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Remains were of 7 dabbling duck and 6 diving duck species (Table 14). Dabbling ducks represented a greater proportion (94%) and diving ducks a lesser proportion (6%) of remains than expected from their proportions in the breeding population (X² = 103.65, 1 df, P < 0.01). We report only data for all years combined because annual comparisons of species found dead with those in the breeding population were nearly identical. We detected differences in proportions of individual species found dead among dabbling ducks, relative to their occurrence in breeding populations (X² = 118.94, 6 df, P < 0.01), but not among diving ducks (X² = 4.36, 4 df, P = 0.36). There were nearly one-third more dead mallards and nearly twice as many dead northern pintails as expected from their relative abundance in the breeding population. Conversely, American wigeons, blue-winged teals, and northern shovelers were less abundant among dead ducks than expected.

We determined sex of 501 dead ducks (Table 15). More females than males were tallied each year among all species of dabbling ducks, except for mallards in 1984, gadwalls in 1985, and a few species for which samples were small. Disparities in the expected 50:50 ratio of females to males were significant in mallards (X² = 20.25, 3 df, P < 0.01) and northern shovelers (X² = 8.07, 2 df, P = 0.02), but not in gadwalls (X² = 0.82, 2 df, P = 0.66), blue-winged teals (X² = 4.52, 2 df, P = 0.10), or northern pintails (X² = 5.33, 3 df, P = 0.15). Among mallards, significantly more females than males were found dead in 1983 (X² = 14.63, 1 df, P < 0.01) and 1985 (X² = 5.63, 1 df, P = 0.02), but not in 1984 (X² = 0.0, 1 df, P = 1.00). Among northern shovelers, significantly more females than males were found dead in 1983 (X² = 6.40, 1 df, P = 0.01), but not in 1985 (X² = 1.67, 1 df, P = 0.20); only 10 northern shovelers were found dead in 1984.

Cause of mortality seldom could be determined because dead ducks were represented mostly by scattered feathers or feathered body parts, but predators were strongly implicated. Nearly all dead ducks had been fed on by predators, and nearly all fresh carcasses that we examined had predator-inflicted wounds with recent hemorrhaging; all dead females found at nests showed evidence of predation. No other causes of mortality were indicated except for a few (n = 11) collisions with vehicles or overhead wires. We observed 22 instances of raptors killing ducks or feeding on fresh duck carcasses—10 by Swainson's hawks, 9 by northern harriers and 1 each by a ferruginous hawk, falcon (species unknown), and great horned owl.

The incidence of dead female mallards in relation to size of breeding populations on individual study areas provided insight into the extent of mortality that occurred. We found an average of 0.27, 0.14, and 0.22 dead mallard females/km² on all study areas during 1983-85, respectively. Based on annual breeding population estimates of 8.3, 3.7, and 3.2 mallard females/km² (Table 1 of Appendix A), we estimated that 3.3, 3.7, and 6.9% of available female mallards were found dead during 1983-85, respectively.