Northern Prairie Wildlife Research Center
|aEstimated by the method of Mayfield (1961) as modified by Johnson (1979).|
Daily mortality rates were tested for differences among ages, years, and habitats by using the daily mortality as the dependent variable in analysis of variance. Observations were weighted by exposure days. This is essentially the method proposed by Johnson (1979) for testing differences between 2 classes, but extended for testing differences among more than 2 classes. Daily nest survival rates were 0.89, 0.92, and 0.94 for AHY, ASY, and SY birds, respectively. These survival rates were not significantly different; therefore ages were combined. Cropland was almost completely rejected as nesting habitat (Table 5); only 3 nests were found, all in 1978, and none of these clutches hatched. We therefore eliminated cropland from the analysis of success by habitat class.
There was a significant habitat × year interaction apparent in the success estimates for right-of-way and hayland (Table 9). Tests for differences among habitats in the individual years failed to detect significant differences, although the among-habitats effect approached significance in 1978 and 1979. Our inability to detect differences in nest survival rates was probably the result of the small sample sizes involved, which is readily apparent from the confidence limits (Table 9).
Klett and Johnson (1982) recommended a sample of 50 nests in each unit to be compared, and our sample is far short of that goal. Our estimates by habitat are consistent with those of Higgins (1977) who estimated nest success in several of the same habitats as ours. We obtained his raw data and transformed it to our classification. Nest success for 3 of our classes derived from his data was 2.2% for cropland, 7.9% for right-of-way, and 17.9% for odd area. These estimates, though higher than ours, are in the same order.
Survival of nests in hayland is primarily a function of mechanical destruction by machinery. To account for this fact, we reran the above analyses but considered destroyed nests only those destroyed by predation. The survival rate was much higher in hayland (Table 10). Again we were unable to detect differences in the means for all years combined, but there was a significant habitat-year interaction. Hayland success was significantly higher than right-of-way or odd area success in 1978 and was near significance in 1979 (P= 0.054).
|Nest success (%)||95% CI|
Alfalfa received heavy use late in the nesting season and success was high prior to the cut, which makes the cutting date a crucial factor in determining nest success. In 1978, wet weather delayed mowing of alfalfa and allowed clutches to hatch. The result was the 50% hatch rate observed in that year. In general, even a 2-week delay in cutting can greatly improve nest success.
Nest success for wetland nests was 6.7% (CI= 1.3-24.5%), unexpectedly low in view of the 54% success (well outside our confidence limits) reported by Krapu et al. (1979b). Success was only 14% for 8 nests observed over water in this study. The Scirpus-Typha life form within wetlands showed excellent success in wet years 1978 and 1979 and very poor nest success in 1980 when most of the ponds were dry. J. T. Lokemoen (unpubl. rep., Sixteenth Delta Waterfowl Seminar, Portage La Prairie, Manitoba, 1983) has observed extreme fidelity of successful hens to specific habitats and nest sites. It is possible that hens nesting successfully in overwater sites return to similar sites in dry years and may suffer high nest loss.
Success of right-of-way nests (3%) was next to lowest for the habitats studied and far below the average of 38% calculated from Klett and Johnson's (1982) estimates for an interstate highway in the same area. Those authors attributed the high nest success to the flow of traffic on the interstate, which may repel certain predators. Most of our right-of-way nests were along secondary gravel roads with little traffic. Mowing of right-of-ways was later than for hayland, and no right-of-way nests were destroyed by mowing.