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Igl, Lawrence D. and Douglas H. Johnson.  1997.  Changes in breeding bird
     populations in North Dakota:  1967 to 1992-93.  The Auk 114(1):74-92.

Igl, Lawrence D. and Douglas H. Johnson.  1997.  Changes in breeding bird
     populations in North Dakota:  1967 to 1992-93.  The Auk 114(1):74-92.
     Northern Prairie Wildlife Research Center Online. 
     http://www.npwrc.usgs.gov/resource/birds/birdpop/index.htm
     (Version 31JUL97).

Table of Contents

• Introduction
• Study Areas and Methods
  • Study Area
  • Methods
  • Species Classification
  • Calculation of Population Estimates
  • Breeding Bird Survey Trends
  • Land Use Changes
• Results
  • Species Composition
  • Frequencies of Occurrence on Sample Units
  • Statewide Population Estimates
  • Population Changes
  • Breeding Bird Survey Trends
• Discussion
  • Species Composition
  • Patterns of Population Changes
• Acknowledgments
• Literature Cited

Tables

• Table 1. -- Breeding bird populations in 128 randomly selected quarter-sections in North Dakota.
• Table 2. -- Composition of breeding birds and mean number of indicated pairs on 128 randomly selected quarter-sections in North Dakota, by year, breeding habitat, and migratory status.
• Table 3. -- Land use changes on the 128 quarter-sections surveyed in 1967, 1992, and 1993, and percentage change between the two periods (1992 and 1993 averaged).

Appendices

• Appendix 1. -- Statewide frequencies of occurrence (confidence interval) of common bird species (i.e. species with frequencies greater than 10%) in North Dakota.
• Appendix 2. -- Statewide breeding population estimates (1,000's of pairs with confidence intervals) of common species of birds (i.e. species with statewide frequencies greater than 10%) in North Dakota.

Introduction

Although the BBS is the best source of quantitative data on trends of breeding bird populations in the midcontinent, the BBS has several limitations, including sparse coverage in central North America and biases associated with roadside surveys. Several studies have attempted to verify population trends from the BBS with data from independent, long-term surveys (e.g. Breeding Bird Atlas: Robbins et al. 1989; checklists: Temple and Cary 1990; Christmas Bird Count: Hagan 1993; migration counts: Dunn and Hussell 1995). Parallel trends derived from studying the same populations in different ways may provide corroborating evidence and strengthen the assessment of population trends of the BBS. Compared with eastern deciduous forests (e.g. Johnston and Hagan 1992), however, long-term data sets on breeding bird populations in the midcontinent are scarce.

Though a paucity of comparative information exists in the midcontinent, historic surveys provide an often overlooked source of baseline data on breeding bird populations (e.g. Graber and Graber 1963). These include the extensive survey of breeding birds in North Dakota conducted by Stewart and Kantrud (1972) in 1967 to obtain estimates of statewide breeding bird abundances and frequencies of occurrence. Data from the Stewart-Kantrud survey provided a unique opportunity to evaluate changes in breeding bird populations in the northern Great Plains. In 1992 and 1993, we repeated the Stewart-Kantrud survey using the same sample units and methods. Our objectives in this paper were to (1) examine changes in breeding bird populations in North Dakota; (2) compare patterns in breeding bird population changes with trends from the BBS; and (3) assess the likelihood that population changes may be influenced by changes in land use in North Dakota.

Study area

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.

Methods

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.

Species classification

Calculation of population estimates

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

Land Use Changes

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).

Results

Species Composition

The composition of breeding birds in North Dakota in 1992 and 1993 was similar to that in 1967 (Table 1). One hundred twenty-two species were recorded in all three years, 21 species were observed in two of the three years, and 18 species were detected in only one of the three years (Table 1). One species was recorded only in 1967, four species only in 1992, and 13 species only in 1993. Thirty-two species were observed only in 1992 and/or 1993, but not in 1967.

Most (92%) of the 161 breeding bird species are migratory, and only a few species are considered permanent residents that show little or no seasonal movements in North Dakota (Table 1 and Table 2). Of the species that migrate, 86 species are short-distance migrants and 62 are long-distance migrants. Moreover, migrants constituted over 96% of the total number of indicated pairs detected each year. Short-distance migrants composed over one-half of the observed indicated pairs each year.

Among breeding habitat associations, wetland species composed the largest proportion (32%) of species, followed by species associated with open woodland or edge and grassland habitats Table 1 and Table 2). In contrast, grassland birds composed the largest proportion of observed breeding pairs, accounting for 38% or more of the indicated pairs recorded in each year.

Frequencies of Occurrence on Sample Units

Statewide Population Estimates

Population Changes

Of the 161 breeding bird species observed, 46 (29%) had consistent and significant (P </= 0.10) population changes between 1967 and 1992 and between 1967 and 1993 (Table 1, Appendix 2). Of these 46 species, 11 long-distance migrants, 15 short-distance migrants, and 8 permanent residents exhibited increasing population changes and 7 long-distance migrants, 5 short-distance migrants, and no permanent residents showed decreasing population changes. Twenty-four of the 34 (71%) species with significantly increasing population changes were associated with woody vegetation or human-made structures. In contrast, 11 of the 12 (92%) species with significantly decreasing population changes were associated with wetlands or grasslands. Grassland species, however, had equal numbers of significantly increasing (4) and decreasing (4) population changes.

Breeding Bird Survey Trends

Land Use Changes

Discussion

Species Composition

Another characteristic of grassland bird assemblages in North America is the prevalence of short-distance migrants (MacArthur 1959, Willson 1976). In this study, short-distance migrants composed more than one-half of the observed species and indicated pairs in each year. These patterns are comparable to those of other grasslands in North America but contrast greatly with northeastern deciduous forests, where long-distance migrants and permanent residents dominate the breeding avifauna (MacArthur 1959, Willson 1976).

One species, Northern Waterthrush (Seiurus noveboracensis), was recorded only in 1967, compared with 32 species detected only in 1992 and/or 1993. Although differences in observer abilities between 1967 and 1992-93 may account for the increase in the number of species between the two periods (Table 1), many other recent or irregular events may have contributed to this increase. These include food-based nomadism (Pine Siskin [Carduelis pinus], DuBowy 1983; Short-eared Owl [Asio flammeus], Stewart 1975; Long-eared Owl [Asio otus], Marks et al. 1994), increased nest box availability (Hooded Merganser [Lophodytes cucullatus], Doty et al. 1984; Eastern Bluebird [Sialia sialis], Sauer and Droege 1990), successful reintroduction and management programs (Wild Turkey [Meleagris gallopavo], Johnson and Knue 1989; Canada Goose [Branta canadensis], Lee et al. 1984), and increased habitat availability (species associated with woody vegetation, see below). Nonetheless, most of the 32 species that were not observed in 1967 are considered rare, uncommon, localized, or irregular breeders in North Dakota (Faanes and Stewart 1982, DeSante and Pyle 1986).

Patterns of Population Changes

Our results (Table 1 and Table 2; Appendix 1 and Appendix 2) are consistent with earlier reports showing that breeding bird populations in grassland ecosystems are tremendously dynamic, exhibiting considerable annual variation in abundance and frequency of occurrence (Graber and Graber 1963, George et al. 1992, Zimmerman 1992). Within breeding habitat associations, our observed patterns of population change were remarkably similar and consistent among species with different migratory behaviors. First, many species associated with woody vegetation (e.g. House Wren [Troglodytes aedon]) and human-made structures (e.g. Barn Swallow [Hirundo rustica]) generally increased between 1967 and 1992-93 (Appendix 1 and Appendix 2). Second, many grassland (e.g. Savannah Sparrow) and wetland (e.g. Wilson's Phalarope) species declined between 1967 and 1992-93, though many exhibited slight (e.g. American Coot), moderate (e.g. Western Meadowlark), or dramatic (e.g. Common Yellowthroat) increases between 1992 and 1993.

The two patterns of long-term population change evident in our data were consistent with trends from the BBS. For both surveys, most species with significantly increasing population changes were associated with human-made structures or woody vegetation, and most species with significantly decreasing population changes were associated with wetlands or grasslands. This concordance illustrates that both independently derived measures of population change likely were recording the same phenomena. Moreover, of the 46 and 64 species that had significant population changes in our survey and the BBS, respectively, 26 species had significant trends in both data sets; all 26 species were considered common species in North Dakota (Appendix 1 and Appendix 2). Only one of the 26 species, Grasshopper Sparrow, had a population change that was opposite in sign. This discrepancy could be attributed to many factors, including differences in survey methodology or trend analysis. However, the long-term trend from the BBS may have masked the recent population increase in Grasshopper Sparrows in the Great Plains; these increases have been associated with the increased availability of perennial grassland habitat established by the Conservation Reserve Program (Reynolds et al. 1994, Johnson and Igl 1995).

The causes of the above patterns of population change may involve conditions in the breeding, migration, or wintering seasons (Sherry and Holmes 1992). The prevalence of parallel patterns of population change among habitat associations, but not among migratory behaviors, suggests that these population changes may be caused, at least in part, by conditions on the breeding grounds (Finch 1991). A common feature of breeding birds observed in this study is their dependence on habitats on the breeding grounds; most of these species breed in the northern Great Plains but winter elsewhere. For example, if breeding ground conditions are primary influences on bird population dynamics, then populations of long-distance migrants should show similar patterns of change as short-distance migrants. The examples of Blue-winged Teal (Anas discors) and American Coot for wetlands, Baird's Sparrow (Ammodramus bairdii) and Savannah Sparrow for grasslands, and House Wren and Blue Jay for open woodlands are illustrative (Appendix 1 and Appendix 2).

What conditions or factors might limit populations on the breeding grounds in the northern Great Plains? Habitat loss and changes in land use have been viewed as major potential causes of population change of many breeding birds in the Great Plains (Knopf 1988, 1994; McNicholl 1988). Within the last century, the landscape of the Great Plains has been greatly modified by a number of human-induced changes. The once abundant grasslands and wetlands of the Great Plains have been drastically reduced, altered, and fragmented by intensive agriculture, roads, tree plantings, encroachment by woody vegetation, invasion of exotic plants, and other human activities. Knopf (1994) described several historic, contemporary, and continuing influences on breeding birds and their habitats in the Great Plains, including: (1) removal of native grazers and transformation to intensive, domestic livestock grazing; (2) cultivation of native grasslands; (3) loss of wetlands; and (4) woody development in the form of tree plantings and ecological invasions. The first three influences were implicated in the recent declines of grassland bird populations, as well as wetland associates. The fourth provided increased nesting opportunities for species associated with woody vegetation that generally were lacking or were limited in presettlement times. Likewise, Herkert (1994), Vickery et al. (1994), and Warner (1994) implicated fragmentation of grasslands in the recent declines of breeding birds in eastern and midwestern grasslands.

Before European settlement, the landscape of North Dakota included vast expanses of grassland, consisting primarily of northern mixed-grass prairie with some tallgrass prairie on the extreme eastern edge of the state (Risser et al. 1981). Moreover, the grasslands of the prairie pothole region (i.e. Drift Plains and the Missouri Coteau; Fig. 1) were dotted by millions of shallow wetland basins. Since settlement, North Dakota has lost about 49% of its wetlands (Dahl 1990) and 75% of its native grasslands (Samson and Knopf 1994), much of this before 1967. We compared the statewide changes in bird populations (Table 1 and Table 2, Appendix 1 and Appendix 2) with the overall changes in land use area (Table 3) between 1967 and 1992-93. Long-term population changes in our study and BBS were consistent with the notion that these changes were, in part, caused by changes in land use on the breeding grounds (e.g. Knopf 1994). Declines in wetland species were commensurate with declines in wetland area. The increases in species associated with human-made structures and woody vegetation were consistent with the overall increases in area of those habitats. The declines in grassland birds paralleled the decrease in the combined total area of grassland and secondary grasslands (hayland and planted cover). Bernstein et al. (1990) repeated the historic survey of Kendeigh (1941) in northwestern Iowa and found similar declines in grassland birds and increases in species associated with woody vegetation.

Land use area, however, changed very little between 1992 and 1993, but our results suggest that many grassland and wetland species, as well as several other species, shifted similarly by increases in frequency and abundance between the two recent years (Table 1 and Table 2, Appendix 1 and Appendix 2). Droege and Sauer (1989) and Peterjohn and Sauer (1993) suggested that breeding bird population declines on BBS routes in the late 1980s and increases in the early 1990s reflect extreme drought conditions and amelioration of drought conditions, respectively. Extended periods of below-average rainfall are known to affect populations of grassland and wetland birds (e.g. Johnson and Grier 1988). In this study, the increases between 1992 and 1993 corresponded with the amelioration of the long-term drought conditions and are consistent with observations of behavioral flexibility and opportunism in habitat selection (Cody 1985) and weather-related shifts in distribution and abundance by grassland (Wiens 1974) and wetland (Johnson and Grier 1988) species. George et al. (1992) also noted rapid recoveries of grassland bird densities after drought. Reproductive success of grassland birds varies annually, with very poor productivity during some drought years (George et al. 1992).

In conclusion, our results indicate that BBS data reflect real population changes for many common species in North Dakota. The increase in species associated with human-made structures and woody vegetation and decreases in wetland and grassland species in agricultural landscapes have been reported previously, both in North America (McNicholl 1988, Knopf 1994, Herkert 1995) and elsewhere (Böhning-Gause and Bauer 1996). Our data suggest that land use changes, and probably drought conditions, influenced some breeding bird population changes in North Dakota between 1967 and 1992-93. Although these factors would not necessarily affect different species to the same extent, the fact that our surveys detected significant population changes for several common species (e.g. Northern Shoveler [Anas clypeata]) that the BBS did not detect and vice versa (e.g. Sora [Porzana carolina]) deserves further investigation. Furthermore, we cannot attribute all population changes to conditions on the breeding grounds. However, because many birds in the Great Plains are short-distance migrants, many of the factors driving population changes on the Great Plains are likely North American processes (Knopf 1994, Herkert 1995). Nonetheless, our data indicate that factors on the breeding grounds are having a major effect on breeding bird populations in the northern Great Plains.

Acknowledgments

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Table 1

^aAsterisks following species names indicate peak breeding period during which counts were made: * early-nesting species, peak breeding period 24 April to 7 June; ** mid-nesting species, peak breeding period 14 May to 10 July; *** late-nesting species, peak breeding period 22 May to 21 July.

^bLDM: Long-distance migrant; SDM: Short-distance migrant; RES: Permanent resident.

^cBreeding habitat associations defined as: GRAS: Grassland; WETL: Wetland; WOOD: Woodland; SHRU: Shrubland; OPTR: Open habitat with scattered trees or shrubs; OPWO: Open or semi-open deciduous woodland and edge; RESI: Residential (rural development, urban, human-made structures); OTHR: Other habitats (mostly unvegetated habitats including clay buttes, cliffs, banks).

^dPopulation change when the difference between 1967 and 1992 estimates (Appendix B) and the difference between 1967 and 1993 estimates were both significant at P </= 0.10 and only if both differences were in the same direction: D = decreasing, and I = increasing.

^eAverage percent annual change on Breeding Bird Survey routes in North Dakota between 1967 and 1993: D = (decreasing) at P < 0.10, DD = at P < 0.05, and DDD = at P < 0.01; I = (increasing) at P < 0.10, II = at P < 0.05, and III = at P < 0.01; NA = not available.

Table 2 and 3

Appendix 1

Appendix 2