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Most studies reporting body mass, growth curves, or measurements of ducklings have used data from captive birds (e.g., Oring 1968, Rhymer 1988b, Lightbody and Ankney 1984). Variation in body mass among captive ducklings tends to be small (Rhymer 1988b), probably because of uniform environmental conditions and access to ad libitum food. Information on variability in body mass under natural conditions is sparse, however, and few researchers have assessed this variability within or among broods. Low variability in body mass within broods may be expected because brood mates are genetically similar and have encountered similar environmental conditions and foraging opportunities. However, two studies reporting within-brood variability found sizable differences in body masses. Dzubin (1959) reported within-brood differences of 5 g (11% of mean body mass) at hatch, 16 g (17%) in nine-day-old, and 120 g (12%) in 50-day-old wild Canvasback (Aythya valisineria) ducklings. In wild Gadwall (Anas strepera), Mallard (Anas platyrhynchos), and Blue-winged Teal (Anas discors) ducklings 19-41 days old, differences in body mass within broods ranged as high as 140 g for females and up to 95 g for males (Lokemoen et al. 1990).

Canvasback nests and broods in the prairie pothole region are often parasitized by Redheads (Aythya americana) (Stoudt 1982, Serie et al. 1992). The effect of interspecific brood parasitism on duckling growth and body mass is unknown, although some researchers have assessed its effect on survival to fledging. Canvasbacks reared in broods parasitized by Redheads had lower survival rates than Canvasbacks reared in unparasitized broods (Leonard 1990). In contrast, survival of Common Goldeneye (Bucephala clangula) ducklings was not affected by interspecific parasitism (Eadie and Lumsden 1985).

Data collected from Canvasback and Redhead ducklings during banding operations in the 1970s provided an opportunity to assess body mass of 20- to 50-day-old wild ducklings. Because Canvasback nests were commonly parasitized by Redheads (Serie et al. 1992), data included broods of both single and mixed (containing Canvasback and Redhead ducklings) species. My objectives were to (1) assess variability of duckling mass within and among broods of Canvasbacks and Redheads, (2) evaluate factors influencing duckling mass, and (3) determine whether ducklings raised in a single-species brood are heavier than ducklings in mixed-species broods.