing habitat. Species using a broad range of habitats (e.g. Song Sparrow [Melospiza melodia], Common Yellowthroat [Geothlypis trichas]) were classified according to their principal habitat type.

Calculation of population estimates

We estimated population means and totals, and their standard deviations, using standard methods for stratified random samples with proportional allocation (Cochran 1977). We calculated Bayesian confidence intervals (95% confidence limits; Box and Tiao 1973) in lieu of the usual confidence intervals using the methods described in Johnson (1977). Bayesian intervals exploit the prior knowledge that the means of bird densities and of total numbers of birds are non-negative. Population estimates are given for only those species with statewide frequencies of occurrence of 10% or higher (i.e. common species). Results for 1967 in this paper may vary slightly from those given in Stewart and Kantrud (1972) due to differences in sample size (i.e. 130 vs. 128 quarter-sections).

Statewide population estimates were compared between 1967 and 1992-93 with z-tests. A significant change was claimed only if the difference between 1967 and 1992 values and the difference between 1967 and 1993 values were both significant at P < or = to 0.10 and only if both differences were in the same direction.

Biases associated with the bird survey were not quantified (see Stewart and Kantrud 1972). In 1967, Stewart and Kantrud made efforts to minimize apparent biases in methodology through adjustments in census techniques. In the recent surveys, we endeavored to conduct our surveys as similarly as possible to the methods used in 1967. We recognize that the size of the breeding population for certain species may be over- or underestimated. For example, we assumed that all males were mated, although some territorial males may have been unmated (e.g. Dickcissel [Spiza americana], Fretwell and Calver 1970; Ovenbird [Seiurus aurocapillus], Gibbs and Faaborg 1990). Also, population estimates of wide-ranging species or species with large territories or home ranges may have been overestimated. For polygynous (e.g. Yellow-headed Blackbird [Xanthocephalus xanthocephalus]) and polyandrous (Wilson's Phalarope) species, the number of indicated pairs represents, in terms of breeding mates, a minimum population. Undoubtedly, biases related to differences in observers, years, weather, sampling time, etc. were present, but biases associated with methodology should be relatively consistent among years.


Breeding Bird Survey Trends

We obtained trends in abundance from the BBS for North Dakota during the period 1967- 1993 (J. R. Sauer and B. G. Peterjohn pers. comm.). Trends are based on statistical methods described by Geissler and Sauer (1990) and are presented as the average percent annual change between 1967 and 1993. In North Dakota, the BBS began in 1967, the same year that Stewart and Kantrud (1972) conducted their survey.

Land Use Changes

Comparable data on land use and cover were available each year for every quarter-section. To evaluate overall changes in major habitats, we digitized land use and cover by drawing vectors over scaled rasters of scanned aerial photographs using Map and Image Processing System (MIPS) software (MicroImages, Inc. 1992). Eight major land use or cover classes were delineated:
  1. Cropland (all land used for the production of annual field crops and land under summer fallow);
  2. Hayland (areas that have been plowed and seeded to mixtures of grasses and legumes for forage or seed production);
  3. Grassland (natural grassland regardless of the condition of the grassland and regardless of disturbance regime [e.g. grazed, mowed, idle] and areas planted to introduced species);
  4. Planted Cover (mixtures of grasses and legumes planted for wildlife cover or soil conservation [e.g. Conservation Reserve Program]);
  5. Woodland And Shrubland (native and artificially stocked tree and shrub stands and plantings.);
  6. Wetland (areas classified as wetlands by Stewart and Kantrud [1971] as well as permanent and semipermanent riparian areas);
  7. Human-Made Structures (human-made structures, fence rows and field borders, and road and railroad rights-of-way); and
  8. Clay Buttes (portions of clay buttes that are mostly unvegetated).

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