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Another confounding problem is that, until recently, most nesting studies did not recognize the bias of nest success estimates based on samples of nests that have already survived various periods of time before being found. Mayfield (1961) proposed an analytic technique to overcome some such problems, but his method was little-used until recently (Miller and Johnson 1978).

Losses of entire clutches have been attributed to predation, abandonment, agriculture, weather, and human disturbance. These may operate in concert. Because causes of loss are often not reliably known, some reports discuss nest success in general, others describe losses attributable to specific causes.

1. Age, Breeding Experience, and Condition of the Bird

Abandonment may be more likely among young or less experienced females (Alliston 1979a, Heusmann 1984) or those in poor physical condition. Braithwaite (1982) indicated that a failure in food supplies may be the major cause of abandonment by Black Swans.

2. Nesting Habitat

Although nest losses to predation should obviously vary with predator numbers, few studies (e.g., Johnson, Sargeant and Greenwood 1989) have documented this, usually because of the difficulty of indexing predator numbers. Study areas from which predators have been excluded, however, consistently show high nest success (Balser, Dill, and Nelson 1968; Duebbert and Kantrud 1974; Duebbert and Lokemoen 1980; Lokemoen et al. 1982).

Some sites offer protection from predators. Predation is often lower on islands than on mainland (Mihelsons 1968; Schamel 1977; Giroux 1981a, b; Duebbert, Lokemoen, and Sharp 1983), especially islands farther from shore (Giroux 1981a) or farther from gull colonies (Odin 1957). Predation may be lower on nests well-concealed by vegetation (Odin 1957, Hill 1984b, Sugden and Beyersbergen 1987), but not all investigators (e.g., Kalmbach 1937, Hammond 1940, Keith 1961) found the anticipated effect. Johnson, Sargeant, and Greenwood (1989) found predation rates on duck nests to vary according to precipitation patterns and wetland conditions, which they suggested might relate to improved vegetation around the nest or increased numbers of buffer prey. Afton (1984) and Cowardin, Gilmer, and Shaiffer (1985) detected higher nest success of Lesser Scaup and Mallards, respectively, in years or areas with many wetlands. In an experiment, Bowman and Harris (1980) found reduced foraging efficiency by raccoons with increased spatial heterogeneity of nesting habitat.

Buffer prey, usually small mammals, can reduce depredation on upland nests (Hanson and Smith 1950, Barry 1967, Byers 1974, Eisenhauer and Kirkpatrick 1977, Weller 1979, Summers 1986). Crabtree and Wolfe (1988) reduced predation on waterfowl nests by experimentally providing alternative foods for striped skunks. Conversely, Ryder (1961) found little evidence that American Coot clutches buffered losses of duck nests in marshes. An indirect buffering was proposed by Bousfield and Syroechkovskiy (1985), who observed that in years of low lemming populations, fewer arctic foxes had litters, so they ranged more widely and inflicted damage to more Snow Goose nests.

Waterfowl sometimes nest among larids or other, usually colonial, birds. Blomqvist and Elander (1988) showed that King Eider nests were significantly clustered near those of Long-tailed Skuas. Arctic Terns, Black Turnstones, or larids may provide protection against avian predators for ducks and geese nesting among them (Barry 1967, Mihelsons 1968, Evans 1970, Mickelson 1973, Kistchinski and Flint 1974, Schamel 1977, Young and Titman 1986, Götmark and Åhlund 1988), although some larids themselves will depredate eggs or ducklings (Dwernychuk and Boag 1972). Canada Geese may offer some protection for ducks (Giroux 1981b) or Graylag Geese (Fabricius et al. 1974). A similar interspecific phenomenon involved an Eider colony growing rapidly under protection afforded by tethered husky sledge dogs (Meltofte 1978).

The land use of a nesting habitat can affect nest success. Waterfowl nests constructed in hayland are vulnerable to mowing losses (Labisky 1957, Evans and Wolfe 1967, Mihelsons 1968, Livezey 1981) or predation because of increased exposure (Labisky 1957, Mihelsons 1968). Nests in areas subjected to grazing may be trampled (Moyle 1964, Gjersing 1975), and those in cropland sometimes suffer plowing or other agricultural destruction (Meanley and Meanley 1959, Higgins 1977). In addition, nests are destroyed by fires set by farmers (Stoudt 1971, Fritzell 1975) or grassland managers (Kruse and Piehl 1986).

3. Density and Parasitism

Abandonment rates can be high in dense nesting situations (Newton and Campbell 1975; Duebbert, Lokemoen, and Sharp 1983) or when parasitism is substantial (Dement'ev et al. 1967, Brown and Brown 1981, Giroux 1981c, Eriksson and Andersson 1982). Primary causes of nest abandonment in dense nesting situations include competition for nest sites and interference from other birds (Grenquist 1963, Anderson 1965, Titman and Lowther 1975). Ewaschuk and Boag (1972) considered attacks by neighboring males to cause abandonment of Canada Goose nests. Duebbert, Lokemoen, and Sharp (1983) attributed high abandonment rates of Mallards and Gadwalls on a crowded island to harassment of nesting females by males, rather than to inter- or intraspecific parasitism. Gauthier and Smith (1987) suggested that in one year desertion by Buffleheads was density-dependent among first-time nest box users.

4. Humans and Weather

Weather phenomena leading to abandonment include snowstorms (Heusmann 1984), heavy rains (Dement'ev et al. 1967, Heusmann 1984), and late ice breakup (Fredga and Dow 1984). For nests in low-lying situations, flooding can be an important cause of abandonment (Andrews 1952, Hildén 1964, Joyner 1977, Livezey 1980).

5. Other Factors

Visits to nests by predators can lead to abandonment (Bellrose, Johnson, and Meyers 1964; Cowardin, Gilmer, and Shaiffer 1985; Haramis and Thompson 1985; Greenwood et al. in prep.), as can trampling by cattle (Thomas 1980). Partially depredated nests may be more likely to be abandoned (Haramis and Thompson 1985). The death of a hen away from the nest will cause the appearance of nest abandonment (Livezey 1980).