Northern Prairie Wildlife Research Center
Changes in Breeding Bird Populations in
North Dakota: 1967 to 1992-93
Results
Species Composition
Frequencies of Occurrence on Sample Units
Statewide Population Estimates
Population Changes
Breeding Bird Survey Trends
Land Use Changes
Species Composition
One hundred and sixty-one breeding bird species (not including subspecies) were
recorded within the 128 quarter-sections surveyed in at least one year, including
129 species in 1967, 144 in 1992, and 153 in 1993. These 161 species represent
about 72% of the 223 species of North Dakota's recognized breeding avifauna
(Faanes and Stewart 1982). Most of the remaining species that were not observed
on the quarter-sections are either rare, uncommon, localized, or irregular breeders
in North Dakota (Faanes and Stewart 1982, DeSante and Pyle 1986).
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
Sixty-nine species (i.e. common species) occurred in 10% or more of the sample
quarter-sections in one or more years, including 48 species in 1967, 51 in 1992,
and 64 in 1993 (Appendix 1). The increase in the number
of common species between 1967 and 1992-93 was related primarily to the increase
in statewide frequencies of some permanent residents and some species associated
with human-made structures and woody vegetation (Table 1,
Appendix 1). In decreasing order, the five most frequently
occurring species were Horned Lark (Eremophila alpestris), Brown-headed
Cowbird, Western Meadowlark (Sturnella neglecta), Red-winged Blackbird,
and Eastern Kingbird (Tyrannus tyrannus; Appendix
1). All but one (Downy Woodpecker [Picoides pubescens]) of these
69 common species occurred in all three years. Stewart and Kantrud (1972) commented
that the absence of the Downy Woodpecker in 1967 was "somewhat surprising,"
given the species' widespread distribution in North Dakota.
Statewide Population Estimates
The projected statewide population estimates for breeding birds in North Dakota
were 25.5 million breeding pairs in 1967, 24.1 million in 1992, and 27.4 million
in 1993. Projected statewide population estimates are given in Appendix
2 for the 69 species that had statewide frequencies of occurrence of 10%
or higher in one or more years (i.e. common species). These 69 common species
composed 95% of the projected statewide breeding bird population in 1967, 92%
in 1992, and 92% in 1993. By this criterion, more than one-half (92 species)
of the species observed in the three years were uncommon breeders. In decreasing
order of abundance, the five most common species were the Horned Lark, Chestnut-collared
Longspur (Calcarius ornatus), Red-winged Blackbird, Western Meadowlark,
and Brown-headed Cowbird. Collectively, these species accounted for 31-41% of
the estimated statewide breeding population in the three years. The Horned Lark,
a species that is most characteristic of cropland and heavily grazed grassland,
accounted for over 10% of the projected statewide populations in each year.
Population Changes
Annual variation in breeding bird populations was considerable (Table
2 and Table 3, Appendix
1 and Appendix 2). Annual breeding bird density for
all species varied from 141 indicated breeding pairs per 100 ha in 1967 to 133
in 1992 and 151 in 1993. An evaluation of population changes of each species
is beyond the scope of this paper. Rather, we restrict our evaluation to two
broad patterns of population change evident in our data. First, most wetland
species and many grassland species occurred less frequently and were less abundant
during the dry year, 1992, than during the wetter years, 1967 and 1993. This
pattern appears to be most pronounced and consistent for species associated
with open water, such as waterfowl and the American Coot (Fulica americana).
Second, many permanent residents and many species associated with human-made
structures and woody vegetation generally increased in frequency and abundance
between 1967 and 1992-93.
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
Sample sizes were sufficient to estimate statewide BBS trends from 1967 to 1993
for 151 of the 161 observed species. Of the 64 species with statistically significant
trends, 14 long-distance migrants, 15 short-distance migrants, and 7 permanent
residents exhibited increasing trends. In contrast, 9 long-distance migrants,
18 short-distance migrants, and 1 permanent resident showed significantly decreasing
trends. Twenty-eight of the 36 (78%) species with significantly increasing trends
were associated with human-made structures or woody vegetation. Most (23) of
the 28 species with significantly decreasing trends were associated with wetlands
or grasslands. Grassland species, however, had equal numbers of significantly
increasing (5) and decreasing (5) population trends.
Land Use Changes
Cropland and grassland were the dominant land uses in the study area, covering
about three-quarters of the total area on the 128 quarter-sections in all three
years (Table 3). The area of cropland increased
by 2% between 1967 and 1992- 93. In contrast, grassland, wetland, and hayland
declined by about 3%, 27%, and 52%, respectively, from their 1967 levels; all
declines reflected nationwide trends (Samson and Knopf 1994, Dahl 1990, Herkert
1991). Woodland, human-made structures, and unvegetated portions of clay buttes
increased by about 19%, 13%, and 6%, respectively, between 1967 and 1992-93.
Planted cover increased by 40%, which largely reflects the Conservation Reserve
Program, a long-term, federal cropland retirement program that began in 1985
(Johnson and Igl 1995). Land use changes between 1992 and 1993 were relatively
small.
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