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Habitat Distribution and Territoriality In the Dickcissel and Red-Winged Blackbird

Methods


A 22 acre grid was set in the oldfield by placing small flags at 100 foot intervals in the field. A scale map of this grid was then drawn for use in plotting positions of territorial males. The marsh was marked with a two acre grid by flags set at 50 foot intervals. A scale map was drawn of this grid also. No grids were placed on either portion of the prairie but scale maps were drawn of both areas using aerial photographs. Each prairie area contained sufficient numbers of natural landmarks to allow mapping of birds without the use of a grid.

Censuses of each area were taken on a week-by-week basis. Positions of territorial males of both species of birds were plotted on the maps. By observing and following each male, a fairly accurate outline of his territory could be drawn on the map. In this way accurate censuses could be obtained for each species in each area. A careful record was also kept on the mated status of each male. Thus, a week-by-week sex ratio could be calculated. Areas of territories were computed by tracing their outlines with a compensating polar planimeter.

Identification of individual birds was aided by the use of color bands which were placed on the legs of captured birds. Birds were captured in mist nets by Roger Packard. Birds that were not banded could usually be identified (in the case of dickcissels) by size and shape of the characteristic throat patch and often by song. Unbanded redwings were difficult to identify on the basis of physical appearance or song patterns, but in most cases there were not enough birds on the study area to make identification a problem. Females were generally not identified individually. Observations were simply made on the number of females occupying each male's territory.

Oldfield vegetation heights were collected during one census at the peak of dickcissel density. Measurements were made with a 2-meter stick at each point of the grid. Heights could then be categorized by observing in which territory they fell.


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