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A longer span of breeding allows for more nesting attempts and potentially greater reproduction. In some instances earlier breeding is advantageous for selection of a nest site (Grice and Rogers 1965, Findlay and Cooke 1982).

1. Species and Geographic Variation

Within a species, the timing of breeding often varies geographically. The onset of breeding is less fixed, and the duration often longer, in areas with favorable climate (reviewed by Newton 1977). Southern-breeding populations of a species typically initiate nesting earlier and continue longer than their northern-breeding counterparts. This holds for the Canada Goose (Klopman 1958, Mickelson 1973), Wood Duck (Cunningham 1968, Haramis 1975), Black Duck (Reed 1970), Mallard and Pintail (Calverley and Boag 1977), captive Mallard (Batt and Prince 1978), and several dabbling and diving duck species (Toft, Trauger, and Murdy 1982). Vaisanen (1974) found that nesting by early breeders, but not late breeders, was progressively delayed the farther north the breeding area. In addition, breeding may be later at higher altitudes (Krohn and Bizeau 1980 for Canada Geese).

2. Age, Breeding Experience, and Condition

Despite starting earlier, older birds may persist with laying attempts longer than younger ones (Olson 1964 for Canvasback; Lokemoen, Duebbert, and Sharp [1990] for Gadwall). The earlier and more prolonged nesting by older, more experienced waterfowl has been variously attributed to their better condition (Mihelsons et al. 1968, Ely and Raveling 1984), greater food-gathering abilities (Duncan 1987a), earlier arrival on breeding grounds (Johnson 1978), and superior home ranges (Dzubin 1969).

There is both direct and indirect evidence that condition of the female influences the timing of nesting. Earlier laying by females in better condition was indicated by Spurr and Milne (1976) for Eiders, J. T. Barnett and J. K. Ringelman (pers. commun.) for Mallards, and Hepp et al. (1987) for Wood Ducks. Captive Mallards on restricted diets initiated laying later than those on unrestricted diets (Pattenden and Boag 1989). Indirect evidence that condition affects timing of nesting includes late nesting the following year by Canada Geese that had had larger broods (Lessells 1986), late laying dates of Mute Swans following a severe winter (Bacon 1980), and late breeding by Black Swans after a winter of reduced food supplies (Miers and Williams 1969).

3. Wetland and Nesting Habitat

The availability of nest sites can influence timing of breeding. Nesting on islands is often deferred until they are no longer ice-bound (Hildén 1964, Ahlén and Åndersson 1970), probably as a safeguard against terrestrial predators (Lack 1950, Schamel 1977). Nest sites on upland areas must be free of snow, ice, and meltwater before nesting commences (Barry 1967, Ryder 1967, Mednis 1968, Eisenhauer and Kirkpatrick 1977, Kerbes 1986). Waterfowl that nest in vegetation may delay breeding until vegetation reaches an appropriate stage for nest construction or security (Mendall 1958, Meanley and Meanley 1959, Havlín 1966b, Jarvis and Harris 1971).

4. Weather

Early-nesting species are more often affected by weather than late-nesting ones (Mendall 1958, Bezzel 1962, Newton and Campbell 1975). Conversely, Hammond and Johnson (1984) showed that the later-nesting Gadwall and Blue-winged Teal were affected as much as the Mallard by cool temperatures during their normal initiation period. The difference between early- and late-nesting species may simply reflect the greater chance that early-nesting species will encounter adverse weather during nest initiation.

Nesting may be prolonged into the summer by certain weather features, such as adequate precipitation (Hammond and Johnson 1984 for Mallard and Blue-winged Teal, Lavery 1967 for Wandering Whistling Duck and Grey Duck), and by above-normal temperatures in spring (Hanson and Eberhardt 1971 for Canada Goose). Delayed start of breeding due to inclement spring weather may result in an extended nesting period (Sowls 1955 for Mallards and Pintails; Cowardin, Gilmer, and Shaiffer 1985 for Mallards).

5. Other Factors

Some social factors have been proposed as influences on the timing of breeding: time of pairing the previous winter for Eiders (Spurr and Milne 1976), and social stimulation for Eurasian Pochards (Bezzel 1969).