Louis B. Best

Habitat Selection by Birds of Riparian Communities: Evaluating Effects of Habitat Alterations

Avifauna of riparian communities were studied in Iowa during late spring and early summer. Birds were censused on 28 study plots representing a habitat gradient from hayfields to closed-canopy woodlands. An index of nesting niche breadth was determined for 18 open-nesting bird species on the basis of vegetation life form(s) used for nesting, and for 10 cavity-nesting species on the basis of type(s) of nest-cavity support. Cavity-nesters preferred soft snags as nest sites. Floodplain woodlands supported higher densities of breeding birds than upland woodland or herbaceous habitats. Bird species richness increased (P < 0.01) with the width of wooded riparian habitats. Wooded habitats supported a maximum of 32 species; herbaceous habitats, 8. Observation frequencies of 41 bird species in 6 general habitat types were used to calculate indices of tolerance to habitat alteration. Microhabitat characteristics selected by each species were determined by comparing bird observation frequencies with 36 vegetation variables, using stepwise multiple regression. The potential effects of 6 alterations to wooded riparian habitats on the 41 species are predicted. J. WILDL. MANAGE. 44(1):1-15 Riparian habitats are critical to wildlife, especially in regions with intensive agriculture where streamside habitat is being depleted rapidly. Nongame birds constitute a major component of riparian wildlife communities, but most research concerning nongame bird management has been local in nature or concerned with endangered species (Robbins and Erskine 1975). Recently, contributors to several symposia have emphasized the need for research on management of all nongame birds (Smith 1975; DeGraaf 1978a,b). Such research is essential to assess the impacts of habitat alterations on the associated wildlife (Lennartz and Bjugstad 1975). Our study is an attempt to satisfy part of that need. Our objectives were to quantify factors critical in nest-site and habitat selection by breeding birds of riparian communities over a range of habitat types and to use the results to evaluate effects of habitat perturbations on avian communities. The results are applicable to both riparian and upland habitats of the Central Plains and, with caution, could be used in other areas as well. We are grateful to the landowners of Guthrie County, Iowa, for their cooperation in all phases of the field work. T. Rosburg and R. Deitchler assisted in collecting field data. D. F. Cox, L. Wolins, M. Hand, and B. J. Meador provided help with the statistics and computer analysis. J. J. Dinsmore, R. B. Dahlgren, and R. Q. Landers reviewed an earlier draft of the manuscript. The manuscript also benefited from the suggestions of 2 anonymous referees.

A Review and Synthesis of Habitat Use by Breeding Birds in Agricultural Landscapes of Iowa

Existing information on bird species composition, abundance and nesting sta- tus during the breeding season (May through July) was compiled for habitats characteristic of the agricultural landscapes of Iowa. Data were derived from 60 sources for 144 bird species in 20 habitats. Total numbers of breeding bird species were highest in floodplain forest (107 species) and upland forest (85), and lowest in small grains (31) and herbaceous fencerows (27). Species abundances were standardized and categorized on a scale from 0 (absent) through 5 (very abundant with >250 individual birds/census count/100 ha). Bird species abundances are lowest in some agricultural habitats (e.g., tilled row crops and small grains) and highest in narrow, strip-cover habitats (e.g., railroad rights-of-way, wooded fencerows and farmstead shelterbelts). Species abundance patterns in natural habitats (forest, marsh and prairie) are intermediate between those in agricultural and strip-cover habitats. Twenty-five species occurred only in forest habitats and 14 only in marshes. Other species selectively (through not exclusively) use tilled row crop, grassland or wooded habitats. Principal com- ponents analysis was used to assess the relative similarities in use of the 20 habitats by the assemblage of breeding birds in Iowa. Predicted numbers of nesting species increased from 18 to 93 over four landscape scenarios representing a progression from an intensively farmed row-crop monoculture to a diverse mosaic of crop and noncrop habitats. Although laborious, the approach developed in our study has been useful for standardizing and synthesizing a diverse literature in efforts to conduct ecological risk assessments for farmland birds. It can also provide valuable baseline data for landscape-level research.

Use of cornfields by birds during the breeding season: the importance of edge habitat

Birds were censused in the center and perimeter of cornfields and in adjacent edge habitats to assess how edge habitat affected cornfield use by birds during the breeding season. Cornfields were bounded either by predominantly herbaceous vegetation or by woodland. Fifty bird species were observed in woodland edges compared with 23 in herbaceous edges; bird abundance in woodland edges was more than seven times that in herbaceous edges. Although the number of bird species and the number of birds were less in cornfields than in adjacent edge habitats, neither bird species richness nor total bird abundance in cornfields was influenced significantly by the type of edge habitat (herbaceous vs. woodland). In contrast, bird species composition within cornfields bordered by herbaceous habitat differed from that in cornfields adjacent to woodland habitat. More bird species and about five times more birds used the perimeters of cornfields than the centers, demonstrating a significant edge effect. Consequently, bird abundance in cornfields decreases logarithmically as field size increases. Most of the bird species that used cornfields regularly or occasionally are ground-feeding omnivores during the breeding season, whereas the species that rarely or never frequented cornfields are mainly insectivores that forage on woody vegetation. Continuing land-use practices that increase crop field size and eliminate woody vegetation from edge habitats will affect both the species richness and abundance of the avifauna associated with cornfields.

Potential effects on grassland birds of converting marginal cropland to switchgrass biomass production.

Abstract Habitat loss is a major reason for the decline of grassland birds in North America. Five habitats (pastures, hayfields, rowcrop fields, small-grain fields, Conservation Reserve Program fields) compose most of the habitat used by grassland birds in the Midwest United States. Growing and harvesting switchgrass ( Panicum virgatum ) as a biomass fuel would create another habitat for grassland birds. Bird abundance information from studies conducted in Iowa and adjacent states and land-use data for the Rathbun Lake Watershed in southern Iowa were used in a Geographic Information System to model the potential effects on bird abundances of converting rowcrop fields to biomass production. Abundances of bird species that are management priorities increased in both biomass scenarios. Common yellowthroat ( Geothlypis trichas ) abundance in the watershed also increased greatly in both scenarios. Other species (e.g., horned lark [ Eremophila alpestris ], killdeer [ Charadrius vociferous ]) were more abundant in the existing land use than in the biomass scenarios, and conversion of fields from rowcrop to biomass production could be detrimental to these species. In general, biomass fields will provide habitat for grassland birds that are management priorities, but future monitoring of birds in such fields is needed as conversion of rowcrop fields to biomass production continues.

Breeding Ecology of Vesper Sparrows in Corn and Soybean Fields

The breeding ecology of vesper sparrows (Pooecetes gramineus) was studied along fencerows between corn and soybean fields in central Iowa. Territories included portions of both corn and soybean fields, usually within 80 m of the fencerow. Breeding densities were greater along more shrubby fencerows. Nesting success in corn and soybean fields was low, 9% before 15 June and 28% thereafter. Nest losses resulted primarily from agricultural operations (27 % of all nest losses) and predation (29 % ). Nest predation was higher near agriculturally nonproductive areas. Late in the season, when most of the successful nesting by vesper sparrows occurred, washes were heavily used as nest sites. Nest placement shifted seasonally, probably in response to changes in food availability and cover for nesting activities. Renestings during the middle of the breeding season were farthest from nonproductive areas. Annually, vesper sparrows produced an average of 2.8 young/pair, probably below replacement levels. Vesper sparrow breeding success likely would be greater if the number of tillage operations was reduced and crop residue was retained on the

Short-term bird response to harvesting switchgrass for biomass in Iowa.

The Conservation Reserve Program (CRP) provides habitat for grassland birds, but as contracts expire, some CRP fields might be returned to rowerop production. One alternative to returning CRP fields to rowcrops is to produce switchgrass (Panicum virgatum) for use as a biomass fuel. Because the biomass is harvested during the fall and winter, breeding birds would not be directly affected by mowing the fields but might be influenced by changes in vegetation structure resulting from the harvest. We evaluated bird abundances and nest success in totally harvested, partially harvested (alternating cut and uncut strips), and nonharvested CRP switchgrass fields in southern Iowa, USA, in 1999 and 2000. Species richness did not differ among harvest treatments. Abmidances of most species (16 of 18) were not affected by the harvesting of switchgrass fields, and strip width did not affect bird numbers in strip-harvested fields. Grasshopper sparrows (Ammodramus savannarum) were mole abundant in barvested portions of fields, and more sedge wrens (Cistothorus platensis) were recorded in nonharvested areas. The residual vegetation in nonharvested areas provided nest cover for species that begin nesting early in the season (e.g., northern harrier [Circus cyaneus] and ring-necked pheasant [Phasianus colchicus]). Nest success rates of grasshopper sparrows and common yellowthroats (Geothlypis trichas) were similar to those reported by other studies in switchgrass fields and might be sufficient to maintain stable populations. In general, switchgrass biomass fields create breeding habitat for some grassland birds, and a mixture of harvested and nonharvested fields would be more beneficial to grassland birds than totally harvesting or partially harvesting all switchgrass fields.

Influence of landscape composition on bird use of rowcrop fields

We evaluated the influence of landscape composition on bird use of rowcrop (corn and soybean) fields in 6 watersheds in Iowa from mid-May to late July 1993 and 1994. We counted birds within 50-m-radius circular plots positioned randomly within rowcrop fields and determined coverages for 21 habitats within 800-m-radius circles centered on each bird census plot. We evaluated the relationships between bird abundances in rowcrop fields and the habitat coverages in the landscape by using 2 multivariate procedures. We derived 3 landscape scenarios from a cluster analysis of the original habitat variables; the abundances of 7 bird species differed significantly among the 3 scenarios. Species abundances in rowcrop fields were greater in landscapes with more grassland block-cover and/or more wooded block-cover and strip-cover. Principal component analysis illustrated the responses of bird species to landscape composition; species responses depended upon the relative use (ranging from resident to occasional) that the birds made of the rowcrop fields. Habitat selection and use in birds is a multiscale phenomenon, and the landscape context should be considered when evaluating bird use of rowcrops.

Bird Abundance and Species Richness in Grassed Waterways in Iowa Rowcrop Fields

-Grassed waterways have been used for decades to prevent soil erosion in agricultural cropland, but their benefits to wildlife had not been evaluated prior to our study. We documented bird species composition and relative abundance during the breeding season in 44 grassed waterways in central Iowa. The waterways were planted predominately to smooth brome (Bromus inermis) and were in cornfields and soybean fields. Forty-eight bird species were observed in waterways, compared with only 14 in the surrounding crop fields. Red-winged blackbirds (Agelaius phoeniceus), dickcissels (Spiza americana), barn swallows (Hirundo rustica), western meadowlarks (Sturnella neglecta), brown-headed cowbirds (Molothrus ater), grasshopper sparrows (Ammodramus savannarum) and song sparrows (Melospiza melodia) were the most abundant bird species in the grassed waterways. Total bird abundance in the grassed waterways averaged 2198 birds observed/census/100 ha compared with 682 in crop fields. Temporal patterns in bird abundance were attributed primarily to aspects of the waterways and surrounding cropland that changed over time, such as vegetation height. Because most (53%) of the bird species were at peak abundance in the waterways during 4-22 July, grassed waterways should not be mowed until the end of August or early

Avian Abundance in CRP and Crop Fields during Winter in the Midwest

Abstract We compared the abundance and species composition of birds in Conservation Reserve Program (CRP) fields with the same aspects in row-crop fields during the winter (January and February) over several years (1992–1995) for six midwestern states (Indiana, Iowa, Kansas, Michigan, Missouri and Nebraska). Field techniques were standardized in all states. CRP fields consisted of either permanent introduced grasses and legumes (CP1) or permanent native grasses (CP2), and the plant species seeded in CRP fields differed within and among states. Vegetation characteristics of CRP fields varied considerably from state to state, but vertical density and total canopy cover (primarily grasses) were particularly high in Nebraska. Mean annual total bird abundance ranged from 0.1 to 5.1 birds per km of transect in CRP fields and from 0.1 to 24.2 in row-crop fields. The total number of bird species recorded in CRP fields in the six states ranged from 6 to 32; the range for row-crop fields was 8 to 18. The most abundant species in CRP fields differed among states but included the ring-necked pheasant, American tree sparrow, northern bobwhite, dark-eyed junco and American goldfinch. The most abundant species in row-crop fields included the horned lark, American tree sparrow, European starling, mourning dove, lapland longspur, meadowlarks and Canada goose. Some of the most abundant bird species wintering on CRP fields have been undergoing long-term population declines, thus this program has the potential to mitigate population losses.

GRASSLAND BIRD USE OF RIPARIAN FILTER STRIPS IN SOUTHEAST IOWA

Abstract The U.S. Department of Agriculture (USDA) under its Continuous Enrollment Conservation Reserve Program (CRP) has actively promoted establishment of conservation buffers. Although these programs are intended to benefit wildlife in addition to protecting soil and water resources, benefits to grassland birds may be compromised by narrow widths, presence of woody vegetation, and high predation pressure. During 2001 and 2002, we surveyed breeding grassland birds and searched for nests in 33 CRP filter strips that varied in planting mixture (cool-season vs. warm-season grasses) and adjacent edge type (non-wooded vs. wooded). The most abundant species in filter strips were red-winged blackbird (Agelaius phoeniceus), dickcissel (Spiza americana), song sparrow (Melospiza melodia), and common yellowthroat (Geothlypis trichas). Relative abundances of birds and nests were similar between cool-season and warm-season planting mixtures. Dickcissels and red-winged blackbirds and their nests were relatively less abundant at wooded than non-wooded sites. Our nest success estimates generally were low in all treatments, and nest success varied little with the variables we studied. Predation was the major cause of nest failure; 62% of all nests were depredated. Although the most common birds using filter strips are generalists, filter strips also have potential to provide breeding habitat for some species of management concern.