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1. Species

Among the North American anatids, geese tend to have higher survival rates than ducks, and the single estimate for swans is highest of all (Table 14-1). Larger species tend to have higher survival rates. Survival and several components of reproductive rate appear to be allometrically scaled to size in mammals (Western 1979, Stearns 1983), and probably birds as well (Lindstedt and Calder 1976, Sæther 1987).

Blandin (1982) and Krementz et al. (1987) calculated lower survival estimates for Black Ducks than the average estimates for Mallards presented by Anderson (1975b). These estimates were not strictly comparable, however, as they represented different geographic areas and time periods. Nichols, Obrecht, and Hines (1987) found no consistent differences in survival rates between the two species for birds banded in the same areas and time periods (also see Heusmann 1981 for a similar comparison).

2. Age

Bandings in winter of Canada Geese, Mallards, and Canvasbacks have provided insight into when age-specific survival differences occur. Tests for age-specificity of survival rates of winter-banded Canada Geese have yielded diverse results. Most tests (e.g., Trost, Rusch, and Anderson 1981; Stotts 1983) have found no evidence of age-specific survival differences. Kasul and Wright (1980), however, found that survival rates of young banded during winter in southern Illinois were significantly lower than those of adults during two out of three time periods tested. The 1960-77 period, with the largest banded samples, yielded estimates of 0.77 for young geese and 0.82 for adults (Kasul and Wright 1980).

Although winter-banded Mallards showed no evidence of age-specific differences in survival rates (Hopper, Funk, and Anderson 1978; Rakestraw 1981; Nichols and Hines 1987), radiotelemetry mortality studies of female Mallards in the Mississippi Alluvial Valley during late fall and winter did detect age-specific differences in mortality (Reinecke, Shaiffer, and Delnicki 1987). We conclude that by late winter any mortality differences between first-year and older Mallards are small and that large age-specific differences observed for preseason-banded Mallards occur primarily between August and February, including the fall migration and hunting season (also see Nichols and Hines 1987).

Analyses of Canvasbacks banded on Chesapeake Bay during winter also suggested lower survival rates for young birds (G. M. Haramis, pers. commun.). Because the tests were based on capture-recapture data, age-related differences in permanent emigration may have contributed to the difference.

Age-specificity in survival beyond the first year has not been investigated thoroughly in waterfowl. Although age-specific survival rates can be estimated for virtually any number of initial age classes using standard band recovery and capture-recapture models, these models have been little used beyond the two-age (young and adult) case because they require that newly banded birds be assigned to more than two age classes. However, the H2 versus H3 test of Brownie et al. (1985) can detect, but not estimate, differences in survival or recovery rates between second-year and older birds, using data from birds aged to only two age classes. This test occasionally provides evidence of age-specificity beyond two years in data sets for ducks, and somewhat more frequently with Canada Geese, but two-age models fit the data adequately in the large majority of cases (J. D. Nichols, unpubl. data).

Capture-recapture data for known-age cohorts can be used to estimate age-specific survival rates for virtually any number of age classes (Buckland 1982, Parkin and White-Robinson 1985, Loery et al. 1987). Resulting survival estimates are much less robust to heterogeneous survival and capture probabilities than those based on data not separated by cohort (Buckland 1982; Johnson, Burnham, and Nichols 1986; Loery et al. 1987). Nevertheless, cohort analyses can still yield useful estimates and inferences about age-specific survival beyond two age classes (Loery et al. 1987). Parkin and White-Robinson (1985) found for Canada Geese in England an increase in annual survival for the first three to five years and a possible decline in survival probability beyond seven years of age. This analysis is one of the few to provide evidence of senescent decline in survival for any bird species (also see Loery et al. 1987).

3. Sex

Sex differences in survival appear to depend on the roles of the sexes in incubation and brood-rearing. For Mallards, and probably other ducks, very high mortality rates of hens during spring and summer are primarily responsible for the lower annual survival of females (Johnson and Sargeant 1977; Sargeant, Allen, and Eberhardt 1984; Cowardin, Gilmer, and Shaiffer 1985; Blohm et al. 1987). Among geese and swans, however, males generally remain in attendance at the nest and with the brood. Such nest- and brood-guarding likely reduces mortality risks to the female and results in similar nesting-season risks for both sexes. Hunting mortality rates also differ between the sexes in many species and may influence sex-specific survival differences (Johnson and Sargeant 1977).

Differences between sexes in annual survival rates tend to be less marked for young birds than adults, e.g., Wood Ducks (F. A. Johnson et al. 1986, Nichols and Johnson 1990), Mallards (Anderson 1975b), Black Ducks (Krementz et al. 1987), Pintails (Rienecker 1987), and Blue-winged Teal (L. D. Schroeder unpubl. report). Among young Ring-necked Ducks, Conroy and Eberhardt (1983) found instances of higher survival rates for males than females. For several duck species (see section III of this chapter), young birds make less reproductive effort than adults, so they may have reduced mortality risks, which are similar for young males and females, and higher breeding-season survival.

4. Condition and Social Status of the Bird

Investigating the effect of condition on vulnerability to hunting, Hepp et al. (1986) and Blohm et al. (1987) reported that banded Mallards with low condition indices in late fall showed higher probabilities of being shot and recovered during the subsequent hunting season. Greenwood, Clark, and Weatherhead (1986) found lower body weights for Mallards shot over decoys than those obtained by pass-shooting, suggesting that condition might affect susceptibility to certain kinds of hunting.

The relationship between social status and survival probability has been little studied in any bird species (Ketterson and Nolan 1983). Among waterfowl, behavioral dominance can lead to greater access to defendable food resources in some situations (Raveling 1970, Scott 1977, Alexander and Hair 1979, Patterson 1982), which conceivably could enhance survival. In the only test of this idea that we are aware of, Patterson (1982:75) found no consistent difference between the proportions of high- and low-ranking Northern Shelducks "disappearing" between years.

5. Geographic Variation

Black Duck breeding and wintering populations in the Lake Erie-Lake Ontario region tended to survive at a lower rate than those in other areas (Krementz et al. 1987), possibly because of changing land use patterns and competition with Mallards. Krementz et al. (1987) also noted that young Black Ducks seemed to exhibit greater geographic variation in survival rates than adults.

Lower survival rates for northern than southern Wood Duck populations were reported within both Mississippi (Bowers and Martin 1975) and Atlantic (F. A. Johnson et al. 1986) flyways. Nichols and Johnson (1990) speculated that these survival differences may be associated with the different migration habits and corresponding mortality risks of northern (migratory) versus southern (sedentary) Wood Ducks.

Nichols and Haramis (1980) found that female Canvasbacks wintering in a Delaware-Maryland-Virginia area exhibited lower survival rates than those wintering in California. Conroy and Eberhardt (1983) found lower survival probabilities for female Ring-necked Ducks banded in Mississippi Flyway wintering areas than for those banded on Atlantic Flyway wintering grounds; the reverse held for males.

6. Wetland Habitat and Weather

Seasonal, rather than annual, survival rate estimates provide better opportunities for investigating relationships between environmental variables and mortality risks, because annual rates are confounded by a variety of risks operating throughout the year. Reinecke, Shaiffer, and Delnicki (1987) found year-to-year variation in winter survival rates of Mallards in the Mississippi Alluvial Valley, with the highest survival rate occurring in a year of heavy winter rains (good water conditions) and mild temperatures; the lowest survival occurred during a year of favorable precipitation but freezing temperatures that prevented access to shallow water areas. Spring-summer survival rates for five years for Mallards in prairie habitat were not clearly related to May or July pond numbers (R. J. Blohm, R. E. Reynolds, and J. D. Nichols, unpubl. data).

Weather affects survival directly as well through its impact on habitat, it has direct effects as well. Severe winter weather may result in increased waterfowl mortality. The usual evidence provided in support of this relationship involves birds found dead following such weather events (e.g., Harrison and Hudson 1964).

7. Population Density