Weight Loss of Birds during Nocturnal Migration

SEW•.AL attempts have been made to estimate energy metabolism of birds during migratory flight (Odum, 1960; Nisbet, 1963; Raveling and LeFebvre, 1967). Estimates of weight lost by migrants during flight are relevant to these studies because weight loss must be reconciled with related physiological processes, including energy utilization. In this paper I am concerned primarily with weight loss during migratory flight, the estimates being based on weights of Veeries (Hylocichla ]uscescens) and Ovenbirds (Seiurus aurocapillus) attracted to the lighthouse at Long Point, Ontario during nocturnal migration. Ideally we would like to be able to weigh an individual bird before and after a migratory flight of known duration to determine its weight loss. So far such direct measurements have not been achieved with wild birds, and other, indirect methods have been used to estimate weight loss. One method is to weigh samples of birds from what is believed to be the same population, before and after a migratory flight whose duration can be estimated (Helms, 1959; Nisbet et al., 1963). Another approach, the one used here, is to take a series of samples of migrants as they pass a fixed point on the migratory path, and to assume that differences between the mean weights of birds arriving at different times are the same as the mean weight changes of individual birds. flying during the same time intervals. This method is based on that used by Graber and Graber (1962) to estimate weight loss during nocturnal flight of thrushes killed at a television tower in Illinois.