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Foraging Ecology and Nutrition

IV. Timing of Nutrient Acquisition

Waterfowl employ numerous strategies to meet their nutritional needs for breeding. Some species acquire most of their nutrients for reproduction while on wintering and/or staging areas, and others depend primarily on daily intake of nutrients during the nesting season (Owen and Reinecke, 1979).

A. Swans and Geese

North American swans probably derive a significant part of their nutrient requirements for reproduction from wintering grounds and/or spring staging areas. Tundra Swans of the eastern population leave mid-Atlantic wintering grounds relatively lean, but stop at intermediate staging areas where nutrient reserves important to reproduction are thought to be acquired (Bortner, 1985, p. 32). Information is lacking on patterns of nutrient acquisition by Trumpeter Swans.

Most North American goose populations depend on nutrients imported to the breeding grounds to provide a significant part of their requirements for reproduction (Table 1-1). Canada Geese breeding in interior regions have adapted to harsh climatic conditions on their temperate, subarctic, and arctic breeding grounds prior to, and during nesting, by acquiring nutrient reserves before leaving temperate wintering and staging areas. Giant Canada Geese deposit sufficient nutrient reserves to satisfy protein and lipid requirements for egg production before leaving their wintering grounds in southern Minnesota (McLandress and Raveling, 1981a). Midcontinent populations of Interior Canada Geese and Lesser Snow Geese acquire most of their nutrients for reproduction during migratory stopovers in temperate and subarctic regions (Hanson, 1962; Raveling and Lumsden, 1977; Wypkema and Ankney, 1979; Thomas and Prevett, 1982a; Alisauskas, 1988). Among Lesser Snow Geese, most fat reserves are deposited in the northern prairie region, whereas protein storage occurs early in spring migration and after geese arrive on staging areas along southern Hudson Bay (Alisauskas, 1988). Some arctic-nesting geese with widely spaced spring staging and nesting areas feed intensively after arrival on their breeding grounds, presumably to replenish depleted energy reserves. Greater Snow Geese acquire fat reserves principally on staging areas in the St. Lawrence River estuary (Gauthier et al., 1984a), but females still forage 75% of the time during the extended prelaying period (Table 1-2). Brant that follow an inland route to their breeding grounds in the arctic northwest of Hudson Bay deposit large fat reserves prior to departure from their coastal wintering grounds on Long Island, New York (VanGilder et al., 1986). However, Brant also feed intensively on the breeding grounds and derive part of their nutrient requirements for reproduction there (Ankney, 1984). Greater White-fronted Geese of the Pacific Flyway population deposit fat reserves during their annual spring stopover in the Klamath Basin. Spring weight gains of western White-fronts are somewhat less than those of most other species of arctic-nesting geese (Ely and Raveling, 1989), and the population breeding on the Yukon-Kuskokwim Delta, Alaska, acquires part of the energy and nutrient reserves necessary for reproduction after arrival on the breeding grounds (Budeau, 1989).

Some species or populations of geese derive a major part of the nutrients required for egg production from foods obtained on the breeding grounds (Tables 1-1, 1-2). Female Dusky Canada Geese nesting in a relatively mild climate on the Copper River Delta in southeastern Alaska feed intensively prior to and during egg laying (Table 1-2), and acquire an estimated 66% of their protein requirements for egg production from foods available on the nesting grounds (Bromley, 1984, p. 57). For smaller species such as Cackling Canada Geese and Brant, nutrient reserves are the primary source of lipid and protein for egg production (Table 1-1). However, daily food intake is important as a secondary source of protein for egg production and as a primary source of energy for maintenance, providing an estimated 60% of the requirements of female Cackling Canada Geese (exclusive of the energy content of eggs) between arrival and initiation of incubation (Raveling, 1979a).

The extent of foraging by female geese during incubation also varies among species and populations. Female Lesser Snow Geese, Ross' Geese, Emperor Geese (Table 1-2), and Interior Canada Geese feed little during incubation and experience marked losses of body weight (Ryder, 1967; Harvey, 1971; Raveling and Lumsden, 1977; Thompson and Raveling, 1987). In contrast, female Brant feed regularly during incubation (Table 1-2), and obtain an estimated 78% of their energy requirements from foods available on coastal marshes near the nesting site (Ankney, 1984). Feeding during incubation is necessary for Barnacle Geese nesting in Spitzbergen; females that fall below median food intake rates generally fail to complete incubation (Prop et al., 1984). All species of geese forage intensively during the brood-rearing period to recover from their emaciated condition (Table 1-2). Female Lesser Snow Geese feed 85% of the daylight hours during brood-rearing (Harwood, 1977), and Cackling Canada Geese feed almost continuously after the clutch hatches (Raveling, 1979b).

Geese apparently acquire most of the calcium needed for reproduction after arriving at breeding areas (Table 1-1). Femur calcium levels in Lesser Snow Geese increased 80% during spring migration, mostly while the birds staged in North Dakota and southwestern Manitoba, but calcium reserves in medullary bone accounted for only 17% of the calcium required for egg production (Campbell and Leatherland, 1983). Cackling Canada Geese deposited medullary bone during the last 6-7 days of rapid ovarian development, after arrival on Alaskan breeding areas (Raveling et al., 1978). Changes in ash content in carcasses of Cackling Canada Geese between arrival and the onset of incubation are sufficient to account for all of the calcium needs for egg production (Raveling, 1979b).

The relative importance of endogenous and dietary calcium to reproduction in geese is difficult to assess because the pattern of calcium deposition and mobilization is more complex than the pattern for fat. The contribution of fat reserves to reproduction can be determined by regressing reproductive fat on body fat, because fat reserves are deposited prior to nesting and then progressively depleted during nesting. In contrast, there is a dynamic equilibrium between calcium supplies in the blood, bone, and shell gland of female birds during egg laying (Sturkie, 1988). Calcium is absorbed from medullary bone when requirements for shell formation exceed dietary intake, and deposited in medullary bone when dietary intake exceeds requirements for shell formation. Data on turnover rates of medullary bone in female waterfowl are needed before the importance of endogenous calcium can be assessed adequately.

B. Ducks

Patterns of nutrient acquisition for reproduction in the Subfamily Anatinae differ substantially from those of the geese (Table 1-1). Female Anatinae generally obtain protein and calcium for egg production from exogenous sources through intensive foraging during the laying stage (Table 1- 3). The timing of fat deposition and mobilization is more variable. Wood Ducks (Tribe Cairinini) nesting in Missouri acquire fat reserves for reproduction after arrival at their breeding areas (Drobney, 1982). Mallards (Tribe Anatini) and Ring-necked Ducks, Canvasbacks, and Lesser Scaup (Tribe Aythyini) breeding in the Midcontinent Region of North America imported most of the fat used for production of their initial clutches (Krapu, 1981; Hohman, 1986; Barzen and Serie, 1990; Afton and Ankney, 1991). Mallards deposit large fat reserves in March prior to departure from wintering grounds and/or staging areas in Nebraska (Jorde, 1981) and Missouri (Heitmeyer, 1988). Of the perching, dabbling, and pochard species studied thus far, most satisfy their fat requirements for egg production (renesting excluded) primarily from endogenous reserves (Table 1-1).

Most species of the Tribe Mergini breed in areas of limited wetland fertility and probably import a significant part of their nutrient requirements for reproduction. Patterns of nutrient acquisition and utilization vary widely, however. White-winged Scoters forage intensively during prelaying and laying (Table 1-3) to satisfy nutrient requirements for egg production from daily food intake and use fat and protein reserves primarily during incubation (Dobush, 1986). Female Common Eiders deposit fat and protein reserves for egg production prior to nesting and rarely forage during egg laying or incubation (Milne, 1976; Korschgen, 1977). The strategy used by Common Eiders is unusual among the Anatinae and apparently has evolved to limit exposure of unattended nests to avian predators (Milne, 1976) rather than to overcome nutrient limitations on breeding areas. Eiders usually select small islands as nest sites, apparently for protection from red fox (Vulpes fulva) and arctic fox (Alopex lagopus), not primarily for their proximity to good feeding sites (Parker and Holm, 1990). Little information is available concerning nutrient acquisition by King, Spectacled, or Steller's Eiders, but the large clutch sizes of these species (Palmer, 1976) suggest that they are less dependent on endogenous nutrients for clutch production than the Common Eider. The Mergini also compensate for limited food abundance on breeding areas by extending the period of rapid follicular growth and by decreasing rates of egg production.

Ruddy Ducks (Tribe Oxyurini) rely primarily on food resources available on the breeding grounds to obtain nutrients for egg production, and utilize fat reserves deposited earlier to provide energy during incubation (Table 1-1). Ruddy Ducks also compensate for their dependence on breeding ground foods by extending the period of rapid follicle development to 11 days (Gray, 1980, p. 63), which is 4-5 days longer than most dabbling ducks and pochards. Extending the period of rapid follicle growth reduces the maximum daily energy requirement for egg production by 15% (Tome, 1981, p. 47).

There currently are no data to indicate whether widely distributed species exhibit intraspecific variation in the magnitude of endogenous nutrient reserves acquired before arrival on the breeding grounds. For example, Wood Ducks nesting in southeastern Missouri acquire fat reserves after arrival at the breeding area (Drobney, 1982), but populations nesting farther north may not have adequate time or food available after arrival to deposit comparable reserves.

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