After the old bridge on Highway 171 over the Kickapoo River was replaced in the spring of 2003, Cliff Swallows began breeding under the new concrete structure. Cliff Swallows are one of six species of swallows breeding in Wisconsin and are highly colonial birds with a complex social life. Each fall they migrate to South America and each spring return to North America, making a round-trip journey of more than 16,000 miles. They winter in southern Brazil, Uruguay, and Argentina, leave their wintering range starting in early February, and arrive in Wisconsin in May to select mates, build complex domed mud nests, lay eggs, raise their young, and begin the journey south again, usually by the end of July. Their nests are repaired or rebuilt each year, and the birds forage together in groups to gather suitable mud, carrying it in their bills back to the nesting site; one pair was observed bringing 44 mud pellets in a 30 minute period. Originally built on vertical cliff surfaces, these enclosed gourd shaped nests are now more commonly found on the undersides of concrete bridges and culverts, and less often on buildings and cliffs, where the more stable concrete surfaces appear to make them more secure and protected. The interiors help keep occupants warmer in cool weather and cooler in heat. Mud composition and strength vary from colony to colony, so some nests last for 11 years while others fall apart within a season. These small birds, weighing little more than one half ounce, eat, drink, and bathe entirely in flight, feeding on swarming insects including farm crop pests such as corn borer moths and grasshoppers, as well as mosquitoes and many other insects.