Leopold Center Completed Grant Reports Leopold Center for Sustainable Agriculture

1995 Wildlife use of and productivity in diverted cropland Louis Best Iowa State University

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Recommended Citation Best, Louis, "Wildlife use of and productivity in diverted cropland" (1995). Leopold Center Completed Grant Reports. 59. http://lib.dr.iastate.edu/leopold_grantreports/59

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Abstract The alt l-grass prairie ecosystem once domi nated much of the Midwest; Iowa was in the center of this immense grassland. A diverse birdlife used the abundant prairie pothole wet lands and upland tall-grass cover. Although early agriculture produced a landscape that briefly increased wildlife diversity, the steady shift toward larger farms has resulted in re duced wildlife habitat.

Keywords Animal Ecology, Wildlife and recreation

Disciplines Biodiversity | Population Biology

This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/leopold_grantreports/59 COMPETITIVE GRANT Leopold Center REPORT

LEOPOLD CENTER FOR SUSTAINABLE AGRICULTURE 92-24

Wildlife use of and productivity in diverted cropland

Principal investigator Background The expansion of soil conservation practices Louis B. Best The tall-grass prairie ecosystem once domi­ in the Midwest, which may benefit wildlife by Animal Ecology nated much of the Midwest; Iowa was in the leaving more food and/or cover on the soil Iowa State University center of this immense grassland. A diverse surface for longer periods, is enhanced by the Budget birdlife used the abundant prairie pothole wet­ Conservation Reserve Program (CRP), a pro­ $11,720 for year one lands and upland tall-grass cover. Although vision of the 1985 Federal Food Security Act $11,720 for year two early agriculture produced a landscape that (Farm Bill) that allows the removal of highly briefly increased wildlife diversity, the steady erodible and environmentally sensitive crop­ shift toward larger farms has resulted in re­ land from production for 10 years. More than duced wildlife habitat. 36 million acres have been enrolled in the CRP nationwide; 2.3 million of those are in Iowa. Row-crop habitats are attractive to some bird Studies of the effects of land retirement and species; in fact, some of these were less abun­ set-aside programs on wildlife have concen­ dant in native tall-grass habitat—for example, trated on game species, particularly the ring- the horned lark, red-winged blackbird, and necked pheasant and northern bobwhite. But vesper sparrow, and introduced species such other surveys have noted dramatic declines in as the European starling and ring-necked pheas­ some grassland bird species. Because of the ant. Even so, row-crop habitats may reduce CRP's long-term nature, its benefits to wild­ bird productivity below levels needed to sus­ life are expected to be substantial. However, tain populations without influx from "source" this merit must be quantified with care. The habitats. Along with nest parasitism and pre­ primary goal of this research was to determine dation, farming activities further reduce nest the effects of the CRP on bird populations in success. central Iowa by (1) documenting bird abun­ dance, species composition, and nesting suc­ cess in CRP and row-crop fields; and (2) Numbers of dick­ evaluating the influence of differences in the cissels are currently declining. vegetation structure and composition of CRP land on bird use.

Approach and methods Research was conducted in Marshall County, Iowa. About 80% of the land is tillable; corn and soybeans constitute the main crops. Ten 40-acre CRP plots were paired with ten 40­ acre row-crop plots for study in 1991-1993. Pairings were based on similarities in topogra­ phy and edge habitat. Because exotic, cool- season grasses and legumes are the predomi­ nant CRP cover types in Iowa, only these types of CRP fields were used in the study. Row- crop plots were managed with conservation tillage methods. All CRP land in the study had

102 Volume 4 (1995) been enrolled since 1987. Plots were bounded grass, or alfalfa/grass mixtures. Smooth brome on at least two sides by roadsides, farmsteads, was the most dominant species in CRP plots. or adjacent fields. The other two edges were Broadleaf plant (forb) coverage was substan­ positioned within the CRP or row-crop fields tial in some plots due to weed invasion. Study such that the plots were 400 meters (m) on a plots planted to orchard grass had the most side. All fields were at least one kilometer diverse vegetation. Four plots were planted apart to reduce risk of overlapping bird use. exclusively to smooth brome and alfalfa; these were the most homogeneous because broad­ Vegetation characteristics of CRP plots—ver- leaf weeds were sprayed and/or mowed. Two tical density, height, canopy coverage, and other plots were planted to several grass spe­ species—were measured twice during the cies that resulted in greater vertical patchiness. growing season. Nest site vegetation data Alfalfa and weedy forbs dominated in two were collected after nest termination to charac­ plots. Vegetation in another plot had a low terize the vegetative structure used by species percent forb coverage but a greater diversity of breeding in CRP grassland. forb species because chemicals were not used to control weeds. Investigators conducted bird counts in all 20 plots twice during the breeding season by Bird species diversity: Thirty-three bird spe­ walking along grid lines until the plots had cies were recorded in CRP plots; 34 were been completely traversed. All birds seen or observed in row-crop fields (Table 1). Ten heard within 25 m of the line were recorded. species were unique to CRP land; the horned Abundance differences between CRP and row- lark was the only row-crop species absent crop land were analyzed statistically.

Nests were located and their outcomes deter­ Table 1. Mean bird abundance (birds/census count/100 hectares mined. Teams of individuals walked abreast [247 acres]) in CRP and row-crop fields in Iowa, 1991-1993. across each plot; these searches were repeated Values represent the mean of all plots over the three year period. twice during each breeding season to obtain a Only species with abundances greater than 1 bird/100 ha are more complete sample. All nests were marked shown. and revisited every 2-3 days to determine their CRP Row-crop outcome. Species Ring-necked pheasant 6.3 N* 2.4 A nest was considered successful if it fledged Killdeer 0.8 2.4 Upland sandpiper 1.0 N 0.1 at least one young. Other nesting outcomes Mourning dove 0.6 N 0.3 included nests lost to predation, weather, or Eiastern kingbird 1.0 0.9 brown-headed cowbird parasitism. Nest fates Horned lark 0.0 12.0 N EJarn swallow 5.5 4.2 were tabulated to show the relative importance American crow 0.4 1.9 of the various causes of nest failure. Nest Sedge wren 2.6 N 0.0 densities also were estimated for each plot. American robin 0.2 1.5 Common yellowthroat 10.8 N 0.8 Dickcissel 58.4 N 0.3 Three-hundred and one nests were used in Savannah sparrow 7.8 N 0.1 analyzing nest fates. Of these, only red-winged Grasshopper sparrow 48.5 N 1.0 Vesper sparrow 0.6 N 12.0 N blackbird, dickcissel, grasshopper sparrow, Song sparrow 3.4 N 0.9 ring-necked pheasant, and vesper sparro w nests EJobolink 37.7 N 0.0 (more than 10 active nests/species) were used Western meadowlark 5.6 N 0.6 Red-winged blackbird 109.0 N 20.0 in calculating species-specific daily nest-sur- Common grackle 0.5 5.9 vival rates. EJrown-headed cowbird 10.1 N 11.0N American goldfinch 1.9 N 0.6 Findings Total abundance 315.0 84.0 Total no. of species 33 34 Vegetation measurements: CRP plots were initially planted to smooth brome, orchard *N = species nesting in CRP or rowcrop plots.

Leopold Center Progress Reports 103 from CRP plots. The most abundant species in Mean nest density of all species combined was both habitats v/as the red-winged blackbird, greater in CRP fields than in row-crop fields, which accounted for 35% of all birds in CRP although nest densities increase in row-crop and 24% in row-crop fields. The dickcissel, habitats with an increase in crop residue. grasshopper sparrow, bobolink, common yel­ lowthroat, brown-headed cowbird, savannah Nesting success and nest-site selection: The sparrow, and ring-necked pheasant were the major cause of nest loss for all species was next most abundant species in CRP plots. predation, accounting for more than 50% of These eight species represent 92% of the aver­ nest losses in both field types. Still, predation age bird abundance from 1991 to 1993. Ex­ rates varied considerably, probably largely cept for the ring-necked pheasant and brown- attributable to off-site phenomena. Predator headed cowbird, all of the above species were species included fox, raccoon, skunk, farm more abundant in CRP than in row-crop fields. cats, rodents, birds, and snakes. Many of these species were more abundant in plots closely Bird abundance and density: Bird abun­ associated with farmsteads. Large mammals dance varied among CRP plots, probably be­ accounted for 89,88, and 85% of the predation cause of differences in the vegetation charac­ on grasshopper sparrow, red-winged black­ teristics of the plots. Individual bird species bird, and dickcissel nests, respectively, and abundances were correlated with vegetation 100% for ring-necked pheasants. Farming characteristics Dickcissel abundance was activities such as mowing and weed spraying positively correlated with vertical vegetation in CRP fields also caused some nest losses. density and percent forb coverage. This re­ Overall nest success of ground-nesting spe­ flects this species' need for structurally sound cies (pheasant and grasshopper sparrow) was nesting substrates. Grasshopper sparrow abun­ twice that of above-ground nesting species dance was negatively correlated with the ver­ (dickcissel and red-winged blackbird) in CRP tical density of the vegetation, consistent with plots. Weather and predation accounted for this species' preference for relatively short, nearly all the additional above-ground nest clumped grasses. losses.

Habitat selection by grasslands birds is strongly Nest success and causes of nest failure in CRP influenced by fluctuations in temperature and fields should be compared to studies of other precipitation. The three years of this study agriculturally associated habitats to determine encompassed temperature and moisture pat­ the relative benefits of CRP to nesting bird terns that included drought, record high mois­ species. The success rate for red-winged black­ ture, and temperatures well below normal. birds in CRP habitat was intermediate be­ The dickcissel is one species that will readily tween that in grassed waterways and road­ relocate to areas with less harsh conditions. sides, as determined in other Iowa studies. Dickcissel abundance was low in 1992 be­ Thirty percent of the redwing nests in grassed cause of dry conditions, and in 1993 because waterways are lost to mowing, compared with of excessively wet and cool weather. The less than three percent in this CRP study. bobolink increased in abundance over the three- Ring-necked pheasant nest success in CRP year study period, probably because it requires fields was much greater than that reported for high grass coverage and thick litter for nesting, linear agricultural habitats. and these increase over time in CRP fields. Plots dominated by tall, thick vegetation were From 1991-1993, 16 species nested in CRP likely to contain more above-ground than fields. Red-winged blackbird nests repre­ ground nests because this vegetation provided sented 48% of all nests found. The vesper for the structural needs of dickcissels and red- sparrow and horned lark were the only species winged blackbirds. The nest sites of these two found nesting in row-crop fields; vesper spar­ species had similar characteristics, although row nests represented 87% of row-crop nests. red-winged blackbird nests were found in a

104 Volume 4 (1995) greater variety of plant species. Managing Various factors affect vegetation structure and vegetation to favor use by dickcissels would composition on CRP land. Some factors are likely encourage redwings also. Grasshopper environmental; others involve administration sparrow nest sites had much higher grass cov­ of the CRP—its enforcement as well as varia­ erage but lower total canopy coverage than tions in plant species from state to state. Deci­ pheasant, dickcissel, and redwing nest sites. sions on the plant species should be based in Managing CRP land for grasshopper spar­ part on attributes to which birds respond. rows, savannah sparrows, and upland sand­ pipers should be achievable, as all these prefer Weed control policies that require farmers to relatively short and diverse grass structure. mow or spray will decrease plant diversity and Redwing numbers would be controlled in this negatively impact populations of bird species scenario, and late-season mowing could be that rely on heterogeneous habitats. Variable used to open up tall vegetation to grasshopper enforcement of such policies causes dispari­ sparrow nesting. ties in CRP's benefits to wildlife, sometimes from one field to another. Implications Any sustainable food production system must Wildlife concerns can be addressed in future consider its impact on the associated biotic federal land set-aside policies. Increased com­ community. The CRP has contributed to an munication among farmers, CRP officials, and increase in abundance of many grassland bird wildlife ecologists is critical as ten-year CRP species in central Iowa, as the row-crop habitat contracts begin expiring. Greater vegetation it replaced has lower bird abundance and fewer diversity and more weed management options nesting species. This research on CRP land will enhance bird use of CRP land. shows that it may support greater nest densi­ ties and nesting success than roadsides, grassed This project was also supported by the Max waterways, and other agriculturally associ­ McGraw Wildlife Foundation, the Iowa De­ For more information ated nesting habitats in Iowa. These differ­ contact L B. Best, partment of Natural Resources Nongame Wild­ Animal Ecology, Iowa ences are attributable to diverse vegetation, life Program, and the Iowa Agriculture and State University, Ames, large habitat blocks, and reduced agricultural Home Economics Experiment Station/Re- Iowa 50011, (515) activity. gional Research. 294-7477.

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