Seasonal distribution of migratory tree bats (Lasiurus and Lasionycteris) in North America

Despite evidence of migration in North American tree bats (genera Lasiurus and Lasionycteris), details regarding seasonal movements in these widely distributed species are few. This study attempted to clarify patterns of seasonal distribution by mapping museum occurrence records by month. Monthly changes in the distribution of records indicate seasonal movements of tree bats. Northward migration during spring by Lasiurus cinereus appears to stem from wintering grounds in California and Mexico. During summer, male L. cinereus is mainly distributed in western North America, whereas females dominate samples from eastern regions. Lasiurus borealis winters in southeastern areas and expands its range into more northern regions during warmer months. Seasonal dispersal of Lasiurus blossevillii from California is apparently limited, and it is unclear if this group mixes with others to the south and east. Lasionycteris noctivagans also shows movement at the continental scale, although movements may differ between western and eastern groups. Museum records provide little evidence of major movement by any species between North and South America. Despite inherent biases, museum records are a viable means of investigating bat migration.