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1. Seasonal Decline in Clutch Size

One result of this seasonal decline is that clutches tend to be smaller than normal when breeding seasons are delayed. This tendency is pronounced where seasons are constricted, such as the far north.

2. Age and Breeding Experience

Individuals with previous breeding experience were shown to produce larger clutches than novices among the Mute Swan (Birkhead, Bacon, and Walter 1983), Redhead (Alliston 1979a), and Hooded Merganser (Morse, Jakabosky, and McCrow 1969). Cooke, Bousfield, and Sadura (1981) noticed smaller clutches among Snow Geese for first- or second-time breeders than for more experienced ones. Increases in clutch size from breeding year 1 to 2 and from 2 to 3 were found for individual Eiders (Gross 1938), Goldeneyes (Dow and Fredga 1984), and Red-breasted Mergansers (Wilhjelm 1938, cited in Klomp 1970:12). In an analysis that separated the effects of age and experience, Heusmann (1975) found that clutch sizes of Wood Ducks nesting for the first time as two-year-olds were smaller than those of experienced two-year-olds.

Senescent declines in average clutch size were suggested for captive Hawaiian Geese more than seven or eight years old (Kear 1973) and for Goldeneye females past their sixth breeding season (Dow and Fredga 1984)

3. Condition of the Bird and Wetland Habitat

Clutch size and condition of the female are not always related. Wood Ducks in Missouri depleted their fat reserves very little during incubation, which may have permitted them to lay more eggs (Drobney 1980). Duncan (1987a) found no relation between clutch size and body mass of wild Pintails during incubation, and the clutch size of captive Pintails declined during the season despite ad libitum food supplies. Dobush (1986) indicated that clutch size of White-winged Scoters did not depend on endogenous body reserves. Rohwer (1984) questioned the concept of nutrition as the proximate factor limiting clutch size in prairie ducks; his experiments indicated that some captive Mallards extended laying if eggs were removed.

That diet affects clutch size was indicated by Krapu and Swanson (1975) and Eldridge and Krapu (1988), who observed lower egg production by Pintails and Mallards, respectively, fed low-protein diets. Clutch size in turn depends on the quality of food in wetlands used by females. Bengtson (1971) noticed reduced clutches in several species during a year when chironomid larvae were greatly reduced. Bengtson (1972a) suggested that the small clutch size of Harlequin Ducks was an adaptation to the restricted food base found in the fast-flowing streams they favor. Conversely, Pattenden and Boag (1989) detected no difference in clutch size of captive Mallards on restricted versus unrestricted diets. Salyer (1962) found no difference in clutch sizes of prairie ducks between a drought and a non-drought year. And Gauthier (1987a) noticed no correlation between clutch size and food density in territories of Buffleheads.

4.Nesting Habitat and Weather

Delayed nest site availability due to inclement weather may result in reduced clutch size. Clutch sizes appear to decline with the calendar date, regardless of the stage of the nesting season, so clutches tend to be smaller in late years. This relation has been recognized mostly in northern goose and swan populations (reviewed by Newton 1977) but was also demonstrated among Mallards in North Dakota (Krapu and Doty 1979) and Redheads in Quebec (Alliston 1979a).

5. Parasitism and Other Factors

Clutch size sometimes varies geographically within a species. In the northern hemisphere, clutches are often smaller at high latitudes, and those of waterfowl nesting on oceanic islands are typically smaller than those of the same or closely related species on nearby mainland (Lack 1970, Weller 1980).

Evidence for density effects on clutch size is scant. Coulson (1984) noticed that clutch size of Eiders on an island decreased as the adult population increased in the latter years of his study. Findlay and Cooke (1983) suggested population density as one factor affecting clutch size, possibly by increasing follicle resorption rates or through competition for nest sites. Most investigators reported no relation between clutch size and either population density (Hildén 1964 for Tufted Duck and Greater Scaup, Bengtson 1971 for several duck species, Eriksson 1979 and Fredga and Dow 1983 for Goldeneye) or nest density (Yocom, Buechele, and Harris 1956 for Canada Goose). Within nesting colonies, larger clutches are often found in the areas of highest nest density (Marshall 1967, McLandress 1983), probably because those areas host the earliest nests.

Clutch size may be partly heritable. Year-to-year repeatability of clutch size within individual females has been noted for the Canada Goose (MacInnes and Dunn 1988a), Mallard (Batt and Prince 1979, Swanson et al. 1986), White-winged Scoter (Koskimies 1957), and Goldeneye (Dow and Fredga 1984). Lessells, Cooke, and Rockwell (1989) estimated that about 15% of the variation in Snow Goose clutch size is heritable. Findlay and Cooke (1983) indicated that although females tended to produce clutches of consistent size, some female were more sensitive than others to environmental influences. Laurila and Hario (1988) found repeatability of clutch size among Eiders to be 0.33. Birkhead, Bacon, and Walter (1983) unsed starch-gel electrophoresis to show an association between genotype of males and large clutches in Mute Swans.