Northern Prairie Wildlife Research Center
The results of our investigations suggest that CRP cover is providing benefits for some grassland-nesting birds. For ducks, we found nest success in CRP cover and planted cover in 1992-93 to be 6-18 percent higher than that reported for planted cover in 1980-84 by Klett et al. (1988) (Table 2). Nest success in CRP for three principal species was 2-9 percent higher than that believed necessary to maintain populations (i.e., 15 percent for mallard, 20 percent for gadwall and blue-winged teal) (Cowardin et al. 1985, Klett et al. 1988). Our estimates of nest success in CRP cover were comparable to that reported for CRP cover by Kantrud (1993) for North Dakota and Minnesota combined in 1989-91. For seven combined species of ducks, he reported higher nest success in CRP (23 percent) than in planted cover (8 percent). His study was conducted during drought years, and CRP fields were farther from ponds than were planted cover fields. Kantrud speculated this may have been a cause of the difference in nest success because predator activity probably is greater near wetlands. We did not find evidence of a difference in nest success between CRP and planted cover in 1992-93. We purposefully selected study sites that were not affected strongly by drought. Although we did not measure distances from our fields to the nearest pond, it was common to have ponds within and adjacent to both our CRP and planted cover fields. If what Kantrud (1993) speculated is correct, it may partially explain why our nest success estimates were similar in CRP and planted cover.
We can only speculate as to why nest success in planted cover was higher in our study than that reported by Klett et al. (1988) for the 1980-84 period. It is possible that the increased amount of grass cover provided by the CRP had a positive effect on nest success in planted cover and other cover types by dispersing nests or providing a larger prey base (Lysne 1991). If this is true, then benefits of CRP to grassland nesting ducks extend beyond the CRP cover itself.
Another explanation is that the expanding coyote (Canis latrans) populations and declining red fox populations in our study area during the pre- and post-CRP periods (Sovada 1993, Sargeant et al. 1987) may have increased nest success in all cover types. Sovada (1993) found that nest success of upland-nesting ducks was about 15 percent higher in her study areas where coyotes were active and there was little or no activity by red fox compared with study areas where the reverse was true. On study areas where foxes were active, however, average nest success (17 percent) was higher than that reported by Klett et al. (1988) for planted cover in 1980-84. Sovada's study was conducted during 1990-92 after most CRP cover had been established. She speculated that CRP may have elevated nest success in all cover types. Thus, increased CRP cover and changing canid populations may be working in concert to yield increased nest success for ducks. This high nest success combined with the attractiveness and availability of CRP cover suggests that this Program has great potential for increasing duck production in the Prairie Pothole Region of the U.S.
Breeding Bird Survey data indicate that some grassland-nesting species show population increases coincident with the post-CRP period, reversing the negative trends of the pre-CRP period. We believe that CRP provides substantial benefits for certain species, especially those restricted to grassland habitats during the breeding season. For example, the population status of grasshopper sparrow (Ammodramus savannarum) and lark bunting (Calamospiza melanocorys) improved markedly during the post-CRP period. These two species were reported by Johnson and Schwartz (1993b) to be the most abundant breeding birds in CRP fields in four states including North Dakota, and their increasing populations reflect the increased availability of nesting habitats offered by the CRP in this region.
Not all grassland-obligate species increased during the post-CRP period, indicating that factors other than breeding habitat availability may be strongly influencing their population trends. For example, bobolink (Dolichonyx oryzivorus) and dickcissel (Spiza americana) are neotropical migrants, and their declines during the post-CRP period may be the result of conditions encountered during their migrations or on their wintering grounds in South America. Additionally, species with specialized habitat requirements, such as Baird's sparrow (Ammodramus bairdii), may be able to occupy only a small proportion of the CRP habitats that have been created. However, their presence in CRP cover, relative to the alternative (cropland) (Johnson and Schwartz 1993b) indicates that these species still benefit from the CRP.
Three considerations should be made when interpreting these results. First, any grouping of birds is subject to criticism because of the unique life-history characteristics associated with each species (e.g., Mannan et al. 1984). We believe these results are robust to minor changes in the groupings, and encourage interested readers to evaluate the patterns demonstrated in Table l for alternative groupings. Second, regional patterns of population changes are influenced by many factors, including drought. Between 1987 and 1992, the number of ponds declined in most of North Dakota as a result of drought (Hunnicutt and Reynolds 1993). We believe this extended drought could negatively affect some species irrespective of other changes such as increased habitat availability provided by the CRP. Finally, our analyses considered only North Dakota, and the species investigated also occur elsewhere. Therefore, it is possible that the population changes observed were at least partly a result of redistribution of birds from other areas. In conclusion, we feel that CRP is benefiting both upland-nesting ducks and some species of grassland-nesting nonwaterfowl birds.
Most of the vegetative cover associated with the CRP became progressively available from plantings during the period 1987 to 1992. Unfortunately, this same period coincided generally with drought conditions across much of the northern plains. Low soil moisture resulted in less than optimal conditions for vegetative growth and runoff was insufficient to fill most prairie wetlands. These conditions prevented upland-nesting ducks and likely some nonwaterfowl birds from taking full advantage of the increased availability of grass cover provided by the CRP. Additionally, emergency haying has been allowed on a significant portion of the CRP acres in recent years which has reduced vegetation used by birds arriving in early spring. Since summer 1993, precipitation has increased and the outlook is favorable for improved wetland conditions and increased vegetation growth in 1994. Our evaluation of CRP cover and nesting ducks is scheduled to continue for two more years. We look forward to providing additional insight into the importance of CRP to grassland-nesting birds.