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Foraging Ecology and Nutrition

V. Food Habits

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Waterfowl exploit a greater variety of food resources than any other family of aquatic or semiaquatic birds. Freshwater and marine macroinvertebrates, fish, and various parts of aquatic and terrestrial plants all are important foods. Swans and geese generally are herbivores. Ducks have more varied diets; some specialize on plant or on animal foods, and others are omnivorous. In this section, we examine the dietary patterns of waterfowl during periods when nutrients are acquired for reproduction. Hereafter, we refer to the time before arrival at the breeding area as the prebreeding period and the time after arrival as the breeding period.

A. Swans and Geese

Limited information available on food habits of swans during the prebreeding and breeding periods suggests that plant foods dominate the diet. Swans feed on roots, tubers, stems, and leaves of submerged and emergent aquatics (Banko, 1960; Frith, 1967, p. 96; Owen and Kear, 1972, p. 63). Pondweeds (Potamogeton spp.) are particularly important, but many other plant species also are consumed (Owen and Kear, 1972, pp. 63-66). Waste grain has become an important food of Tundra Swans during winter (Tate and Tate, 1966; Bortner, 1985), and observations suggest that use of cereal grains has increased during spring migration in the Prairie Pothole Region.

Most species of North American geese now forage principally on agricultural lands during spring staging periods in temperate regions. Waste corn (Zea mays) is a major food of Atlantic Canada Geese in the St. Lawrence River Valley (Reed et al., 1977) and of mid-continent White-fronted Geese in the Great Plains during spring (U.S. Fish and Wildlife Service, 1981). Giant Canada Geese feed principally on waste corn during winter in southern Minnesota, but shoots of bluegrass (Poa pratensis) become more important during the month preceding spring migration (McLandress and Raveling, 1981a). Quantitative data on the prebreeding diets of most other subspecies of Canada Geese are lacking. However, most, if not all, forage on agricultural lands during spring stopovers in temperate regions (e.g., Raveling and Lumsden, 1977).

Lesser Snow Geese feed primarily on agricultural foods, particularly corn, during stopovers on temperate staging areas in midcontinent North America (Alisauskas, 1988). Until recently, Greater Snow Goose populations were relatively small and their principal spring staging areas were natural tidal marshes on the St. Lawrence River estuary in Quebec. Rhizomes of three-square (Scirpus americanus) were the principal food source of Greater Snow Geese during spring, but limited availability and a growing goose population have caused an increasing proportion of the geese to occupy cordgrass (Spartina spp.) marshes (Gauthier et al., 1984a). Greater Snow Geese now also feed extensively on agricultural lands adjacent to the bulrush and cordgrass marshes (Bédard et al., 1986). During winter and spring migration, Brant feed primarily on native marine and salt marsh vegetation, including sea lettuce (Ulva lactuca), saltmarsh grass (Spartina spp.), and eelgrass (Zostera marina) (Penkala, 1975; Buchsbaum et al., 1986). Brant also forage on agricultural lands (Charman, 1979; St. Joseph, 1979) and even on roadsides and suburban lawns during periods of food shortage (Nelson, 1978).

In Europe, food habits of prebreeding geese are similar to those of North American species. Diets of prebreeding Bean and Pink-footed Geese, for example, include a substantial proportion of cereal grains in Scotland, Finland, and Denmark (Newton et al., 1974; Lampio, 1984; Fruzinski, 1977; Madsen, 1985a). The diet of Dark-bellied Brant in the Wadden Sea region of Denmark, Germany, and the Netherlands, where nearly the entire population gathers during spring migration (Teunissen et al., 1985), consists primarily of marine and salt marsh vegetation. Eelgrass, the preferred food of Brant in Europe and North America, is depleted during fall by migrant waterfowl and therefore is less important in the diet during spring (Charman, 1979). Like the Brant, Barnacle Geese feed principally on salt marsh plants during the prebreeding period (Owen, 1980, p. 142).

During the breeding season, information from visual observations and from examination of limited numbers of esophageal samples indicates that geese feed principally on vegetation, with grasses, particularly Puccinellia spp., and sedges dominating the diets of most North American species (Table 1-4). Before these foods become available, aquatic plants, roots, berries, and lichens are used where available. Diets of geese generally are influenced by patterns of plant growth more than by reproductive status of the birds (e.g., Buchsbaum and Valiela, 1987), but laying females may be more selective than geese at other reproductive stages (Murphy, 1988). Male geese generally utilize new food sources earlier than their mates, who incubate almost continuously. Both sexes of adults and young eat succulent shoots of grasses and sedges during brood-rearing (Mickelson, 1975; Harwood, 1977; Sedinger and Raveling, 1984).

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B. Ducks

The prebreeding food habits of ducks are known for only a few species. Mallards and Northern Pintails feed primarily on plant foods during winter but may increase their intake of macroinvertebrates prior to spring migration (Heitmeyer, 1985; Miller, 1987; also see LaGrange and Dinsmore, 1989; Gruenhagen and Fredrickson, 1990). During spring stopovers on pools of the Upper Mississippi River, Canvasbacks feed primarily on storage tissues (e.g., tubers, rootstocks, and buds) of pondweed and wild celery (Vallisneria americana); macroinvertebrates comprise about a quarter of the diet (J. Barzen and C. Korschgen, unpublished data).

The food habits of most North American dabbling ducks and pochards have been thoroughly studied during the breeding season. Results of these studies indicate that macroinvertebrates account for a high proportion of the diet, especially among females during the laying stage (Table 1-5, Fig. 1-1A). Insects, particularly larvae and nymphs of aquatic species in the orders Diptera, Coleoptera, Odonata, and Trichoptera, are the principal animal foods consumed by nesting females (Fig. 1-1B). The dipteran family Chironomidae is particularly important in the diets of dabbling ducks, pochards, and Ruddy Ducks during the period of egg formation (Fig. 1-1B).

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Mollusks, especially snails (Class Gastropoda), are included in the diets of most Anatinae during the breeding season (Fig. 1-1A), but usually in smaller quantities than insects. Esophageal samples from female Blue-winged Teal and Northern Shovelers collected in North Dakota contained 36% and 40% snails, respectively (Swanson et al., 1974, 1979). Crustaceans are another important food of breeding ducks (Fig. 1-1C); water fleas (Order Cladocera) dominate the diet of female Northern Shovelers during laying and are a major food of Gadwalls (Fig. 1-1C). In the Prairie Pothole region, fairy shrimp (Order Anostraca) are an important food of laying female Northern Pintails and Mallards early in the breeding season, particularly during years when temporary and seasonal wetlands are plentiful (Krapu, 1974a; Swanson et al., 1985). Scuds (Order Amphipoda) are the primary food of breeding White-winged Scoters and Lesser Scaup. One species of scud, Hyalella azteca, provided 97% of the diet of female White-winged Scorers nesting in Saskatchewan (Brown and Fredrickson, 1986). The genera Hyalella and Gammarus are important foods of Lesser Scaup breeding in marshes in Manitoba and Saskatchewan (Rogers and Korschgen, 1966; Bartonek and Hickey, 1969; Dirschl, 1969; Afton and Hier, 1991), and in the Northwest Territories (Bartonek and Murdy, 1970).

Mergansers eat mostly fish, and are the only waterfowl that feed primarily on vertebrates. Studies in Europe and North America indicate that sticklebacks (Gasterosteus aculeatus) (Bengtson, 1971a) and juvenile salmonids (Oncorhynchus spp.) (White, 1957; Wood, 1986) are major foods of the Red-breasted and Common Mergansers, respectively.

Plant foods provide a significant percentage of the breeding diets of female ducks of many species, particularly during prelaying and incubation (Table 1-5, Fig. 1-1D). Vegetative plant parts are most important in the diet of American Wigeon (Fig. 1-1D), the only species of North American dabbling duck specialized for grazing. In Australia, the Maned Duck (Tribe Cairinini), another grazer, apparently feeds primarily on legumes and grasses during the breeding season (Kingsford, 1989). Females of some species (e.g., Northern Pintails breeding in the Prairie Pothole Region) eat macroinvertebrates during egg laying and then switch to plant foods after the clutch is completed, whereas females of some other species continue to feed on animal matter (Table 1-5). Natural selection may favor use of plant foods by certain species during incubation because plants are more stable food resources than macroinvertebrates. This hypothesis is supported by data showing that Northern Shovelers, which feed on small crustaceans, take more breaks to forage during incubation than do many other anatids (Afton, 1980).

Male ducks generally forage less intensively than their mates (Table 1-3). When actively feeding pairs of Blue-winged Teal were collected in North Dakota, the average food intake of males was only half that of females (Swanson et al., 1974). Despite differences in time spent foraging, diets of males and nonlaying females usually contain similar proportions of plant and animal foods (e.g., Jarvis and Noyes, 1986). During laying, however, females eat more macroinvertebrates than males; laying female Gadwalls ate nearly twice the amount of Diptera, three times the amount of Coleoptera, and less plant matter than their mates feeding at the same sites (Serie and Swanson, 1976).

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