Date of this Version
Red-winged blackbirds (Agelaius phoneiceus) have been studied intensively in the United States and Canada by research biologists of the Bureau of Sport Fisheries and Wildlife for more than a decade. The detailed information on popula¬tion dynamics of the species that has been acquired is now being assembled and assessed. Definite patterns in the distribution of some of the eastern and mid-western populations have become apparent (Dyer et al., 1072; Power, 1069, 1970). A very complex distribution of territorial densities occurs in the western Lake Erie Basin of the U.S. and Canada (Dyer et al., 1972), where there are significant differences in density, both among widely dispersed areas and within isolated sections of single areas. These differences are caused by differences in availability of preferred habitat types and by differences in spacing distances selected by the territorial birds when preferred habitat is available. The pattern was made even more complex by changes in density within single study areas during a 3-year period while comparable areas in the region showed no such change. In some areas in the Lake Erie Basin, numbers of males dropped and then recovered; in one area, they increased steadily; and in another area they declined slightly but steadily.
Some progress has been made in the development of unifying theories to explain the apparent vagaries of population density and change noted for the red¬wing. For example, a relationship between distributional trends, latitude, and tem¬perature led Power (1969, 1970) to postulate that distributions were related to ther-moregulatory efficiencies in the populations he studied. If this postulate is correct, the speculation can be carried further to suggest that phenotypic variants with dif¬fering thermoregulatory efficiencies occupy isolated but adjacent breeding sites without the more obvious barriers often considered necessary for isolation (Mayr, 1963:ch. 16). Dyer (1964:143) presented data that suggest that such isolated red¬wing populations exist. The theory is supported further by the finding of Orians (1969) that the quality of the energy base in the breeding habitat is a principal fac¬tor in determining polygynous ratios and the ultimate productivity of the redwing population. In areas where the energy base is high, the ratio is low.
Therefore, it is becoming more apparent how interactions of genotype, phen¬otypic adaptation to the environment of the breeding area, and total energy avail¬able on the breeding grounds determine the success of any given redwing population. The present paper adds to this picture by describing population densities of territo¬rial males in a well developed and stable agricultural community, and relates the results to population theory.