Reproductive success in birds often is measured in terms of the proportion of nests that hatch or fledge young. We assessed survival of 35 Mallard (Anas platyrhynchos) broods near Yorkton in east-central Saskatchewan, Canada, from 1990 to 1993. Brood survival was higher for offspring that hatched early in the breeding season in three of four years when wetland abundance declined over the summer. Seasonal patterns of wetland abundance were reversed in 1993, and early hatching broods suffered higher mortality. A more accurate measure of reproductive success is the number of young that survive to breed, i.e. recruitment. Ducklings were tagged at Yorkton and on the St. Denis National Wildlife Area (1985 to 1995), enabling us to identify individuals that returned to breed in subsequent years. Female recruits were more likely to originate from early hatched nests at both sites. With respect to recruitment, we also tested for an additional advantage of hatching early, over and above that associated with duckling survival. There was no statistical difference in the seasonal distribution of nests producing recruits compared with broods that successfully fledged at least one young. Thus, the selective advantage to nesting early for Mallards likely is driven by a seasonal decline in duckling survival, which in turn may be associated with decreasing wetland density.
TIMING OF BREEDING is considered an adaptive trait that evolved to maximize reproductive output (Lack 1968). Survival of offspring born later in the breeding season often is lower than that of young born earlier in the season(e.g. mammals, Iason 1989, Virgl and Messier 1997; fish, Schultz 1993). Similarly, among birds, young that hatch early in the breeding season have higher survival to independence than young that hatch late in the season (e.g. Perrins 1965, Murton and Westwood 1977, Dow and Fredga 1984, Newton and Marquiss 1984, Burger et al. 1996). Survival to fledRing or independence often is used as an estimate of reproductive success; however, this measure may lead to improper inferences about reproductive fitness (Cooke et al. 1984). Reproductive success is best measured by the number of offspring that enter the breeding population (i.e. recruitment; Howard 1979, Hepp et al. 1989). Sciences and Engineering Research Council, the University of Saskatchewan Alumni Association, and the Isabel Lopez and Dennis Pattinson memorial scholarship funds. Ducks Unlimited (Canada) provided logistical support. We greatly appreciate the time spent in the field by the many assistants and our families. The work would not have been possible without the cooperation of landowners in both areas. Comments by R. Cox, D. Mauser, J. Rotella, and two anonymous reviewers improved the manuscript.