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Conservation Reserve Program:
Benefit for Grassland Birds in the Northern Plains

Ronald E. Reynolds, Terry Shaffer, John R. Sauer, and Bruce G. Peterjohn


This resource is based on the following source (Northern Prairie Publication 0911):
Reynolds, Ronald E., Terry L. Shaffer, John R. Sauer, and Bruce G. 
     Peterjohn.  1994.  Conservation Reserve Program: benefit for 
     grassland birds in the Northern Plains.  Transactions of the North 
     American Wildlife and Natural Resources Conference 59:328-336.

This resource should be cited as:

Reynolds, Ronald E., Terry L. Shaffer, John R. Sauer, and Bruce G. 
     Peterjohn.  1994.  Conservation Reserve Program: benefit for 
     grassland birds in the Northern Plains.  Transactions of the North 
     American Wildlife and Natural Resources Conference 59:328-336.  
     Jamestown, ND: Northern Prairie Wildlife Research Center Online.  
     http://www.npwrc.usgs.gov/resource/birds/benefit/index.htm
     (Version 01JUL03).

Table of Contents

Tables and Figures


Introduction

During the past few decades numbers of some species of upland-nesting birds in North America have declined. Duck species such as mallard (Anas platyrhynchos), northern pintail (A. acuta) and blue-winged teal (A. discors) have declined since the early 1970s and have remained low since 1985 (Caithamer et al. 1993). Some grassland-dependent nonwaterfowl species also have declined since 1966, as indicated by the North American Breeding Bird Survey (BBS) (Robbins et al. 1986). For prairie-nesting ducks, population declines can be attributed mostly to low recruitment, partially as a result of low nest success. Klett et al. (1988) concluded that nest success (probability of ≥1 egg of clutch hatches) in much of the U.S. Prairie Pothole Region was inadequate to maintain populations of the five most common upland-nesting duck species studied, and that predators were the most important cause of nest failure. Over the years, as grassland areas have been converted to cropland, ducks have concentrated their nesting in the remaining areas of available habitat, where predators such as red fox (Vulpes vulpes), striped skunk (Mephitis mephitis) and badger (Taxidea taxus) forage (Cowardin et al. 1983).

The reasons for declining populations of grassland nonwaterfowl birds are not clear but the loss of suitable grassland-nesting habitat probably is an important factor. Currently, approximately 95 percent of the land in North Dakota is used for agricultural purposes, of which over 60 percent is used for annual crop production (Haugse 1990). Of the grassland that remains, 95 percent is used for livestock production. This probably had a severe impact on grassland bird species that seek idle grass cover for nesting.

The 1985 and 1990 U.S. Farm Bills include provisions under the Food Security Act to fund a cropland-idling program called the Conservation Reserve Program (CRP). Over 36 million acres have been enrolled nationwide in the CRP since 1985 (Osborn 1993), and up to 25 percent of cropland in some counties has been converted primarily to grass. In North Dakota, nearly 3 million acres have been enrolled. Over 90 percent of the CRP plantings in North Dakota are grass and grass-legume mix composed primarily of wheatgrass (Agropyron spp.), smooth brome (Bromus inermis), alfalfa (Medicago saliva) and sweetclover (Melilotus spp.). Mixes of these species have been reported to attract high densities of nesting ducks (Duebbert and Kantrud 1974). According to the CRP provisions, the land must remain idle for the 10-year contract period, with the exception of emergency provisions for haying or grazing. CRP appears to have great potential for benefiting many species of grassland-nesting birds.

There have been efforts to document the importance of the CRP to migratory birds in the Upper Great Plains of the U.S. Kantrud (1993) studied duck nest success in CRP cover and concluded that nest success was higher than in planted cover on U.S. Fish and Wildlife Service (FWS) Waterfowl Production Areas (WPAs). Johnson and Schwartz (1993a) measured the use of CRP fields by nonwaterfowl birds and reported that several species have responded positively by colonizing CRP fields. They concluded that CRP has the potential to help reverse the population declines of several species.

We investigated the importance of CRP to upland-nesting ducks and certain other grassland-nesting birds. For ducks, we compared nest success in CRP cover with nest success in planted cover on WPAs in the same period (1992-93) and with that of an earlier period (1980-84). For nonwaterfowl, we used BBS data to compare the trends in populations of certain species found in CRP, for the periods 1966-86 (pre-CRP cover establishment) and 1987-92 (post-CRP cover establishment) in North Dakota.

Study Area and Methods

For duck nest success, our study area included that portion of North and South Dakota east or north of the Missouri River (Figure 1). This area corresponds roughly to the Prairie Pothole Region of North and South Dakota and also was studied by Klett et al. (1988). For nonwaterfowl bird population trends, our study area included all of North Dakota.

Figure 1: Map of North Dakota and eastern South Dakota showing locations of study sites.
Figure 1.  Locations of study sites (dots) in eastern South Dakota and North Dakota used to estimate duck nest success. Study area used for nonwaterfowl bird population trends analyses is indicated by hatching.

Duck Nest Success

In the spring/summer 1992 and 1993, we located duck nests (scrape or bowl containing ≥1 egg) in CRP cover and planted cover on WPAs using methods described by Klett et al. (1986). For our study area, we obtained a sample of 2-mile by 2-mile (3.2 km by 3.2 km) blocks from another study (see Cowardin et al. 1988). From this sample, we selected blocks that met two criteria: (1) a minimum of 40 acres (16.2 ha) of CRP cover; and (2) sufficient ponds to attract ≥20 breeding pairs of mallards estimated from a pair-pond regression model similar to that described by Cowardin et al. (1988).

For each block, we selected the nearest WPA (farthest WPA was 9 miles [14.5 km]) that had ≥40 acres (16.2 ha) of planted cover. Planted cover on WPAs consisted of a vegetation mix similar to CRP cover. Each block and associated WPA constituted a study site (Figure 1). For each study site, fields to be searched by treatment (CRP or planted cover) were selected randomly from all fields available until the last field selected resulted in ≥200 acres (80.9 ha) of that treatment for the study site. For study sites with ≤200 acres of a treatment, all fields of that treatment were searched. Each field was searched three times.

We calculated daily nest survival rates (DSR) for each treatment by study site the modified Mayfield method of Johnson (1979). We modeled DSR as a function of treatment, year and location of each study site. For spatial effects, we considered linear and quadratic terms involving universal transverse mercator (UTM) coordinates of Easting and Northing values and their cross products. Four regression models, corresponding to the four combinations of treatment and year, were developed and statistically compared for each species. The method of weighted least squares, with weights equal to the number of exposure days for each DSR estimate, was used to fit the model.

To compare our results with those of Klett et al. (1988), we required overall nest success estimates for the North Dakota portion of our study area (Klett et al. 1988 did not report estimates for the remainder of our study area). We used our model of DSR to estimate the average DSR for each of the nine Wetland Management Districts (Districts) in North Dakota, using the centroid UTM coordinates for each District as an explanatory variable. Some of the nine Districts (four in 1992 and two in 1993) had UTM coordinates that were farther west than our westernmost study sites (Easting = 482296 and 369094, respectively). To avoid extrapolating beyond the geographic range of our data, we truncated our models at the westernmost study site and used the estimated DSR for that study site as a constant for Districts farther west. Nest success by District was estimated by raising the District DSR to the power equal to the mean laying plus incubation period for successful clutches (Klett et al. 1986). Nest success by year for North Dakota then was estimated by weighting each District's nest success by the proportion of breeding pairs that occurred in that District, as estimated from surveys conducted by the FWS, and averaging. Overall nest success was estimated by averaging the individual year values weighted by population estimates in each year.

Nonwaterfowl Bird Populations

We estimated population trends from the BBS (Peterjohn and Sauer 1993) during the pre- (1966-86) and post-CRP (1987-92) periods for grassland-nesting nonwaterfowl birds regularly observed in CRP fields in North Dakota. We used all species reported in Table 2 of Johnson and Schwartz (1993b). We estimated trends (a percentage change per year estimated as a weighted average of slopes of linear regression on each route [Geissler and Sauer 1990]) for each species in each period.

To evaluate the effects of CRP on these species, we classified them into two groups: species primarily nesting within grassland habitats whose trends may be associated directly with the availability of CRP grasslands (CRP-influenced species), and species nesting in various habitats in addition to grasslands whose trends may not be associated necessarily with the availability of CRP grasslands (CRP-neutral species, Table 1). We then used t-tests to determine whether the difference in mean trends (trendPost-CRP - trendPre-CRP) for CRP-influenced species was similar to those of CRP-neutral species. Rejection of this null hypothesis in favor of a one-sided alternative hypothesis would indicate that CRP-influenced species were more likely to have positive population changes than CRP-neutral species between the pre- and post-CRP periods.

Table 1.  Trends in abundance of grasslands-nesting nonwaterfowl birds for the periods 1966-86 and 1987-92 in North Dakota with associated significance levels and sample size (n of routes)
Species 1966-86 1987-92
Trenda n Trenda n
CRP-influenced species
    Bobolink (Dolichonyx oryziverus) –0.15 37   –1.46 42
    Western meadowlark (Sturnella neglecta) –0.94 37     1.98 up 43
    Chestnut-collared longspur (Calcarius ornatus) –0.86 29     8.26 3 arrows up 32
    Savannah sparrow (Passerculus sandwichensis) –3.14 35   –6.03 41
    Baird's sparrow (Ammodramus bairdii) –1.19 25 –15.29 3 arrows down 24
    Grasshopper sparrow (Ammodramus savannarum) –7.86 3 arrows down 37   10.06 2 arrows up 42
    Dickcissel (Spiza americana) –6.51 3 arrows down 25   –9.86 24
    Lark bunting (Callamospiza melanocorys) –7.70 3 arrows down 29   21.73 3 arrows up 35
CRP-Neutral species
    Killdeer (Charadrius vociferus)   1.92 arrow up 37 –13.02 3 arrows down 43
    Mourning dove (Zenaida macroura)   4.17 3 arrows up 37     6.51 3 arrows up 43
    Eastern kingbird (Tyrannus tyrannus)   5.13 3 arrows up 37     7.79 3 arrows up 43
    Western kingbird (Tyrannus verticalis)   5.85 3 arrows up 36   12.61 3 arrows up 43
    Horned lark (Eremophila alpestris)   1.09 37     0.74 43
    Brown-headed cowbird (Molothrus ater)   4.15 3 arrows up 37   –2.98 43
    Red-winged blackbird (Agelaius phoeniceus) –1.38 arrow down 37 –10.89 3 arrows down 43
    Vesper sparrow (Pooecetes gramineus)   2.59 2 arrows up 37     3.22 42
    Clay-colored sparrow (Spizella pallida) –3.88 3 arrows down 36   –1.46 42
    Barn swallow (Hirundo rustica)   5.09 3 arrows up 37   –4.68 2 arrows down 43
    Common yellowthroat (Geothlypis trichas) –0.36 37   –2.86 43
    Sedge wren (Cistothorus platensis) –1.15 22 –10.07 23
a Percentage change per year, arrow up(arrow down) P<0.10; 2 arrows up(2 arrows down) P<0.05; 3 arrows up(3 arrows down) P<0.01.

Results

Duck Nest Success

In 1992 and 1993 we searched 9,567 acres (3,872 ha) in 137 CRP fields and 5,745 acres (2,325 ha) in 95 planted cover fields on 52 study sites (Figure 1). We found 1,197 duck nests in CRP cover and 624 duck nests in planted cover, of which 1,121 and 578, respectively, could be used for estimating DSR. Principal species in CRP cover were blue-winged teal (32.1 percent), gadwall (A. strepera) (28.3 percent), mallard (21.6 percent), northern shoveler (A. clypeata) (8.6 percent) and northern pintail (8.4 percent). The species composition in planted cover was blue-winged teal (51.4 percent), gadwall (22.9 percent), mallard (12.2 percent), northern shoveler (10.3 percent) and northern pintail (3.0 percent). Sufficient information to develop models of DSR was obtained for mallard, gadwall and blue-winged teal.

Mallard.  The four regression lines for DSR of mallard did not differ (F3,66, P = 0.09, P = 0.966), indicating no treatment or year effects. All data were combined into a single model that indicated DSR increased from east to west (F1,66 = 8.34, P = 0.005).

Gadwall.  Daily survival rates of gadwall nests increased from east to west (F1,82 = 4.49, P = 0.037) and the rate of increase did not depend on treatment or year (F3,79 = 0.50, P = 0.681). The regression line for CRP in 1992, however, was lower (P<0.05) than the other three lines. The other three did not differ from each other (P>0.05).

Blue-winged teal.  Daily survival rates of blue-winged teal nests varied from east to west, but not linearly or consistently from year to year (P<0.05). The shapes of the curves for CRP and for planted cover were the same within each year, but differed between years. Tests indicated that DSR was higher in CRP than in planted cover in 1992 but the two did not differ in 1993.

Our overall estimates of nest success for CRP and planted cover in North Dakota were higher than estimates of nest success in planted cover reported by Klett et al. (1988) for 1980-84 (Table 2).

Table 2.  Comparison of estimated nest success in Conservation Reserve Program cover and planted cover in 1992-93 with estimates for planted cover in 1980-84a for mallard, gadwall, and blue-winged teal.
Habitat Nest success (percentage)
Mallard Gadwall Blue-winged teal
1980-84 1992-93 1980-84 1992-93 1980-84 1992-93
CRP b 24 b 22 b 25
Planted Cover 9 24 12 30 12 18
a From Klett et al. 1988.
b Habitat not available in 1980-84.

Nonwaterfowl Bird Populations

Prior to the CRP, all eight of the CRP-influenced species had negative estimates of trends, but post-CRP, four of the species had positive trend estimates (Table 1). In contrast, four of the twelve CRP-neutral species had negative point estimates of trends pre-CRP, but seven had negative trends post-CRP. Evaluation of differences in trend (trendPost-CRP – trendPre-CRP) indicates that the CRP-influenced species were more likely to be increasing during the later period (mean differences: 4.72 [CRP-influenced], -3.19 [CRP-neutral], t = 1.73, df = 18, P = 0.052).

Discussion and Conclusions

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.


Acknowledgments

We thank the many individuals who have participated in the BBS for North Dakota. We also thank members of the nest-searching crews who participated in the duck nest success study. We especially thank R. Renner of Ducks Unlimited, Inc. and R. Greenwood of the National Biological Survey, for their assistance in coordinating the duck nest success study. We acknowledge the financial support provided by the Central and Mississippi Flyway Councils, Ducks Unlimited's Institute for Wetland and Waterfowl Research, and the U.S. Bureau of Reclamation. B. Batt and R. Meeks of Ducks Unlimited, Inc., provided administrative assistance for the duck nest success study. We appreciate the logistical assistance provided by H. Hoistad, R. Hollevoet, F. Giese, R. Gilbert, J. Koerner, S. Kresl, D. Potter, D. Walls, P. VanNingen and their staffs. S. Vaa and M. Johnson provided field assistance. A. Schaff typed the manuscript. We thank W. Newton for statistical advice and D. Johnson, R. Koford, G. Pendleton and A. Sargeant for reviewing the manuscript.


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Ronald E. Reynolds, U.S. Fish and Wildlife Service, Bismarck, North Dakota.
Terry Shaffer, National Biological Survey, Jamestown, North Dakota.
John R. Sauer and Bruce G. Peterjohn, National Biological Survey, Laurel, Maryland.
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