Although the phenology of bat migration has been investigated at the population level, the timing and energy management of individual bats is poorly understood. Early arrival on the summering grounds with ample energy stores may give a fitness advantage to females preparing to raise pups. In contrast, there is no such fitness gain for males because they invest in mating during autumn. We use 3 years of capture data to investigate sex differences in spring migration passage date and body composition of Lasionycteris noctivagans. We predicted that females would arrive earlier in the spring and maintain greater fat stores than males. Females passed through the study site earlier and had more fat than males in 2 of 3 study years. Cold weather appeared to delay female migration and to deplete fat stores but did not appear to affect the passage date or fat stores of males. Our findings indicate that sex differences occur in the timing and energy management decisions of bats during spring migration. We postulate this difference in migration strategy is related to the increased demands of reproduction once females arrive at their summering grounds. Our results also suggest that females' fuel migration with energy acquired en route to a greater extent than males.
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Vol. 97 • No. 6