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

Discussion


Density

The CRP fields attracted the same species as did the WPA fields, with the exception of the Eastern Kingbird, which I did not observe in CRP fields. This species, however, was observed in CRP fields by Johnson and Schwartz (1993). Two species, Eastern Kingbird and Clay-colored Sparrow, appeared to be much more abundant in WPA fields. These species nest in shrubs, which occurred in some of the WPA fields but not in CRP fields. The CRP fields, which had been tilled before being enrolled in the CRP, had not been invaded by much woody vegetation. The dearth of Clay-colored Sparrows in CRP fields contrasts with counts as high as 12 indicated pairs per 100 ha (countywide average) reported by Johnson and Schwartz (1993) for CRP fields in the same general area.

Bobolinks and Savannah Sparrows, whose densities were similar in the two states, occurred in both CRP and WPA fields. These species were two of the most abundant in alfalfa-wheatgrass (Agropyron spp.) mixtures in North Dakota WPAs studied by Renken and Dinsmore (1987). In Minnesota the density of Savannah Sparrows was higher in WPA than in CRP fields. Grasshopper Sparrow densities were not significantly different between states, but in Minnesota they were higher in CRP than in WPA fields. Renken and Dinsmore (1987) found this species to be more abundant in native prairie than in alfalfa-wheatgrass mixtures.


Fledging Success

I found no significant difference in fledging success of ground nesters between WPA and CRP fields, leading me to conclude that fledging success was similar in these two kinds of fields. This similarity indicates that CRP fields provided nesting cover as safe for ground nesters as the other habitat I examined. Patterson and Best (1996) estimated 30% fledging success (assuming a 23-d nesting cycle) for Grasshopper Sparrows in CRP fields in Iowa, which is very similar to the 28% fledging success calculated from the mean daily survival rate of ground nests in CRP fields (Table 3), assuming a 23-d cycle. Studies that have compared fledging success in CRP fields and pastures have found no significant differences between these habitats (Granfors 1992, Klute 1994). Thus, available evidence suggests that CRP fields are equivalent in quality to pastures and WPAs for the grassland birds that are common in CRP fields. In general, pasture and rangeland are much more common in midwestern landscapes than are CRP fields (Koford and Best 1996). The CRP fields appear to be better nesting habitat than hayfields, which also attract grassland birds. Fledging success tends to be low in hayfields because of nest losses from mowing operations (Bollinger et al. 1990).

Demonstrating that some habitats were of equivalent quality during the CRP era is not conclusive evidence that birds nesting in CRP fields had higher fledging success than they would have had in the absence of the CRP. The extensive CRP cover could have affected the distribution of birds and predators, making fledging success in suitable habitats different from what it would have been in the absence of the CRP. If one assumes, however, that recent estimates of fledging success in habitats other than CRP fields are similar to levels of fledging success in those habitats before the CRP era, and similar to what they would have been without the CRP, then the CRP has probably benefited grassland birds. The additional cover provided by CRP fields may have lowered breeding densities in all habitats, with possible benefits if reproduction is density dependent. The additional habitat also may have allowed birds to breed that otherwise would not have, such as second-year birds, thereby supporting higher population growth overall. The effect of loss of CRP habitat would be substantial for grassland species that nest in the dense cover provided by CRP fields (see also Johnson and Igl 1995).

Estimated fledging success of 22-27% (Table 3) appears to be relatively low but may be sufficient to maintain stable populations without immigration. Given this range of fledging success, if all pairs renested up to four times after failed nesting attempts, the percentage of pairs producing at least one fledgling in a nesting season would be 63% (1.0 – [0.78]4) to 72% (1.0 – [0.73]4). If each of these successful pairs fledged 3 young, the average production per pair in the population would be 1.9–2.1 fledglings per season. This level of reproduction is at the low end of the range expected for a stable population. Sherry and Holmes (1995) estimated that a pair of neotropical migrants would have to produce 1.7–4 fledglings per season to balance mortality. Rodenhouse et al. (1995), assuming only two nesting attempts, estimated that production of three fledglings would be necessary. More data are needed on mortality, renesting frequency, and double-brooding before definitive conclusions can be reached regarding the adequacy of fledging success in the CRP and WPA fields studied.

The relatively low estimated fledging success of grassland birds in this and other studies (Rodenhouse et al. 1995) raises questions about whether these estimates are accurate. It is possible that investigator effects caused the fledging-success estimates to be biased (Bart 1977, Westmoreland and Best 1985, Major 1990). Comparisons between treatments, as was done in this study, would be valid even if the estimates of fledging success were biased, assuming that biases were similar in all fields studied.

It is desirable to have studies of fledging success in CRP fields and other habitats from various geographic regions (e.g., Granfors 1992, Millenbah 1993, Klute 1994, Patterson and Best 1996). In parts of North Dakota and Minnesota, at least, this study indicates that the CRP provided nesting cover at least as safe as one other habitat. This suggests that declining species of grassland birds have probably benefited from this program which has converted so much former cropland to attractive nesting cover for grassland birds.


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