Northern Prairie Wildlife Research Center
In an earlier paper, Batt and Prince (1978) failed to detect any difference between clutch sizes for yearling and older birds. Although our samples were small, we suspect that there may be an age effect on clutch size beyond that of date because the difference approached significance.
Our estimates of clutch size are probably too low because Franklin's ground squirrels frequently remove eggs (Klett and Johnson 1982; Klett, pers. commun.), yet the hens continue to incubate the clutches. We even found 1 hen incubating a nest of 3 eggs. To reduce the effect of these small clutches that may represent partial clutches, we used an iterative weighted least squares analysis (Mosteller and Tukey 1977) to estimate clutch size for yearling and older birds as well as the rate of decline of clutch size with date. The procedure reduces the weight given outlying points below the regression line. This analysis yielded the following regression for SY birds: clutch size = 16.7 - 0.06 Julian date. The regression gives a 1 April clutch size for SY birds of 10.9 eggs. The estimate for ASY birds on 1 April was 11.8 eggs. If we combine SY and ASY birds in proportion to the occurrence of the ages in the population, we obtain a linear regression (Fig. 7) falling between the wet and dry year regressions presented by Krapu et al. (1983). Those authors had no data on age effect. Our regression is similar in slope but almost 1 egg lower than the slightly curved regression line presented by Batt and Prince (1979). Our clutches may be smaller because our birds were free-flying wild birds whereas theirs were penned birds fed ad libitum.
The fact that clutch sizes decline as the nesting season progresses compounds the problem of low hatch rate described earlier. Even if hens are successful in renesting, the total eggs produced will be fewer than for the first nests.