Analyses of Weight and Fat Variations in Transient Swainson's Thrushes

This paper represents an attempt to derive useful information from data collected in the process of banding and recapturing migrant birds.The Swainsoh's Thrush (Hylocichla ustulata) was selected for this study because it is a long distance, nocturnal nilgrant and because it is the most common bird in our area in autumn.The paper is a series of analyses of weight and fat variations, occurrence, and fluctuations of both recaptures and previously unhandled birds.A number of hypotheses concerning various aspects of migration are presented and compared with the data.Some of the hypotheses are not entirely consistent with others; several interpretations are possible for some aspects of the data.The data for this study are derived from 5,580 handlings of 4,328 Swainsoh's Thrushes taken in a standardized mist-netting scheme at the Cedar Grove Ornithological Station in the autumns of 1959 through 1962.The Swainsoh's Thrush breeds from California, Colorado, the northern Great Lakes, and West Virginia north to central Alaska and central Canada.The winter range extends from southern Mexico to Peru and northwestern Argentina (abridged from the A. O. U. Checklist, 1957).The breeding range of the Swainsoh's Thrush in Wisconsin is apparently confined to the northern tier of counties (Groinroe, 1963).We have received four recoveries on Swainsoh's Thrushes banded in autumn at Cedar Grove: (1) at Montgomery, Alabama, in early May one and one-half years after banding, (2) at Winnipeg, Manitoba, in early May of the year following banding, (3) at Vernon, British Colmnbia, in early August 23 months after banding, and (4) near Caparrapi, Columbia, South America, in January four months after banding.The Cedar Grove Ornithological Station is located on the west shore of Lake Michigan some 64 km north of Milwaukee, Wisconsin.A 400 m wide strip of beaches abandoned by ancient recessions of the lake separates the intensively farmed red-clay-glacial till from the lake at this point (Figure 1).The thin soils covering the abanddoned beaches support a fairly dense stand of second growth forest (Populus, Betula papyrife•'a, P•unus serotina, Fraxinus) in some areas.Much of the area, however, is covered only by a brushy growth (Co•'nus stolonife•'a, Salix, Rubus, Alnus rugosa) or with grasses and Juncus balticus, and occasional blowouts of sand occur.Agricultural use of this land is usually limited to pasturing.