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The number of young produced can be estimated for local areas by direct counts of broods (Chapter 13). An index to duck production on important North American breeding areas is also obtained on a much larger scale by aerial surveys conducted each July (Henny, Anderson, and Pospahala 1972). These estimates of numbers of young are sometimes translated into brood-pair ratios, which can be thought of as estimates of reproductive rate. The translation generally uses estimates of numbers of adults or pairs on an area at the beginning of the breeding season (see Chapter 13).

Estimates of age ratios (young per adult or young per adult female) at certain times of the year also provide useful indices of reproductive rate. For several species of geese it is possible to distinguish visually young and adult birds at a distance, either on migration stopover areas or on wintering grounds, to estimate age ratio (Hewitt 1950, Boyd 1959, Lynch and Singleton 1964, Jones 1970). For most ducks, young cannot be distinguished from adults in the field, but age ratios among harvested birds are sometimes used to index reproductive rate (Petrides 1949, Bellrose et al. 1961). If adult and young birds available in a population during a hunting season do not have equal chances of being harvested, the age ratio in a harvest sample will provide a biased estimate of the population age ratio. It is often possible, however, to estimate these different probabilities of being harvested independently with band recovery data and to use such estimates to "correct" or adjust harvest age ratio data to yield an estimate of the age ratio in the population at the time of banding (Bellrose et al. 1961, Munro and Kimball 1982).

In addition to these indices of reproductive rate, detailed investigations using marked birds permit direct estimates. Such studies have also yielded inferences about sources of variation in reproductive rate.

1. Species and Geographic Variation

Bellrose et al. (1961:466) adjusted Mississippi Flyway harvest age ratios for differential vulnerability and compared the resulting age ratio estimates (juveniles per adult hen) for 12 species of ducks during 1946-49. Relatively high age ratios were calculated for Mallards, Black Ducks, Northern Shovelers, Ring-necked Ducks, and Canvasbacks. American Wigeon and Green-winged Teal exhibited intermediate age ratios. Age ratios were low for Gadwalls, Pintails, Blue-winged Teal, Redheads and Lesser Scaup. In a comparison of harvest age ratio data from southwestern Ontario (uncorrected for differential vulnerability), Dennis, Fischer, and McCullough (1984) found that Mallard age ratios were consistently higher than those of Black Ducks, suggesting greater Mallard production.

Pospahala, Anderson, and Henny (1974) found that the average number of duck broods (all species combined) per square mile and average number of broods per July pond exhibited a decreasing cline from west to east across the prairie-parkland region of Canada. Smith (1970) used aerial breeding ground survey data and harvest age ratio information to infer that Pintail reproductive rate was lower in northern than in southern breeding areas, a conclusion later corroborated by Calverley and Boag (1977). Using pair and brood counts, Dzubin and Gollop (1972) reported the proportion of hens fledging broods in a Manitoba parkland (0.40) to be nearly twice as large as that in a Saskatchewan prairie (0.21).

2. Age, Breeding Experience, and Social Status

Teunissen, Spaans, and Drent (1985) noted that for Brant competitive status affected reproductive success. For four pairs observed in several years, the highest ranking male raised the most young.

3. Wetland Habitat

Bellrose et al. (1961) plotted the number of May ponds per square mile in Manitoba and Saskatchewan, 1948-59 and the Mallard age ratio (young per adult) in hunter bag checks in the Mississippi Flyway. Ponds and age ratios covaried directly. Heitmeyer and Fredrickson (1981) and Kaminski and Gluesing (1987) used linear regression to investigate the relation of age ratio of Mallards in the Mississippi Flyway harvest in relation to spring population size, spring wetland conditions, and winter wetland conditions. In each analysis, age ratios were positively related to both spring and winter wetland indices.

Other workers have used pair and brood count data in conjunction with wetland counts on local study areas. Smith (1971) and Stoudt (1971) computed brood-pair ratios on their study areas for three time periods: predrought years (1953-58), drought years (1959-63), and postdrought years (1964-65). On the Lousana, Alberta study area, brood-pair ratios were lower during the drought period than during the other two periods for all four species studied: American Wigeon, Mallard, Blue-winged Teal, and Lesser Scaup (Smith 1971). On the Redvers, Saskatchewan study area, however, brood-pair ratios were not reduced during the drought years for Mallard, Pintail, Blue-winged Teal, or Canvasback (Stoudt 1971). Stoudt (1971) concluded that the Redvers area was one of the better waterfowl production areas in Canada, producing ducks even when most other areas were too dry to do so. Kaminski and Gluesing (1987) used brood-pair ratios for Mallards at Redvers during 1952-77 and found no evidence of a relation between this ratio and either May or August pond numbers.

Leitch and Kaminski (1985) computed brood-pair ratios for seven duck species on a Caron, Saskatchewan study area and tested for associations between these ratios and both May and August wetland numbers for 1950-75. Significant positive associations were found with August, but not May, pond numbers for all species (American Wigeon, Gadwall, Mallard, Pintail, Blue-winged Teal, Northern Shoveler, and Lesser Scaup).

4. Weather

Hammond and Johnson (1984) investigated the influence of weather on duck production using brood-pair ratios for two North Dakota study areas. For one area, but not the other, they found a significant positive relationship between the brood-pair ratio and mean temperatures between 23 April and 3 June for Mallard, Blue-winged Teal, and Redhead.

Stoudt (1982) mentioned cold spring temperatures, as well as flooding caused by excessive spring rainfall, as weather factors reducing Canvasback brood-pair ratios during some years. Sleet and cold during the summer were reported to be the "worst single hazard" to Snow Geese breeding at James Bay, and reduced age ratios were found for two years of "very severe" weather on the breeding grounds (Hewitt 1950).

Reeves, Cooch, and Munro (1976) found with remotely sensed imagery that snow and ice conditions in June on the breeding grounds of several species of arctic-nesting geese were related to age ratios in the subsequent harvest.

5. Density of Birds

Density-dependent influences on reproductive rate have also been investigated using long-term data on brood and pair numbers for local study areas. Pospahala, Anderson, and Henny (1974) found linear relationships between duck brood and pair numbers for study areas in Alberta and Saskatchewan, with no indication of the nonlinearity indicative of density-dependent reproduction. Kaminski and Gluesing (1987) used different methods with Mallard data from two Saskatchewan study areas for 1950-75. They found that the brood-pair ratio on one study area, but not the other, was inversely related to the continental wintering population counts.

Ebbinge (1985) detected decreased proportions of successful pairs and brood size among Bean Geese and Brant in years with higher population densities. Patterson, Makepeace, and Williams (1983) noticed reduced age ratios of Northern Shelducks with higher densities of territorial pairs on an estuary.

6. Synopsis