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Effects of Fire on Bird Populations in Mixed-grass Prairie

Field Methods

Each year during 1972-95 (1977-82 for Plot 2) the breeding bird community of each plot was estimated by conducting several surveys and mapping territories. Standard survey methods (Hall 1964, Van Velzen 1972) were used, and annual reports were published in American Birds or the Journal of Field Ornithology. See Johnson (1996) for listing of references. About eight visits were made to each plot during late May through mid-June. Surveys were conducted from just before dawn to late morning. Early-morning surveys emphasized concurrent registrations of indicated pairs of the same species, to define multiple territories. Surveys later in the morning, when vocalizations were reduced, focused on reflushing birds to delineate their territories (Wiens 1969).

In most years, one other observer and I conducted independent surveys and compared results. I estimated the number of territories from the locations plotted on field maps. There were two exceptions. In 1972, P. F. Springer conducted all surveys and estimated territories on Plot 9 for the American Birds report. C. A. Faanes conducted surveys and estimated territories on all plots in 1983. For consistency in the analyses presented here (Best 1975), I reviewed all original field data forms associated with these exceptions and independently estimated the number of territories. To conform with guidelines for publishing results of breeding bird censuses, the number of territories was estimated to the nearest half. Sometimes such rounding either over- or under-estimated the true number of territories in a plot, so I modified the counts with a + or -. For analyses here, such modified counts were adjusted by 0.2 territory; for example, a 1+ was converted to 1.2. Also, partial territories, recorded as + in summary reports, were converted to 0.2 for analysis. Visitor species (those recorded on only one to three of the surveys) were credited with 0.1 territory. For Brown-headed Cowbirds, the average number of females seen during the surveys was used as the number of territories.

The reliability of censuses of small plots is greater for species with small home ranges than for those with large ones. Accordingly, results for sparrows, Red-winged Blackbirds (Agelaius phoeniceus), Bobolinks (Dolichonyx oryzivorus), and warblers may be more reliable than those for wider-ranging species such as Western Meadowlark (Sturnella neglecta) and shorebirds.

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