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Renesting is defined to be nesting that occurs after an earlier clutch is destroyed or abandoned. Therefore, renesting rates are higher when nest success rates are lower. Direct measures of renesting are the number of nesting attempts made per female and the probability of renesting given that a clutch is destroyed. An indirect measure is the renest interval (time between termination of one clutch and initiation of another); shorter intervals permitting more renests, longer ones fewer. Variation among geographic areas in renesting can result from variation in breeding-season duration, which itself is a function of climate. Northern geese probably do not renest at all (Klopman 1958).

Double brooding is the attempt to produce a second clutch after one has been hatched during the same breeding season. Double brooding is not widespread among waterfowl but has been reported occasionally in the Canada Goose at low latitudes (Brakhage 1985), Mallard (Bjärvall 1969, Doty 1975, Titman and Lowther 1975), Pintail (Sowls 1951), and Redhead (Alliston 1979b), and is fairly common in Wood Ducks (Odom 1971, Fredrickson and Hansen 1983, Kennamer and Hepp 1987, Moorman and Baldassarre 1988).

1. Age and Breeding Experience

Prior breeding success does not insure continued breeding effort; MacInnes and Lieff (1968) found that some Canada Geese that had nested successfully in earlier years did not attempt to nest in a subsequent year. Fredrickson and Hansen (1983) indicated that older Wood Ducks were primary contributors of double broods, but Kennamer and Hepp (1987) observed a yearling Wood Duck with two broods.

Age is especially likely to be a factor in crowded situations, where young birds may be less able to compete for limited food or nest sites (Surrendi 1970 for Canada Goose; Grice and Rogers 1965 for Wood Duck; Blohm 1979 for Gadwall; Lokemoen, Duebbert, and Sharp 1984 for Gadwall and Mallard; Mihelsons et al. 1968 and Mihelsons, Mednis, and Blums 1986 for Tufted Duck).

Curio (1983) proposed that young birds reproduce less because of "reproductive restraint," rather than "reproductive constraint." Assuming a survival cost associated with reproduction, Curio (1983:403) concluded that "young breeders with high future prospects of breeding refrain from expending maximal parental effort to forestall the associated risk of dying and, hence, future reproduction." Consistent with this view, Dow and Fredga (1984) indicated for Goldeneye females that first- or second-time breeders that laid early, or first-time breeders with larger clutches, tended to disappear from the breeding population, suggesting a cost of reproductive effort. These trends were not evident in more experienced females, indicating an age-dependent effect. In the North American prairie and parkland, up to one fourth of the breeding female ducks succumb to predation each year (Johnson and Sargeant 1977; Sargeant, Allen, and Eberhardt 1984, Blohm et al. 1987); no age-dependence in this mortality has been noticed.

2. Condition of the Social Status of the Bird

Two studies have found a relation between the number of nests initiated and a measure of condition. Cowardin, Gilmer, and Shaiffer (1985) reported that nesting attempts by Mallard females increased with increasing condition index (body weight/wing length) measured early in the breeding season. Eldridge and Krapu (1988) noticed fewer nesting attempts for captive Mallards under a poorer diet. In contrast, Duncan (1987a) noticed no difference in renesting rates between captive Pintail on diets containing 14% versus 29% protein.

Indirect evidence of the effect of condition derives from lower probabilities of renesting the later that initial clutches are destroyed for Blue-winged Teal (Strohmeyer 1967), and Gadwall (Gates 1962, who further observed that the ability of Gadwalls to renest was lower still after destruction of a renest). Among Canada Geese breeding in Manitoba, 92% of 12 pairs that lost their first nest during laying made a second attempt (Cooper 1978). Of 20 pairs that lost a nest at two days of incubation, 70% renested. None of 11 pairs losing nests later in incubation made another attempt.

Renest intervals are longer for females in poorer condition. Longer renest intervals were noted by Eldridge and Krapu (1988) for captive Mallards on a poorer diet. Renesting female Mallards averaged 74% less lipid content than females making first attempts (Krapu 1981). Swanson et al. (1986) found longer renest intervals for captive Mallards exposed to limited natural food than for those with unlimited food. Various authors have noted lengthened renest intervals for hens with clutches destroyed later in development: Wood Duck (Grice and Rogers 1965), several prairie duck species (Sowls 1955), Black Duck (Coulter and Miller 1968), captive Mallard (Swanson et al. 1986), and Blue-winged Teal (Strohmeyer 1967).

Social status too is highly confounded with, and likely to be determined largely by, age and physical condition. Female Eiders that paired late during the previous winter had a significantly higher incidence of nonbreeding (77%) than females paired earlier (17%) (Spurr and Milne 1976). Less aggressive or lower-ranking individuals are unlikely to secure the best breeding sites (Barry 1967 for Snow Goose; Collias and Jahn 1959 for captive Canada Goose; Mihelsons, Mednis, and Blums 1986 for Tufted Duck).

3. Wetland Habitat and Weather

Nonbreeding because of limited food resources was suggested by Bengtson (1972b). Females of some species return to breeding areas with adequate nutrient reserves for at least one nesting attempt, whereas others rely on food resources obtained on the breeding grounds.

In the far north, short seasons severely constrain reproductive activities, and a lack of adequate nest sites on arrival may preclude breeding (reviewed by Newton 1977).

Habitat restraints may affect only some members of the population, creating a density- or age-dependent effect. Improved wetland conditions tended to increase the proportion of yearling and 2-year-old female Lesser Scaup that nested (Afton 1984).

Wetland conditions can cause geographic differences in nesting incidence. Many ducks that normally breed in the North American prairies and parklands are displaced, usually but not always northward, by drought on their customary breeding range (Johnson and Grier 1988). These displaced birds are less likely to attempt breeding (Hansen and McKnight 1964, Calverley and Boag 1977, Derkson and Eldridge 1980). Calverley and Boag (1977) suggested that some birds migrate directly to these northern areas and will attempt to breed, while others are displaced there by drought elsewhere and may not try to breed that year. Geographic differences in breeding incidence may also reflect the fact that non-breeding individuals distribute themselves differently from breeders (e.g., Kerbes 1986), rather than breeding being a consequence of where the birds spend the breeding season.

In addition to indirect effects through wetland habitat, weather has other effects on the proportion of waterfowl that breed, especially among populations in harsh environments, such as the far north (e.g., McLaren and McLaren 1984). Even in temperate areas, cold and windy weather or snowstorms can reduce nesting (Yocom and Hansen 1960, Swanson et al. 1986), as can hot weather (Reed 1970). Rainfall may provide a stimulus for breeding (e.g., Frith 1967 for Australian Wood Duck, Guiler 1967 for Cape Barren Goose) and inadequate precipitation may reduce nesting efforts (Hammond and Johnson 1984).

4. Density of Birds and Individual Differences

In addition, there is the possibility of variation that cannot be accounted for by external variables. Coulter and Miller (1968) found considerable individual variation in renesting persistence among Mallards, Black Ducks, and Ring-necked Ducks; some hens never renested regardless of how early the clutch was destroyed, others renested as many as three times.