Cover Density of a City Summer and Fall Bronzed Grackle Roost

Ames, Iowa, is one of the many Midwestern cities that have the dubious distinction of having a blackbird roost. During the summer and fall months thousands of birds each evening come to roost in the shade trees in· residential areas. Records kept in 1951 (Bliese, 1953a) indicated that roughly 74 per cent of the birds were bronzed grackles, Quiscalus quiscula. Regularly associated with them were about 22 per cent starlings, Sturnus vulgaris, 3 per cent cowbirds, Molothrus ater, and 1 per cent robins, Turdus migratorius. Observations from 1949 to 1952 also revealed that at times purple martins, Progne subis, red-wings, Agelaius phoeniceus, and English sparrows, Passer domesticus, were present also. One of the objectives of the investigation was to learn whether or not certain readily measurable features of the roost sites were correlated with the extent to which the birds used them. To that end records were kept of the degree of use accorded each tree in the roost area, as described in Bliese (1953 a \u26 b). Degrees of use were: none, light, moderate, and heavy. Several physical measurements were made of the various trees, but only those dealing with cover density are discussed in this paper