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Summer mortality of hens is important with regard to recruitment because when hens are killed their nests fail, and there is no opportunity for renesting. Furthermore, an estimate of summer mortality is required for assessing change in population size resulting from changes in recruitment and survival.

Radio-marked birds used in our study furnished an opportunity to measure survival rates during summer. We calculated the total tracking days for each bird and divided the observed deaths by tracking days to obtain a daily mortality rate. This technique, like that of Mayfield (1961), requires the assumption of constant survival. We suspect that mortality is greater in the first part of the season when hens are on nests; therefore, we used 2 periods—the period when most hens are on the nest (Apr through Jun) and the period when most nesting is complete but some hens either are leading broods, are in mixed sex flocks, or are molting (Jul through Sep). The product of the period means (Table 16) furnishes an estimate of survival during summer.

Differences among ages, years, and periods were tested with the same techniques used for testing nest success. No differences were significant. Despite our inability to detect differences, we suspect that survival is greater in July-September than in April-June. The period differences are consistent among years and are similar in magnitude. Massive sample sizes would be required to achieve statistical significance because of the very high daily survival rates. The overall April-September survival estimate, 0.806, is higher than the 0.692 summer survival reported by Johnson and Sargeant (1977) because perhaps predators, especially foxes, chew on transmitters and cause some to become inoperative. Furthermore, both foxes and mink often take the carcass into an underground den. The resulting loss in signal strength can prevent detection by the tracking aircraft.

Causes of Summer Mortality

The mink (Mustela vison) is the second most destructive predator of mallard hens (Sargeant et al. 1973, Eberhardt and Sargeant 1977). Unlike the fox, the mink takes hens during the latter part of the summer when they are leading broods or molting on wetlands and are vulnerable to predation. Two of the 4 known mink kills were hens that were leading broods. Only 2 hens were known to be lost to raptors, 1 probably a great horned owl (Bubo virginianus) and the other an unknown hawk. Our data suggest that avian predation on the study area was of limited importance.

Disease and accidents also account for minor losses. One bird on a nest was run over by spraying equipment. Although we did not observe hens killed by mowing equipment, such losses, especially in alfalfa, are frequently observed by local farmers. Collision with transmission wires is another type of accidental death that is prevalent on the northern prairies (Krapu 1974; Faanes, pers. commun.). There is some loss of hens to motor vehicles but this is probably infrequent (Sargeant 1981).