Northern Prairie Wildlife Research Center
Mortality During the Breeding Season
III. Mortality Rates
Few studies provide mortality rate estimates for waterfowl
adults, nests, and young during the breeding season; no
data exist for most species in most areas. Here, we
summarize numerous published mortality rates of adult
waterfowl during the breeding season and of their eggs or
nests and prefledged young (Table 12-1, Table 12-2, Table 12-3, Table 12-4, and Table 12-5).
We emphasize recent studies that had well designed
sampling or analysis schemes, studies that we believe
provided unique or particularly important data sets, and
studies including diverse species and natural
conditions. Methods of study, methods of reporting
mortality rates, and factors causing mortality varied greatly
and made it impossible to construct Tables 12-1 through 12-5 with totally comparable data (see footnotes to tables).
Therefore, detailed comparisons of data summarized in
these tables should be avoided. Our purpose is to describe
average conditions and major differences or similarities
among waterfowl groups. Numerous other sources of
waterfowl mortality rate data, especially of eggs in older
studies that were not corrected for biases, are provided by
Weller (1964), Palmer (1976), Bellrose (1980), and Bouffard
et al. (1987).
A. Adults
Published estimates of mortality rates of adult swans and
geese during the breeding season are almost nonexistent. Numerous investigators have noted such mortality
(e.g., Barry 1964, 1967; Ryder 1967; Sherwood 1968; Hanson
and Eberhardt 1971; Harvey 1971; Mickelson 1975; Raveling
and Lumsden 1977), but these reported mortalities represent
infrequent occurrences. This has led to a general
conclusion that, except for subsistence hunting, mortality
rates of adult swans and geese during the breeding season
do not significantly affect population sizes (see Scott 1972,
Owen 1980). In contrast, the limited data for certain
dabbling ducks indicate that mortality rates of adult females
during the breeding season are often high (up to 0.40)
(Table 12-1) and are probably a major cause of disparate sex
ratios common among duck species (Johnson and Sargeant
1977).
B. Eggs
Reported nest mortality rates for swans and geese are
usually less than 0.35 and seldom exceed 0.50 (Table 12-2).
In contrast, nest mortality rates for most ducks are
generally greater than 0.50 and, in some major waterfowl
producing areas, commonly average greater than 0.80
(Table 12-3). The mortality rates reported in nearly all swan
and goose studies, and in most duck studies, are likely low
because of biases associated with calculation of apparent
nest success. The biases are particularly severe for duck
studies that had high predation rates, because many nests
were likely destroyed or abandoned before the searches
were made and were not included in the evaluations.
Although nests of swans and geese tend to survive at
much higher rates than nests of most ducks, lower mortality
does not necessarily mean less impact on populations.
This circumstance occurs because of differences among
species in adult mortality rates, age of maturation, clutch
size, and renesting potential.
C. Prefledged Young
Estimated mortality rates among prefledged waterfowl are
also lower for swans and geese (generally 0.15-0.40) than
for ducks (usually > 0.50) (Tables 12-4, 12-5). For all 3
groups, most mortality occurred during the first 2 weeks
after hatching. As with estimates of nest success, most
estimates of mortality rates among prefledged waterfowl are
biased low because of inability to account for losses of
entire broods (in many studies) and, to a lesser extent,
inability to account for losses of some older (near fledging
age) young. Also, the probability of loss of entire broods
may be inversely related to brood size, as reported for
Lesser Snow Geese by Rockwell et
al. (1987). These biases are probably most severe for ducks,
because their young are generally less visible than are
young of swans and geese. For that reason, we reported
findings from few duck studies in which results were based
on changes in brood sizes. Readers interested in data on
age-related changes in duck brood sizes should consult
species accounts in Palmer (1976) and Bellrose (1980).
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