Variation in Winter Populations of Savannah Sparrows:
R. A. Norris and G. L. Hight, Jr.
Aided by a series of sparrows identified as to subspecies or geographic race, we endeavored to assign race designations to all individuals that were handled. Birds considered racially atypical or non-characteristic were designated as intermediates but were always placed nearer one race or another. We found, as had Johnston (1956), who had assembled a series of specimens from the same area in the previous winter, that the population of Savannah Sparrows included birds referable to five races. Our figures on the relative abundance of these races were as follows: P. s. mediogriseus, 30.1 percent; savanna, 27.9; labradorius, 18.4; oblitus, 17.5; and nevadensis, 6.1. Thus labradorius and oblitus, both dark-races with more northern breeding grounds, comprised about 3 percent of the entire sample. Many of the birds wintering in the Savannah River Plant area had necessarily migrated eastward as well as southward; this “migratory drift” is best illustrated by the western race nevadensis, relatively uncommon in our sample. More than half the total birds examined were considered racial intergrades or non-characteristic individuals. Weights of specimens, averaging about 17 grams in fall and winter, increased to about 19 grams percent as much as those of the heaviest race oblitus. In labradorius the wing was large relative to body weight, in mediogriseus it was small, and in other races intermediacy was shown in this proportion.
In 1955-56, the first autumnal migrant was noted on September 27. Common by early November, the species was abundant throughout the winter. Its numbers declined abruptly in early April, and the last bird was seen on May 16. For the nonbreeding season as a whole, in an abstract community involving about 70 different fields, there was an average of approximately 3.9 Savannah Sparrows per acre (as determined by two census methods). The dark races labradorius and oblitus, while represented among the earliest arrivals in fall, became relatively more prevalent in early spring, and were the only races recorded as late as May. There was no evidence of differential habitat selection or other segregation among the different races. This seemingly complete spatial overlap of the racially different populations, which had “funneled down” from far-flung breeding areas in the northern United States and Canadian regions, is presumed to be selectively advantageous to the species on its wintering ground.
Norris, R.A. and G.L. Hight Jr. 1957. Subspecific variation in winter populations of Savannah sparrows: a study of field taxonomy. The Condor 59:40-52.