Northern Prairie Wildlife Research Center
|Age of hena||N||1977||1978||1979||1980||All years|
|All agesc||177||5.9 Ab||23.6 B||17.4 BC||13.6 BC||15.2 C|
= second-year hens in first year of nesting; ASY = after-second-year
hens in their second or later nesting years.
bMeans followed by the same latter are not significantly different (P > 0.05).
cThe all-age group includes AHY birds whose age could not be determined.
To test how well results obtained on our area reflected conditions over a wider geographic area, we related our hen success estimates to fall age ratios of wings of mallards shot in North Dakota (M. F. Sorensen, S. M. Carney, and E. M. Martin, unpubl. rep., U.S. Dep. Inter., Fish and Wildl. Serv., Off. Migratory Bird Manage., Laurel, Md., 1982). We used the raw data uncorrected for differential age vulnerability because in the wing collection survey there is no way to separate birds produced in North Dakota from birds produced outside the state. It is also probable that the wing sample from North Dakota includes some birds from more northern breeding areas. The correlation between these 2 independent estimates was excellent (Fig. 9), illustrating that yearly variation in our sample did reflect differences over a larger area.
We hypothesized that annual variation in nesting effort is a function of habitat conditions and possibly weather. Krapu et al. (1983) presented data that strongly supported this hypothesis. Cowardin and Johnson's (1979) model, H = fPef(l - P)2 furnished a means of testing this hypothesis because it contains a single parameter (f) for nesting effort, and the data for nest success (P) and hen success (H) were available for each year. It was possible to solve for f. We developed an index to water conditions by assigning wetness codes (Table 15) to ponds on each set of aerial photographs and averaging codes for all ponds in the sample. Wetland indices for class II, III, and IV ponds (Stewart and Kantrud 1971) in the late May and early June flights were correlated with nesting effort. The correlation was strongest for class IV ponds (r = 0.98, P < 0.05) (Fig. 10). Correlations with class II (r = 0.83, P > 0.10) and class III (r = 0.91, P < 0.10) were not as strong and were not significant. Furthermore, the deviation of the data points from the regression for class IV ponds may be correlated with length of nesting season (r = 0.82, P = 0.18). Hen success is a function of nest success and nesting effort. Nest success is primarily a function of predation and nesting effort is a function of water conditions and age of the hen.
|Year||Nesting effort (f)a||Wetness indexb||Nest initiation|
|II||III||IV||Length of seasonc (Julian days)|
on Cowardin and Johnson's (1979) model, H = fPef(1-P)2.
bWetness index = mean of codes; 0 = dry, 1 = one-half full, and 2 = full. Classes are after Stewart and Kantrud (1971): II = temporary, III = seasonal, IV = semipermanent.
cLength = 90% quantile - 10% quantile of nest initiation dates.