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Density and Fledging Success of Grassland Birds in Conservation Reserve Program Fields in North Dakota and West-central Minnesota

Results

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.
State
  Species		 CRP WPA
N Density CI N Density CI
North Dakota
  Eastern Kingbird   0 —   18   3.4    1.9-6.3
  Common Yellowthroat   6 —     0 —  
  Clay-colored Sparrow   4 —   24   4.9    2.4-9.9
  Savannah Sparrow 13   2.4    1.0-5.8 10 —  
  Grasshopper Sparrow 45   8.0    4.1-15.6 14   2.1    1.2-4.0
  Bobolink   7 —     6 —  
  Red-winged Blackbird 58 10.8    5.5-20.7   3 —  
  Western Meadowlark   9 —     4 —  
  Brown-headed Cowbird   7 —   11   2.5    0.8-7.3
Minnesota
  Common Yellowthroat   7 —     1 —  
  Clay-colored Sparrow   1 —   15   4.7    1.5-14.0
  Savannah Sparrow 13   3.6    1.9-6.8 41 12.3    8.8-17.3
  Grasshopper Sparrow 19   4.2    1.6-11.1   6 —  
  Bobolink 13   4.5    1.8-11.3 13   3.7    1.2-11.1
  Red-winged Blackbird 11   5.9    1.3-25.7   2 —  
  Western Meadowlark   0 —     1 —  
  Brown-headed Cowbird 10 —     1 —  
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.


Density

Nine species were fairly common in the grassland study fields (Table 1). In North Dakota, Eastern Kingbirds (Tyrannus tyrannus) and Clay-colored Sparrows, both of which nest in shrubs, were more abundant in WPA than in CRP fields. All of the Clay-colored Sparrows observed in WPA fields were in native-prairie fields that had western snowberry (Symphoricarpos occidentalis) shrubs. Grasshopper Sparrows and Red-winged Blackbirds (Agelaius phoeniceus), by contrast, were more abundant in CRP than in WPA fields. On about half of these CRP fields, Red-winged Blackbirds were by far the most abundant breeding birds; on the other fields their numbers ranged from zero to three birds per field. These differences are largely due to the variation among fields in the presence of vegetation such as sweetclover (Melilotus spp.), which supports nests and serves as song perches. Sweetclover, a biennial, would be expected to decline in abundance over time in many CRP fields.

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.
State
  Species		 CRP WPA
N Exposure Days DSR Fledging Success (%) N Exposure Days  DSR Fledging Success (%)
North Dakota
  Mourning Dovea   7   75.0 0.920   8.9b   3   41.0 1.0     100.0b
  Clay-colored Sparrow   8   81.0 0.975 53.5c 49 422.5 0.934   18.0c
  Vesper Sparrowa   6   59.0 0.949 27.1c   7   71.5 0.944   21.3c
  Savannah Sparrowa   4   30.0 0.933 15.5     4   55.0 0.945 22.0
  Grasshopper Sparrowa 38 347.5 0.950 28.5c 14 116.5 0.914  10.6c
  Song Sparrowa   0 — — —   0 — — —
  Bobolinka   8   78.5 0.975 47.3b   7   60.5 0.901    6.0b
  Red-winged Blackbird 70 634.0 0.921 10.9     9   79.0 0.937 17.1
  Western Meadowlarka 20 207.5 0.952 23.9b 13   79.5 0.874    2.0b
Minnesota
  Mourning Dovea   0 — — —   1     5.0 0.800    0.2b
  Clay-colored Sparrow   1   11.0 0.909    9.2c 24 309.0 0.968  43.9c
  Vesper Sparrowa   1     5.0 0.800    0.4c   1    2.0 0.500    0.0c
  Savannah Sparrowa 12   81.0 0.864  1.9 30 259.0 0.950 24.9
  Grasshopper Sparrowa 13   98.0 0.918  11.9c   1   16.0 1.0     100.0c
  Song Sparrowa   8   74.0 0.946 22.3     9   66.5 0.910   7.8
  Bobolinka 16 154.5 0.922   9.6b 31 269.5 0.926  10.7b
  Red-winged Blackbird 25 153.0 0.856 1.5   1    7.0 1.0     100.0  
  Western Meadowlarka   6   88.5 0.955 26.1b   3   37.5 0.973  45.7b
Note:   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.


Fledging Success

Estimates of fledging success were obtained for 657 nests, including 166 nests of precocial species (mostly dabbling ducks) which were not the focus of this paper. Among the altricial species for which at least 10 nests were monitored, estimated fledging success was variable (Table 2). A total of 263 ground nests, mostly of Savannah Sparrows, Grasshopper Sparrows, and Bobolinks (see Table 2), were pooled and analyzed statistically for differences in daily survival rate. There were no significant differences in fledging success between field types (P = 0.65), between states (P = 0.67), or among years (P = 0.06). None of the interaction terms was significant. I therefore failed to reject the primary null hypothesis of interest, that fledging success of ground nesters was the same in the two types of fields.

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
CRP 0.9458 0.9338 0.9566 22
WPA 0.9527 0.9417 0.9627 27

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