A playback paradox? Nathusius' bats, Pipistrellus nathusii, bypass mating and fueling opportunities on migratory transit flights

In late summer, migratory bats of the temperate zone face the challenge of accomplishing two energy-demanding tasks almost at the same time: migration and mating. Both require information and involve search efforts, such as localizing prey or finding potential mates. In non-migrating bat species, playback studies showed that listening to vocalizations of other bats, both con-and heterospecifics, may help a recipient bat to find foraging patches and mating sites. However, we are still unaware of the degree to which migrating bats depend on con- or heterospecific vocalizations for identifying potential feeding or mating opportunities during nightly transit flights. Here, we investigated the vocal responses of Nathusius pipistrelle bats, Pipistrellus nathusii, to simulated feeding and courtship aggregations at a coastal migration corridor. We presented migrating bats either feeding buzzes or courtship calls of their own or a heterospecific migratory species, the common noctule, Nyctalus noctula. We expected that during migratory transit flights, simulated feeding opportunities would be particularly attractive to bats, as well as simulated mating opportunities which at the same time indicate suitable roosts for a stopover. However, we found that the echolocation call activity of P. nathusii decreased during the playback of conspecific feeding buzzes and courtship calls, yet the call activity remained unaffected when heterospecific call types were broadcast. Our results therefore suggest that while on migratory transits, P. nathusii circumnavigate conspecific feeding and mating aggregations, possibly to save time or to reduce the risks associated with social interactions. This avoidance behavior could be a result of optimization strategies by P. nathusii when performing long-distance migratory flights.