Diurnal activity of a trawling insectivorous bat species, Myotis horsfieldii, in Gunung Mulu National Park, Malaysian Borneo

Bats are typically nocturnal. However, daylight activity (ie between one hour after sunrise and one hour before sunset) is known for a small subset of species, often associated with small oceanic islands with no diurnal avian predators. Here, we describe numerous observations of diurnal activity of Horsfield's bat (Myotis horsfieldii), in Gunung Mulu National Park in Sarawak, Malaysia. Between 2015 and 2020, bats were repetitively observed flying along streams and rivers at 14 localities during various hours of the day. Daylight active bats exhibited a flight pattern concurrent with prey hunting strategies and, through bioacoustics, foraging was confirmed by the presence of feeding buzzes. Despite the occurrence of numerous sympatric avian diurnal predators known to hunt bats (eg bat hawk Macheiramphus alcinus) and avian competitors (eg cave swiftlet Aerodramus spp.), the relatively wide temporal and spatial coverage of the records suggests that diurnal activity of this water-associated bat might be a common phenomenon in the area. We speculate that this atypical behaviour might have arisen to circumvent night-time competition with the high numbers of bats roosting in nearby caves.