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A. The Feeding Ecology of Breeding Ducks

The strengths of early studies of the food habits of ducks were their continental scope, large sample sizes, and long-term data. The weaknesses were limitations resulting from the use of food samples from gizzards or combined gizzards and esophagi of nonbreeding birds, and from the lack of supplemental data needed to assess variation in diet associated with sex or reproductive status. Studies conducted prior to 1960 mistakenly concluded that most species of Anatini and Aythyini depended primarily on plant foods during the breeding season.

The years from 1960 to 1970 were a transition period. For the first time, substantial samples of breeding ducks were collected specifically for research on food use. Comparison of foods from the esophagi and gizzards of these birds indicated that data from the latter were biased toward foods with slower rates of digestion, especially seeds and other plant parts (Perret, 1962; Bartonek and Hickey, 1969). These studies clearly demonstrated that aquatic macroinvertebrates were the most important source of food during the breeding season for species such as the Mallard, Redhead, and Canvasback. Although the occurrence of macroinvertebrates in the diet was initially attributed to increased availability during summer (Perret, 1962), the emphasis later shifted to a nutritional interpretation (Bartonek and Hickey, 1969). Differential use of macroinvertebrates by males and females was recognized during the 1960s, but temporal differences in diet were interpreted with reference to calendar rather than physiological events. The experimental work of Swanson and Bartonek (1970), which confirmed the results of field studies regarding the need for esophageal food samples, separated the period of the 1960s, wherein new research methods and concepts were developed, from the subsequent period during which they were widely applied.

Research on the feeding ecology of breeding ducks expanded further in the 1970s and 1980s. It was discovered that the proportion of macroinvertebrates in the diet of females varied with reproductive status (Krapu, 1974a, b; Serie and Swanson, 1976; Drobney and Fredrickson, 1979), and food use also was related to changes in nutrient reserves (Drobney, 1980; Krapu, 1981; Noyes and Jarvis, 1985). Cumulatively, these studies resulted in an integrated approach to feeding ecology research, wherein esophageal samples were used to interpret food use relative to sex, reproductive status, nutrient dynamics, and site-specific food abundance.

B. The Feeding Ecology of Prebreeding and Breeding Swans and Geese

Feeding-ecology data for prebreeding and breeding swans are limited because biologists have been reluctant to collect swans for food habits research. Food habits of geese also received little attention until recently, probably because of limited perceived competition with agriculture and the remote and relatively undisturbed locations of the breeding grounds of most populations.

Although the history of research on the feeding ecology of geese differs from that of the ducks, events during the 1960s had a strong influence on both. Important observations made during studies of the breeding biology of geese in the 1960s were that dramatic changes in body weight and carcass composition, and low rates of feeding, were characteristic of nesting adults (Barry, 1962; Hanson, 1962). The research stimulated by these observations initially emphasized the size, composition, and use of nutrient reserves by breeding geese rather than the foraging behavior associated with nutrient acquisition. However, more recent studies of prebreeding geese (McLandress and Raveling, 1981a, b; Gauthier et al., 1984a, b; Teunissen et al., 1985) have considered food use during periods of lipid and protein deposition, and the effects of sex, age, and social behavior on patterns of nutrient acquisition. Thus, differences in history and emphasis exist, but current approaches to studying the feeding ecology of prebreeding geese and breeding ducks have much in common.