Northern Prairie Wildlife Research Center
|Table 1. Numbers seen, estimated densities (birds/10 ha), and 95% confidence intervals (CI) for density estimates for birds counted on Conservation Reserve Program (CRP) or Waterfowl Production Area (WPA) fields in Stutsman County, North Dakota, and Stevens and Pope Counties, Minnesota.|
|Note: Estimates are from the first survey, 27 May-15 June 1992, except for Bobolink estimates, which are from the second survey, 11 June-9 July 1992. Densities were estimated only for species in which more than 10 individuals were seen.|
In Minnesota, Clay-colored and Savannah sparrows were more abundant in WPA than in CRP fields. Almost all of the Clay-colored Sparrows seen in WPA fields were in one field; many nested in sweetclover. It is possible that Savannah Sparrows preferred the denser cover associated with WPA fields, which were dominated by warm-season grasses. Most CRP fields were dominated by cool-season grasses such as smooth brome (Bromus inermis).
|Table 2. Estimated fledging success of grassland birds for which at least 10 nests were monitored in 1991-1993 on Conservation Reserve Program (CRP) and Waterfowl Production Areas (WPA) fields in Stutsman County, North Dakota, and Stevens and Pope Counties, Minnesota.|
|N||Exposure Days||DSR||Fledging Success (%)||N||Exposure Days||DSR||Fledging Success (%)|
Number of nests (N), exposure days, daily survival rate (DSR) of nests,
and fledging success (Mayfield estimate) are presented. Nesting cycles
were assumed to be 27 d unless otherwise noted.
a Ground nests.
b 29-d nesting cycle assumed.
c 25-d nesting cycle assumed.
The great variability in daily survival rate of ground nests (Table 2) affected the reliability of the estimated daily survival rates of ground nests in the two field types (Table 3). The standard errors associated with the mean daily survival rates were quite large.
Predation was the overwhelming cause of nest failure. For the nine species in Table 2, predation accounted for 80-96% of the nest losses. For most destroyed nests we have no information on the identity of nest predators. On two occasions we saw garter snakes (Thamnophis sp.) eating nestlings.
|Table 3. Least squares mean Daily Survival Rates (DSR) ± 1 SE for nests of ground-nesting species in Conservation Reserve Program (CRP) and Waterfowl Production Area (WPA) fields in Stutsman County, North Dakota, and Stevens and Pope Counties, Minnesota.|
|Field Type||DSR||DSR 1 SE||DSR + 1 SE||Fledging success (%) for 27-d cycle|