Northern Prairie Wildlife Research Center
The regression of clutch size on initiation date for ASY mallards was significant (F = 17.3; 1, 126 df; P < 0.001) and was consistent among years (Fig. 6). There was an average daily decline in clutch size of 0.027 egg during the nesting season. Average incubated clutch size for all ASY mallards (78 nests) was 8.96 ± 1.38 (mean ± SE) eggs compared to 8.49 ± 1.23 eggs for all SY hens (57 nests). The relationship between age and clutch size was clouded because there was a significant (F = 4.14; 3, 126 df; P = 0.008) interaction between age and year. The reason for the interaction was that clutch sizes of SY mallard hens in 1978 exceeded those of ASY mallard hens.
The clutch size of ASY gadwalls also was significantly (F = 31.5; 1, 155 df; P < 0.001) affected by initiation date with an average daily decline of 0.057 egg (Fig. 6). This regression portrayed the clutch size for all years except 1980 when clutch sizes were significantly (P = 0.05) smaller than average. The average size of incubated resident ASY gadwall clutches was 9.56 ± 1.48 eggs.
For blue-winged teal, overall clutch size averaged 10.4 ± 1.05 eggs for all nests. Clutch size was not significantly related (P > 0.10) to age, year, or age and year variables but was significantly affected (F = 21.4; 1, 60 df; P < 0.001) by nest initiation date. The regression of clutch size on date showed an average daily decline of 0.058 egg (Fig. 6).
|Fig. 6. Regressions of incubated clutch sizes (ICS) for ASY mallard, ASY gadwall, and blue-winged teal on initiation date (ID) from the Koenig study area, North Dakota, 1976-81.|