BIRDS OF WISCONSIN OAK SAVANNAS:
PAST, PRESENT, AND FUTURE
David W. Sample
Michael J. Mossman
Wisconsin Department of Natural Resources
Oak savanna, which once was found in the prairie-forest ecotone that included large parts of southern and western Wisconsin, is now perhaps the most threatened and rare vegetation community in the state. Savannas once covered over 5 million acres in southern Wisconsin. It occurred in a landscape mosaic that consisted of a continuum of habitats from prairie to oak woodland; the continuum aspect of the landscape makes savanna difficult to characterize. We use Curtis' (1959) definition of savanna as a grassland with at least one tree per acre, but with less than 50% tree canopy cover, mostly bur oaks, often mixed with or replaced by other species such as white, swamp white, or black oaks, depending on site moisture and soils (Curtis 1959). Shagbark hickory was often present. Wisconsin's oak savanna communities changed greatly with the onset of agriculture and the cessation of fire. Sites not plowed or grazed succeeded into woods, where oaks were (and are being) replaced by shade-tolerant species. Today only about 500 acres of oak savanna remain.
Bird species that once comprised the avifauna of savannas have either disappeared or have adapted to other habitat types, often habitats altered by human activities. To help set meaningful goals for the recovery of Wisconsin's declining bird populations and for the restoration of the nearly vanished prairie-savanna-oak woodland landscape, we investigated the breeding bird faunas of presettlement and recent landscapes of the prairie-forest ecotone of southern and western Wisconsin.
This study was located at sites scattered throughout the southern and western parts of Wisconsin, south of Curtis' "tension zone". This region includes both glaciated and unglaciated areas, with a variety of topography, soils, and other environmental conditions.
We used the literature to characterize the breeding bird communities of the prairie-savanna-oak woodland landscape, including the scanty presettlement accounts (e.g., Schoolcraft 1821; Featherstonaugh 1847), publications from the early post-settlement period, 1850-1910 (Hoy 1885; King 1883; Kumlien and Hollister 1903) and more recent literature (Schorger 1929; Robbins 1991).
Bird community composition in savannas and related habitats in the current prairie-savanna-oak woodland landscape was assessed by recent habitat-specific breeding-bird surveys conducted as part of the Wisconsin Department of Natural Resources Natural Areas Breeding-Bird Survey (1977-93) and the Wisconsin Grassland Bird Study (1985-93). The Natural Areas Survey includes data collected by several cooperators on over 150 natural areas (including approximately 10 natural areas with savannas), using walk-5-minute/stand-5-minute breeding bird counts (Mossman and Lange 1982). The Grassland Bird Study included two data sets. The first set included three years of intensive surveys on 197, 100 x 200 m plots in a wide variety of grassland habitats (including 5 savanna plots) surveyed for birds three times during the breeding season; detailed habitat measurements also were collected (Sample 1989). In the second set, an extensive set of walk-5-minute/stand-5-minute breeding bird surveys was made during single visits to over 800 sites statewide (over 500 south of the tension zone); habitats included native, mostly overgrown savannas (13 sites), and several savanna-like habitats characterized by open-grown oaks with an exotic grass understory such as savanna pastures (19 sites), parks and golf courses (8 sites), ungrazed disturbed sites with open-grown oaks on public land (9 sites), and related habitats such as shrubby oldfields (69 sites).
It is difficult to accurately describe the presettlement bird communities of savannas from the historical literature and from our recent field studies on remaining savanna-like habitats. Our best estimate is that there were approximately 100 bird species which occurred regularly as breeding species in the prairie-savanna-oak woodland landscape in Wisconsin. One of these species is extinct (Passenger Pigeon), and several more are extirpated from this region (Common Raven) or state (Swallow-tailed Kite, Swainson's Hawk). The Wild Turkey was extirpated, but has since reappeared due to reintroductions. Selected species characteristic of the prairie-savanna-oak woodland landscape (hereafter "savanna species") are presented in Table 1.
Several savanna species exhibited population changes during the settlement period (around 1850 to 1900) which were related to changes in land use and other human activities (Sample 1989). Sharp-tailed Grouse did not tolerate the plowing of the prairies, and Turkeys declined quickly due to factors that included harsh winter weather and hunting. The Greater Prairie-chicken responded well to early agriculture, but declined after 1900. Other species increased during the early settlement period due to succession (savanna to brushy oak woodland to forest) and planting of trees on the prairie (e.g., E. Pewee, E. Kingbird, L. Shrike, Indigo Bunting, Towhee, and Field Sparrow). Species that have increased or remained stable in recent years (e.g., Kingbird, Pewee, Nuthatch, Bunting) have been able to adapt well to alternate habitats with suitable structure such as parks and golf courses, suburban areas, fragmented woods, brushy old fields, farmsteads, and hedgerows. Such surrogate habitats have played an important role in maintaining populations for a number of savanna species.
Recent years have also seen the near disappearance of some species (e.g., Prairie-chicken and Sharp-tail), while others have barely persisted in neglected, often isolated fragments of habitat (e.g., Loggerhead Shrike and Bell's Vireo). A number of species have undergone significant population declines since 1966 on Breeding Bird Survey Routes (e.g., Red-headed Woodpecker, Northern Flicker, Towhee, Field Sparrow, and Bobolink); habitat loss likely has had a negative effect on both of these groups of species (although wintering ground problems deserve close attention as well) (Robbins 1991). Of the selected species in Table 1, three are legally designated as Special Concern in the state, one is Endangered, and two are Threatened.
Savanna species can be categorized according to their management implications. We have proposed three categories: a) extirpated species that we cannot realistically expect to restore in the foreseeable future, b) common species that are widespread in disturbed habitats and which will be easily included in a restored landscape, but which require and may be important indicators of critical structural features, and c) species that have declined substantially and will require management to provide particular habitat features. Examples of category a) are the extirpated species mentioned above. Habitat features of importance for selected species in category b) and c) are presented in Table 1. Examples of species in category b) are Kestrel, Turkey, Pewee, Kingbird, Nuthatch, and Indigo Bunting. Some examples of species in category c) are Prairie-chicken, Sharp-tail, Red-headed Woodpecker, Shrike, Bell's Vireo, Towhee, Field Sparrow, and Bobolink.
Our analysis of bird-habitat relationships suggests structural and compositional features that were probably characteristic of the prairie-savanna-oak woodland mosaic (Table 1), and which should be incorporated as natural or surrogate components into restoration projects. Probably the least recognized of these features is upland brush in a grassland context ("open with shrubs" in Table 1). This component of the prairie-woodland mosaic - open grasslands with areas of scattered shrubs and shrub clumps - is important to a number of uncommon or rare species, including Sharp-tail, Bobwhite, Shrike, Bell's Vireo, Yellow-breasted Chat, White-eyed Vireo, and Field Sparrow.
While it is difficult to accurately describe the bird communities of the presettlement prairie-savanna-oak woodland mosaic, it is apparent that while some species have maintained or increased their populations by adapting to various surrogate habitats, others have been extirpated or have declined severely. Effective management and restoration of savanna bird communities will depend on an understanding of critical habitat components for species, and may require creation or maintenance of landscapes that incorporate a variety of habitat structure at a variety of scales, as the original mosaic once did.
We would like to acknowledge the many volunteer bird surveyors for the Natural Areas Breeding Bird Survey program. P. Rasmussen and R. Rolley provided assistance with data analysis. Funding for this study was provided in part by the Federal Aid to Wildlife Restoration Act under Pittman-Robertson project W 141-R.
Curtis, J. T. 1959. The Vegetation of Wisconsin: An Ordination of Plant Communities. University of Wisconsin Press. Madison, WI.
Featherstonaugh, G. W. 1847. A Canoe Voyage Up The Minnay Sotor. 2 volumes. Richard Bentley, London. Reprinted 1970 by Minnesota Historical Society. St. Paul, MN.
Hoy, P. R. 1885. Man's influence on the avifauna on the avifauna of southeastern Wisconsin. Proceedings of the Natural History Society of Wisconsin (March):4-9.
King, F. H. 1883. Economic relations of Wisconsin birds. P. 441-610 In: T. C. Chamberlin (ed.). Geology of Wisconsin, vol. 1. Commissioners of Public Printing. Madison, WI.
Kumlien, L. and N. Hollister. 1903. The birds of Wisconsin. Bulletin of the Wisconsin Natural History Society 3(1-3):1-143.
Mossman, M. J. and K. I. Lange. 1982. Breeding birds of the Baraboo Hills of Wisconsin: their history, distribution, and ecology. Department of Natural Resources and Wisconsin Society for Ornithology. Madison, WI.
Robbins, S. . 1991. Wisconsin Bird Life. University of Wisconsin Press, Madison, WI.
Sample, D. W. 1989. Grassland birds in southern Wisconsin: habitat preference, population trends, and response to land use changes. M.S. thesis, Univ. of Wisconsin-Madison.
Schoolcraft, H. R. 1821. Journal of a tour into the interior of Missouri and Arkansaw. R. Phillips. London, England.
Schorger, A. W. 1929. The birds of Dane County, Wisconsin, Part 1. Transactions of the Wisconsin Academy of Science, Arts, and Letters 24:457-499.
Table 1. Historical and current abundance, status, Breeding Bird Survey population trend and important habitat features for selected savanna bird species in Wisconsin.