Despite the many species of bats (+ of 1200 around the world), they are almost all active almost exclusively at night ! Only a few animals of oceanic islands such as Noctule des Açoresare known for their diurnal behavior and occasionally, other species of temperate and tropical zones were observed in activity in the light of day. These observations, however, remain relatively rare.
During the day, several problems for bats. First of all, these are in competition with insectivorous birds like swallows (Hirundininae) or swifts (Apodidae). Hunt at night would allow bats to exploit an ecological niche not exploited by birds. Then, bats flying in daylight are likely to be attacked by predators such as birds of prey (Accipitridae, Falconidae). This risk of predation is also the night (former: owls, Hawks) but is much lower. A third assumption is powered in a recent article while Voigt et al. (2011). Sunlight absorbed by the wings of bats would cause an increase in the cost of flying in the light of day. Hunting days would be beneficial only if the energy gain is relative and that the elevated risk of predation is low. The authors propose an evolution of wing color to darker tones that would make the bat less detectable by predators, thus trapping bats in the darkness of the night.
Yann et Meriadeg
C. C. Voigt and D. Lewanski (2011). Trapped in the darkness of the night: thermal and energetic constraints of daylight flight in bats. Proceedings of the Royal Society B. Biological Sciences, 278 (1716) 2311-2317.