Northern Prairie Wildlife Research Center
The effect of age was demonstrated by Cooper (1978), who indicated that egg size of Canada Geese increased with age of the female, up to year 5. Kear (1973) reported peak egg masses of captive Hawaiian Geese among three- to five-year-olds. Conversely, age effects were lacking in captive Mallards (Batt and Prince 1978), Pintail (Duncan 1987b), and Wood Ducks (Hepp et al. 1987). Other factors potentially affecting egg size include social dominance (Lamprecht 1986).
Hen condition is presumed to affect egg size (Bezzel 1969, Ryder 1970, Hepp et al. 1987), but Batt and Prince (1978) found no effect of hen mass on egg mass for captive Mallards, and J. T. Barnett and J. K. Ringelman (pers. commun.) found no relation between Mallard condition at arrival on the breeding ground and egg dimensions. Also, a seasonal decline in egg size has been reported (Cooper 1978, but see Rohwer and Eisenhauer 1989). This decline does not seem related to renesting: Cooper (1978) reported that renests of Canada Geese tended to contain larger eggs than initial nests, as did Batt and Prince (1979) and Eldridge and Krapu (1988) for Mallards. Bezzel (1969) noted that very small Eurasian Pochard eggs did not occur in large clutches, and egg size was less variable in larger than in smaller clutches. From this he argued that the small eggs might be a result of hen exhaustion. Krapu (1979) and Eldridge and Krapu (1988) observed that Mallards on a high-protein diet laid heavier eggs than those on a reduced-protein diet. Captive Canada Geese on an alfalfa diet laid lighter eggs than they had during the previous year, when they were on a more balanced diet (Murphy 1988).
Genotype might influence both average egg size (Manning 1978; Birkhead, Bacon, and Walter 1983; Eldridge and Krapu 1988, Laurila 1988), and variation in egg size. Smaller eggs may be more appropriate in temperate areas, with moderate climate and predictable food supplies for young, than in northern ones, with harsh weather and erratic food provisions (Lack 1968). Ankney and Bisset (1976) found substantial variation in egg masses among individual Snow Geese. Lessells, Cooke, and Rockwell (1989) estimated heritability of egg mass among Lesser Snow Geese to be about 60%. Ankney and Bisset (1976:733) hypothesized that "a wide range of egg-weight genotypes survives because of the annual variation in environmental conditions at hatch."