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Variation in Body Mass of Wild Canvasback and Redhead Ducklings

Discussion


Differences in body mass within broods in this study were similar to those reported for wild Canvasbacks (Dzubin 1959) and wild Mallards and Gadwalls (Lokemoen et al. 1990). As expected, within-brood variation in body mass was generally less than that among broods. However, in some groups, the difference between within and among-brood variation was very small. Some of these exceptions may have been due to intraspecific parasitism, which can be extensive in both species (Sorensen 1990). Tests to assess sources of within-brood variation in body mass for Canvasbacks, however, were inconclusive.

Comparison of variability among broods may be confounded by the use of plumage development for ageing, especially for ducklings around 2-3 weeks old. Lokemoen et al. (1990) suspected the high variation in body mass of Mallards, Blue-winged Teal, and Gadwall within the same plumage class was due in part to inaccuracy of ageing birds from plumage. Schneider (1965) found errors of more than two weeks within and among broods if age at which teleoptiles first emerge (2-3 weeks of age; Southwick 1953) was used for age classification. Once feathering began, the sequence and timing of emergence seemed to be constant. Retarded growth or poor body condition may be reflected in delayed plumage development. We were able to assess accuracy of ageing in this study by comparing 49 ducklings from 12 broods of known hatching date with age classes as outlined by Dzubin (1959). Of these, 14% had been misclassified: one was estimated as one week younger, five as one week older, and one as two weeks older than actual age.

Age class and sex were the only factors affecting body mass in Canvasbacks. Sexual differences in mass became significant after Class IIA (about four weeks of age), somewhat earlier than reported for captive Canvasbacks and Redheads (Lightbody and Ankney 1984, Lightbody 1985). Canvasbacks were similar to Blue-winged Teal (Dane 1965) but differed from Mallards (Rhymer 1988b) in the lack of a seasonal effect on duckling body mass. The nesting period of Mallards is longer than that of Canvasbacks or Blue-winged Teal and provides greater opportunities for renesting and any associated changes in egg or duckling mass.

The lack of a year effect on duckling mass suggests habitat quality and food availability were not limiting, despite differences in water conditions among years (Serie et al. 1992). Canvasback broods moved extensively among ponds of various sizes and types (Austin and Serie 1991). The mobility of the broods allows broods to adapt to changing food availability and thus minimize the effect of food availability on growth and fledging.

In studies using captive ducklings, the greatest variability in duckling growth rates and body mass occurred at about 2-6 weeks after hatch, the period of most rapid growth (Schneider 1965, Prince et al. 1970, Brown and Fredrickson 1983). Brown and Fredrickson (1983) suggested that the high variability of body mass during this period may be related to high energy and nutrient demands and that stress would have its greatest effect on gains in body mass, and possibly survival, during this period. Certainly, one would expect that differences of the magnitude reported in this study (20-30% of mean body mass) could affect survival, particularly during periods of cold stress. Unfortunately, researchers investigating duckling thermoregulation have assessed cold stress only during the first 1-2 weeks after hatch (Untergasser and Hayward 1972, Rhymer 1988b); little information is available about the capabilities of older ducklings relative to body mass.

The lack of differences in body mass between ducklings in pure or mixed broods suggested that mass was not affected by interspecific brood parasitism. However, we do not know whether body mass differed relative to brood type in one- to two-week-old ducklings or whether such differences affected early survival. Leonard (1990) found that lower survival of Canvasback ducklings in mixed-species broods occurred during the first week after hatch; survival did not differ between pure- and mixed-species broods in 2-9 weeks after hatch, which coincides with the ages of ducklings in this study.

Although ducklings within a brood encounter the same environmental conditions and foraging opportunities, they can differ by as much as 34% of average body mass even within sexes. Such large differences cannot entirely be attributed to sex or size at hatch because these differences decline throughout development to nonsignificant levels after 3-4 weeks (Rhymer 1988b, Holmberg and Klint 1991). Other factors that may contribute to body mass variation within broods include behavior, injuries or disease, parasite loads, and different parentage (intraspecific parasitism [Sorensen 1990]). If ducklings differ, for example, in their ability to learn various feeding skills, they may develop differences in their foraging efficiency.

Few data are available to assess within-brood variability in body mass during the first two weeks (Dzubin 1959), when duckling mortality is greatest (Sargeant and Raveling 1992). The influences of low body mass in this early period on body mass and survival later in development are uncertain. Swennen (1989) found that during the first 10 days after hatch lighter ducklings were less alert and reacted more slowly to alarm calls than heavier ducklings; these lighter ducklings were most likely to be taken by gulls. Lighter ducklings in the first 1-2 weeks after hatch are more susceptible to cold stress than heavier ducklings (Koskimies and Lahti 1964, Untergasser and Hayward 1972, Rhymer 1988b).

Insight into the factors influencing body mass of ducklings and, in turn the influence of body mass on survival, would be valuable in our efforts to provide brood habitat and enhance wild duck production. A combination of experimental studies of ducklings in captivity and in the wild is needed to address these questions.


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