Northern Prairie Wildlife Research Center
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).
|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.
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 (>24 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 (<8 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.
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.