echolocation to discriminate heavily among potential prey based on shape and texture of a target. This lack of discrimination may be because of the rapid flight of bats and the short range at which prey can be detected using echolocation, allowing bats only a fraction of a second after detection to capture prey. However, gray bats are believed to discriminate somewhat between insects when foraging in their natural habitat, consuming higher numbers of Lepidoptera, Coleoptera, Diptera, and in some populations Trichoptera, than their proportional prevalence would have otherwise indicated without selective foraging. Because of this tendency to select prey while being largely opportunistic, gray bats have been dubbed ‘selective opportunists’. Scientists believe that food moves quickly through the digestive tract of M. grisescens, with feces being purged from the body within 1–2 hours after ingestion. Migration, hibernation, and reproduction Fall migration occurs in approximately the same order as spring emergence, with females departing first (early September for fall migration) and juveniles leaving last (mid-October). Gray bats may migrate as far as from summer caves to reach hibernation caves, although based on band recovery data and the spatial distribution of summer and winter roosts, a migration range below appears to be the norm. The annual activity period of gray bats is April to October, though female gray bats enter hibernation in September. After arriving at winter caves, copulation occurs. Females immediately enter hibernation, while males may remain active for a few more weeks. Males use this extra time before entering hibernation to replenish fat reserves used during breeding. Males begin hibernation by early November. During hibernation, the body temperature of gray bats drops close to the ambient temperature, allowing the body to conserve fat. These fat reserves must last the approximately six months of hibernation and spring migration. Adult mortality is especially high during spring migration, as bats that do not have sufficient fat reserves have difficulties surviving the stress and energy-intensive migration period. After copulation, females store sperm in their uteri, ovulating only after they have emerged from hibernation. Gestation in gray bats lasts 60 to 70 days, with birth occurring in late May and early June. Gray bat females give birth to one offspring per clutch (bout of reproduction), thus giving birth to one offspring per year. Therefore, gray bats demonstrate an iteroparous life-history strategy. The young clings to the mother for about a week, after which they remain