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A nursery population of M. sodalis was discovered in Indiana and studied for 2 years. The nursery roost was located under the loose bark of a dead tree. Sometimes the bats temporarily moved to bark crevices of a living shagbark hickory tree. Weather strongly affected roost microclimate. The nursery tree was unshaded and received maximal solar warmth during clear, mild or hot weather, whereas temperature at the shaded alternate tree was more stable during spring and autumn cold. The nursery population consisted of adult females and young, and few males were netted in local foraging areas. Each female bore a single young and these incurred eight percent mortality between birth and weaning. Social behaviors included “checking,” mothers carrying young to an apparently warmer portion of the roost tree, and apparent mother-young foraging flights as the young became volant. Unusually cool weather in summer 1974 slowed the growth of young; this delayed recruitment of flying young more than 2 weeks and the completion of migration by 3 weeks. The delay exposed some bats to freezing weather at the nursery and may have affected mortality, autumn mating, or fat storage for winter. No problems were evident during the favorable summer of 1975. Foraging habitat included the foliage of riparian and floodplain trees, and the 50 bats used a 0.82 kilometer (km) linear strip of creek. Similar habitat was used by two other populations in Ohio. Before young were volant, adults fed only about riparian trees; when young began to fly, feeding extended to solitary trees and forest edge on the floodplain. This reduced foraging density from 17 to 11 bats per hectare (ha). Suitable foraging habitat occurs over much of the eastern United States, and use of local trees for nurseries makes a large summer distribution possible. Humans also value floodplain habitat, and a land use conflict exists between the two species.