Seasonal dynamics of body mass of insectivorous passerines breeding on the forested dune ridge, Delta Marsh, Manitoba

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Abstract
A total of 4669 body mass estimates was obtained between 1976 and 1983 from nine insectivorous passerine species that nest in a riparian habitat in southern Manitoba. The mass of individuals of the following species were obtained: eastern kingbird (Tyrannus tyrannus), least flycatcher (Empidonax minimus), house wren (Troglodytes aedon), gray catbird (Dumatella carolinensis), American robin (Turdus migratorius), veery (Catharus fuscescens), warbling vireo (Vireo gilvus), yellow warbler (Dendroica petechia), and northern oriole (Icterus galbula). The mass of most of these species varied during the summer and changes were correlated with breeding activity. Yearly differences in mass were significant for some species, but males and females of the same species did not always show the same pattern of variation from year to year. In species where the sexes could be identified, males were usually heavier than females. Yearling yellow warblers were significantly lighter than older individuals, but this trend was not found in the northern oriole.