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Foraging Ecology and Nutrition

VIII. Effects of Agricultural Development on Food Resources

Annual recruitment rates of waterfowl are increasingly being determined by the extent that bird life histories are compatible with human activities, particularly agricultural development in temperate regions. In this section, we discuss impacts of agricultural practices, both positive and negative, on food resources of prebreeding and breeding waterfowl and consider some of the implications to waterfowl populations.

A. Geese

Goose populations in North America and Europe have exhibited an upward trend in recent decades (U.S. Fish and Wildlife Service and Canadian Wildlife Service, 1986; Madsen, 1991), except where recreational and subsistence hunting mortality have exceeded recruitment rates (Raveling, 1984). Although destruction of natural wintering and staging habitat has occurred on both continents, geese have avoided potential food shortages by increasing their use of agricultural foods, especially cereal grains and domesticated grasses. In North America, the Emperor Goose and Brant still depend primarily on natural foods, but most other species eat agricultural foods during fall, winter, and spring (see section V.A). In Europe, at least 16 of 21 goose populations rely on cultivated lands for a significant part of their food resources during the same periods (Madsen, 1987).

Changes in the diet of Dark-bellied Brant in western Europe from marine plants to pasture vegetation illustrate the trend that has occurred and provide insights regarding the causes. Brant traditionally fed on eelgrass until a major die-off of this plant occurred during the 1930s on both sides of the Atlantic Ocean. In eastern North America, the die-off was associated with a drastic decline in Brant numbers (Cottam et al., 1944). On both continents, Brant responded to the food shortage by increasing their use of salt marshes for feeding (Penkala, 1975; Teunissen et al., 1985). In western Europe, Brant began feeding in croplands when many of the remaining salt marshes were reclaimed for agricultural production (Prokosch, 1979; St. Joseph, 1979). Gradually, Brant expanded their foraging activity from the reclaimed marshes into the uplands. In England, Brant now fly several km inland to glean cereal grains and to graze on managed pastures (St. Joseph, 1979; Madsen, 1987).

Graylag Geese wintering in Europe traditionally fed on tubers and rhizomes of marsh plants (Newton et al., 1974), and occupied a foraging niche similar to that of Snow Geese in North America. However, when drought decreased availability of bulrush tubers in the Marismas of the Guadalquivir River in Spain during 1980-81, an estimated 10,000 Graylag Geese died of starvation. The surviving geese increased their use of winter cereal fields and other agricultural lands (Amat, 1986). Some Graylag populations now use agricultural lands almost exclusively while wintering and staging in temperate regions of Europe.

Rapid switches from natural to agricultural foods by geese probably represent "cultural transmission," wherein young or inexperienced birds learn by accompanying older or more experienced individuals. These short-term learned responses may subsequently be reinforced by higher recruitment rates among populations that are able to transform increased nutrient intake into reproductive effort (Black et al., 1991). Available data indicate that geese increase energy intake dramatically by eating seeds of cereal grains. The estimated daily energy intake of Pink-footed Geese in Denmark during spring was 2,824 kJ when feeding on newly sown barley seed but only 1,267 kJ when grazing in pastureland (Madsen, 1985a). The widespread access of geese to high-energy domesticated plant foods as a result of agricultural development probably has resulted in many populations carrying larger fat reserves to the breeding grounds than occurred formerly.

The rapid population growth of several species of northern-nesting geese in recent decades probably is due, in part, to the contribution of nutrients from agricultural foods to reproductive success. The proportion of female Lesser Snow Geese that nest and their potential clutch sizes, for example, vary with the magnitude of fat reserves carried to the breeding grounds (Ankney and MacInnes, 1978; Davies and Cooke, 1983), and fat reserves of Lesser Snow Geese are synthesized primarily from waste grain (Alisauskas, 1988).

B. Ducks

Cereal grains also are an important component of the diet of some duck species during the prebreeding period and apparently are a major substrate from which fat reserves are produced (Miller, 1986, 1987; Jorde, 1981, p. 60). Fat reserves of the Mallard are the primary source of lipids for egg production in first clutches (Table 1-1), and a similar relationship may exist for the Northern Pintail.

Although cereal grains may increase fat deposition in certain species, the cumulative impacts of agricultural practices in temperate regions have severely reduced macroinvertebrate populations, which are the primary source of protein for breeding ducks (section VI.C). Drainage of wetlands has been particularly destructive. Shallow fertile wetlands used by dabbling ducks and pochards have suffered the greatest losses from drainage because of the potential economic benefits of converting these habitats to cropland (Tiner, 1984; Goldstein, 1988). About 60% and 40% of the original wetlands in the Prairie Pothole Regions of the United States and Canada, respectively, have been lost, mostly to agricultural development (US. Fish and Wildlife Service and Canadian Wildlife Service, 1986).

The common farming practice of tilling shallow wetland basins during summer and fall drawdowns to facilitate spring planting also adversely affects macroinvertebrate populations. Tillage eliminates macrophytes and organic detritus, causing a marked decrease in macroinvertebrate density and availability when reflooding occurs (Swanson et al., 1974). In North Dakota, 29% of the wetland area and 52% of the nondrained wetland basins had tilled bottom soils during 1967 (Stewart and Kantrud, 1973), and intensity of land use has increased markedly since then. In prairie Canada, 59% of the wetland basins and 79% of the margins were degraded by agricultural practices during 1981-85; 38% of the basins and 54% of the margins were tilled (Turner et al., 1987). The negative impact of frequent tillage of basins contrasts markedly with use of the land for livestock grazing, which at moderate levels is thought to be beneficial to wetland macroinvertebrate populations (Kantrud, 1986).

Pesticides and other agricultural chemicals further reduce availability of food to breeding waterfowl. Most insecticides used in the Prairie Pothole Region are toxic to aquatic macroinvertebrates, and readily enter wetland systems because they are applied aerially and drift into adjacent habitats (Grue et al., 1986; Sheehan et al., 1987). Herbicides generally are not as toxic to macroinvertebrates as insecticides, but they can destroy aquatic plants and thereby eliminate habitat for important waterfowl food organisms such as gastropods, crustaceans, and aquatic insects (Sheehan et al., 1987).

The primary impacts of decreased macroinvertebrate availability to breeding ducks in areas affected by agricultural development probably are a reduction in the nesting effort of adults (see chapter 14 of this volume), and a reduction in growth and survival rates of juveniles. The importance of macroinvertebrate availability to recruitment rates of ducks underscores the need to maintain a wetland habitat base capable of supplying abundant food resources to breeding pairs throughout the nesting season and under a wide range of water conditions. Wetland complexes containing a diversity of plant communities, water regimes, and macroinvertebrate populations are particularly well suited for meeting this need.