Weight and Size Variation in the Gray Catbird

Weights of wild birds fluctuate because of various activity patterns governed by either internal or external stimuli.Important short timescale activities include locomotion, feeding, and defecation; those on a longer time-scale include migration, reproduction, and molt.Stimuli controlling these activities include day length, temperature, hunger, fright, and many others, some operating directly and others indirectly.Weight changes in living birds provide clues to other aspects of their biology, to environmental stresses, and to the longer-period activities in which the species or individual is engaged.Because of the many variables determining the instantaneous weight of a bird and the limited number of times a wild bird can be captured and weighed (particularly without inadvertently influencing the weight), useful information on cyclical or systematic changes can best be obtained from statistical analyses of large numbers of weights taken throughout the duration of the cycle of interest.This paper presents an analysis of weight variation in the Gray Catbird (Dumetella carolinensis) and relates these variations to time of day and year, activity patterns, age, and sex.In contrast to weight, linear measurements of individual birds are relatively constant after full growth is attained.Some change takes place in wing and tail length by wear between molts but this is typically slow and gradual (Blake, 1971).Thus, size of the Catbird is documented by age and sex but related only to season.Many authors have reported bird weights and several (Baldwin and Kendeigh, 1938; Becker and Stack, 1944; Wiseman, 1975) have analyzed their data by age, sex, time of day or season.Most data were taken from living birds but some from tower kills (Graber and Graber, 1962; Tordoff and Mengel, 1956).Causes of weight changes and their significance have been discussed previously (Nice, 1938; Blake, 1956).General patterns of weight change are well known but have been documented by adequate data for only a few species such as the American Goldfinch (Wiseman, 1975).Other authors studied weight change in relationship to fat deposition and potential flight distance (Drury and Keith, 1962; Mueller and Berger, 1966; Rogers and Odum, 1966).An attempt was made to locate all previous Gray Catbird weights reported in the literature so that comparisons could be made with data from other seasons and other locations.In addition to the authors cited above, Catbird weights were given by Stewart (1937), Poole (1938), Johnston and Haines (1957), Murray and Jehl (1964), and Stewart and Skinner (1967).Only one series of Catbird measurements was found (Wetherbee, 1934).Those measurements are compared below to data from this study.