Northern Prairie Wildlife Research Center
Field work by the authors on the study area was conducted during May 1978 through August 1988. During 1978, extensive observations were made in the area adjacent to the Platte River from Grand Island west to Lake McConaughy. During 1979-80, breeding bird species within the 13 counties were observed. From 1981 to 1987, most observations have been confined to the Grand Island - North Platte reach, although intensive observations have been made on Mormon Island Crane Meadows near Grand Island. As of 1 April 1989, 212 bird species have been recorded on that 866-ha preserve. Data on relative abundance and distribution are derived largely from our unpublished field notes.
Foremost among the published data sources on bird occurrence and distribution we have examined is the Nebraska Bird Review, the quarterly journal of the Nebraska Ornithologists' Union. Extensive use was made of the breeding season reports for the Southern Great Plains region contained in American Birds (and its predecessors), the field publication of the National Audubon Society. Johnsgard's (1979, 1980) publications dealing with Nebraska ornithology provided important information on dates of occurrence and egg dates.
Extensive use was made of the unpublished nest records obtained by Wilson Tout during 1919-1947. We also examined the bird and egg collections in the Division of Zoology at the University of Nebraska State Museum, and at The Hastings Museum. A limited amount of data was made available by the solicited responses of ornithologists and birders living in the Platte River Valley. The bulk of the data contained in this report are our own largely unpublished field notes and data gathered during the study. In total, we present data on 142 bird species that have been confirmed as nesting in our study area, or are highly suspected of so doing. Data are also presented on an additional 24 hypothetical nesting species.
We provide information throughout the species accounts on the habitats occupied by birds during the nesting season in our area. At times, to provide the reader with additional information on nesting habitat, we have included synopses of published information from other regions. Similarly, where data on nesting dates, clutch size, or other reproductive parameters are lacking, we have incorporated information from Kansas, North Dakota, or other nearby states. Our intent is to provide the reader with an indication of when the reproductive activities are likely to occur and in which habitats in the Platte River valley.
Data on breeding status and distribution was enhanced through the use of North American Breeding Bird Survey routes within the study area. These 39.2-km transect routes are selected within physiographic strata and are surveyed once each year during June. Data from these routes are presented to point out regions of peak populations of several species and, in some instances, to show geographic differences in breeding distributions. In addition, we gathered data for mapping bird distribution by conducting point survey counts (Ferry 1974, Blondel et al. 1970) within 97% of the legal townships (93 km2) within the study area. Briefly, we recorded the presence or absence of individual species within a township by spending at least 15 minutes at representative samples of habitats. These data provided a base for the distribution maps.
Breeding Bird Populations
Censuses of breeding birds were conducted on randomly selected plots of habitat within each of several predetermined strata. The first stratification was formed by the legal boundary of each county. Within counties, the next stratification was the legal township. During selection of study sites, only one plot of a particular habitat type was censused per township. The third stratification was based on the predominant soil type of the region, as determined by Aandahl (1973).
Census plots were then selected at random within these strata. Plot size varied according to habitat complexity. Native prairie and cropland plots were all 16.2-ha, residential and riparian plots were 8.1-ha. Wooded river islands were chosen within the selected 16.2-ha plot. Shelterbelts were censused when they occurred on selected native prairie or cropland plots.
Stewart and Kantrud (1972) used 64.7-ha plots because two observers censused each plot, and because they believed complete counts of birds could be made in a 2-hour census period. Larger plot sizes have considerable merit for investigating large, wide-ranging birds such as waterfowl or raptors. We chose smaller plot sizes because they are probably better suited for censusing smaller, inconspicuous species.
Censusing was accomplished by one observer equipped with binoculars. Birds were counted while the observer followed a zig-zag course within each census plot. We followed the recommendation of Bond (1957), and walked slowly for 2-3 minutes, then stopped for 5 minutes to observe territorial behavior and to note if an individual bird was in or out of the plot. All territorial males were counted as they flushed before the observer, or when their singing location was encountered. The number of females was noted for brown-headed cowbirds and Wilson's phalaropes. The number of indicated pairs of sexually monomorphic species (e.g., swifts, swallows) was derived by halving the total of individuals counted on each plot. Polygamous blackbird populations are estimated based on counts of territorial males and thus represent minimums.
Breeding birds were censused only when wind speeds were < 15 km/h, and only on sunny or partly cloudy days. Censuses involving open habitats and wetlands were conducted during the period of 1/2 before after sunrise to 1/2 hour before sunset. Riparian woodlands were censused only until 11:00. We limited surveys in wooded habitats to early hours because of reduced song output and restricted movements later in the day (Robbins and Van Velzen 1967, Stewart and Kantrud 1972). In wetlands, all ducks in the open were counted when the observer first arrived. Once the number of these had been determined, additional ducks and other marsh species were counted as the observer moved slowly through the vegetation.
Each plot was visited once during the 1979 season. In 1980, about 10% of those plots censused in 1979 were revisited to examine year-to-year variation. Additional plots were censused only in 1980. Single bird censuses are not recommended during intensive studies where precise estimates of populations are required (Enemar 1959) but have been used successfully to derive indices to populations during extensive surveys (Robbins and Van Velzen 1967, Stewart and Kantrud 1972, Rotenberry 1978). Speirs and Orenstein (1967) have shown that in open Ontario habitats, average efficiency of single censuses is 66-76% that of 6-10 censuses in estimating whole populations. Jarvinen and Lokki (1978) reported that nearly 90% of the species occurring on an individual census plot can be recorded in a single visit. Jarvinen and Vaisanen (1981) observed that a single census of a plot is adequate when censusing bird populations over extensive areas. Variability inherent in single visits is probably reduced by obtaining replicate censuses of each habitat.
Breeding Range Maps
The apparent range of each species during the nesting season is shown on maps of their distribution. Each symbol on the maps indicates a township in which at least one nesting season record was obtained. Closed circles refer to the presence of territorial males or pairs in the proper habitat during the nesting season. Half closed circles refer to nests, dependent young, or conclusive evidence of nesting (e.g., food carrying) within the township. Range maps for greater prairie-chicken and sharp-tailed grouse represent our best estimate of each species range based on the availability of suitable nesting habitat, and our knowledge of both species ranges in spring and fall.
Population estimates for breeding birds followed the methods described by Stewart and Kantrud (1972), including the calculation of highest probability density (HPD) intervals (Johnson 1977). These intervals are Bayesian confidence intervals and were used because the number of breeding pairs is known not to be negative. Mean densities of breeding birds are expressed in pairs per km2 for both upland and wetland habitats.
Species diversity indices (H') (Tramer 1969) and equitability (J') (Kricher 1972) were calculated for each habitat type. Species diversity is measured by the Shannon-Weaver formula:
where Pi is the proportion of individuals of species i. This index is influenced by the number of species and the number of individuals within each species. Equitability is calculated using:
where H' = the species diversity index, and H'max is the maximum diversity possible in the sample. Equitability is highest when all species in the sample are as nearly equal in population as possible, and it becomes smaller as single species show more influence or dominance in the population.
The terms used to describe species status in this report are adapted from Faanes (1981) including:
Regular - A species that occurs at some location in the Valley each year.
Casual - A species expected to occur at least once every 3 to 5 years, but not annually.
Accidental - A species that is not expected to occur again or that occurs very infrequently.
Hypothetical - A species that probably occurred in the Valley at least once, but the circumstances of the observation leave the record in doubt.
Introduced - A species that is naturally foreign to this region but has been released in the area as an act of man and is now established and reproducing without additional influence by man.
Extirpated - A species that once occurred naturally in the Valley, has now been eliminated, but still exists elsewhere.
Terms used in describing the occurrence of a species in the Valley include:
Permanent resident - A species that is largely nonmigratory or, if migratory, only a very small proportion of the population departs during a migration period.
Migrant - A species that normally occurs in the Valley only during the well-defined spring and fall migration period.
Breeding species - A species for which a viable clutch of eggs, dependent young in the nest, or young that have left the nest but are still dependent have been observed.
Summer resident - A species that occurs in the Valley during the normal nesting period and in all likelihood nests, but for which there areno confirmed records of eggs or dependent young.
Winter resident - A species that winters in the valley even though greater numbers may occur during migration.
Terms used in relating the relative abundance of each species during migration, winter, or the breeding season relate to its importance to the total avifauna. These terms adapted from Stewart (1975) are described as follows:
Abundant - A species that is observed in very large numbers.
Common - A species that is observed in large numbers.
Fairly common - A species that is observed in fair to moderate numbers.
Uncommon - A species that is observed in low numbers.
Rare - A species whose range includes the Valley but is observed in low numbers.
Very rare - A species that is observed in low numbers and sporadically on the study area.
The taxonomic treatment of birds in this report follows the Sixth Edition of the American Ornithologists' Union Check-list of North American Birds (AOU 1983) and its various updates.
Description of the major plant communities of the Valley follows the classification of Bose (1977) for native prairie communities and Currier (1982) for riparian communities including wet meadows. The common and scientific names of plants mentioned in the text follow Barkley (1986). Voucher specimens of most plants mentioned in the text are housed in the Herbarium at the Northern Prairie Wildlife Research Center.