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Petrželková, K. J., Downs, N. C., Zukal, J., & Racey, P. A. (2006). A comparison between emergence and return activity in pipistrelle bats Pipistrellus pipistrellus and P. pygmaeus. Acta Chiropterologica, 8(2), 381–390.
Abstract: Bats may be vulnerable to predation during evening emergence and morning return to their roosts. Early emergence increases the risk of exposure to raptorial birds, but emerging late confers a risk of missing the dusk peak of aerial insects. Here, both emergence and return activity was studied in detail at the same roosts for the first time. We investigated six maternity colonies of pipistrelle bats (Pipistrellus pipistrellus and P. pygmaeus) in NE Scotland and recorded light levels and time of emergence and return of the bats with respect to sunset and sunrise on the same nights. Parameters of return activity generally occurred at lower light intensities than those of emergence. Therefore, the interval between dawn return and sunrise was generally longer than that between sunset and dusk emergence. Emergence and return were equal in duration. Bats clustered more on emergence in comparison with return during pregnancy and lactation, whereas during postlactation this trend was reversed.