This IBA encompasses Avon Bottoms State Wildlife Area, and includes Avon Bottoms State Natural Area and Swenson Wet Prairie State Natural Area. Avon Bottoms is one of southern Wisconsin?s floodplain forest gems. Located on the floodplain of the Sugar River in southwestern Rock County, this site is characterized by mature silver maples, swamp white oak, and green ash, and is one of only a few sites in the state with southern species such as sycamore. Many shrub and groundlayer species are here at their northern range limit, including wild chervil and obovate beak grain, and the site has dense poison ivy, river grape, woodbine, and moonseed, often in the form of lianas climbing into the canopy. Oxbows and running sloughs are abundant in the floodplain, providing homes for invertebrates, reptiles and amphibians, including the rare Blanchard?s cricket frog (WDNR 2007a). Smaller areas of native and planted prairie, cool-season grassland, and oak savanna also are found here. Swenson Wet Prairie contains high-quality wet-mesic prairie and sedge meadow interspersed with savanna (now largely overgrown) and shallow abandoned river channels. Many prairie grasses and forbs grow here, including two state-threatened species, prairie Indian plantain and round-fruited St. John?s wort (WDNR 2007b).
Avon Bottoms provides habitat for a variety of priority birds, particularly floodplain forest species. This is the most reliable location in the state for Yellow-crowned Night-Heron, and one of the few suspected nesting locations for Yellow-throated Warbler. This site is considered a Cerulean Warbler core, with over 1,000 acres of suitable habitat available. Yellow-billed Cuckoos and Prothonotary Warblers are abundant here. Other breeders include Red-shouldered Hawk, Acadian Flycatcher, Willow Flycatcher, Brown Thrasher, Yellow-throated Vireo, Wood Thrush, Hooded Warbler, and American Redstart. Grassland areas support small numbers of Henslow?s Sparrow, Field Sparrow, Vesper Sparrow, Grasshopper Sparrow, and Eastern Meadowlark. Loggerhead Shrikes are seen in this area every year. This site is a concentration area for migrating landbirds, with estimates of over 10,000 birds passing through the area, particularly in spring.
Agricultural pollution, exotic species, and development are some of the threats facing this site. Much of the surrounding river valley has been converted to agriculture, and clearing and draining continue in some areas. Contaminant and nutrient runoff may affect water quality. Problematic exotic species include reed canary grass, creeping Charlie, and potentially emerald ash borer. Infestations should be controlled aggressively and disturbances that facilitate the establishment and spread of invasives avoided. This area is under increasing pressure for development, which can fragment and degrade habitat and prevent the movement of species along the river corridor. Many of the priority birds at this site are considered area-sensitive. Adding to the property through acquisitions and improving connectivity to other large blocks of floodplain forest will improve the site?s ability to support populations of area sensitive species over the long term, and should be conservation and management priorities here. Management also should focus on maintaining forest structure and tree species, size, and age class diversity. Potential exists here to restore other native habitats, including prairie and savanna, as well (Pohlman et al. 2006).