Biology of White-crowned Sparrows in late summer at College, Alaska

DeWolfe, Barbara

This paper presents data on banding, behavior, body weight, gonad dimensions, molt, and fat in the postbreeding period for Zonotrichia leucophrys gambelii at College, Alaska (lat 64░ 49' N) in 1957. Between 26 June and 27 August, 346 birds were examined. Of these, 254 were banded and released, and 92 were kept for specimens. Thirty-two of the banded birds returned to the traps at least once. Defense of territory began to decline by 30 June, flocks started to form by 9 July, migratory behavior was first seen 7 August, and decline in numbers began 9 August. By 30 August all the observation areas in which Gambel's Sparrows had been followed were vacant. A number of facts indicate that Gambel's Sparrows taken on the breeding grounds in late summer, before any migratory behavior can be detected, are not necessarily birds which bred at the collection site. First, the number of Gambel's Sparrows caught in the trapping area (43 adults and 261 young) far exceeded the theoretical maximum of breeding pairs and their offspring (20 adults and 50 young) which would have been expected to forage there, based on counts of nesting pairs and clutches in May and June. Second, the percentage of each day's catch consisting of unbanded birds remained high throughout the trapping period. During the last 10 trapping days (12 August and later) unbanded birds constituted from 69 to 100 per cent of the total catch, with a mean of 84.3 per cent. Third, the total numbers caught, and the average number of unbanded birds caught per trapping hour, fluctuated markedly, beginning in mid-July, more than three weeks before migrating flocks were seen or a steady decline in numbers, not subsequently replenished, set in. Fourth, intervals between dates of first and last capture of banded birds were short: they average 9.8 days for six adults and 7.7 days for 22 young. The median value for the interval for 28 individuals is six days. Fifth, banding records of adults trapping in May and June provide indirect evidence that breeding birds may have moved away from their nesting grounds within the trapping area by early or mid-July. The data reported here complement those reported for Z. l. gambelii at the same latitude in 1962 and 1963 by King, Farner, and Morton (1965). The timing of the events of the nesting cycle, the lack of correlation between changes in body weight and amount of fat, and the lack of precision in timing of autumnal fat deposition are points of agreement. Chief differences between the two sets of observations involve the data molting birds were first collected, the duration of the period from earliest sign of molt to the dat the last molting bird was taken, and the behavioral events used as an index for the start of migration. Zonotrichia leucophrys has become a favorite subject for physiological studies. It is important to realize that samples taken from the same place on the breeding grounds in late summer may include individuals from several different breeding populations. Hence the variation within such samples may be due not only to individual differences but to intrapopulational differences as well.

Biology of White-crowned Sparrows in late summer at College, Alaska

Series