Why do insectivorous bats in Britain not fly in daylight more frequently?

Predatory attacks, mostly by avian diurnal predators occurred in 13 of 420 observations; in at least five of these instances the attacks were fatal. Large bats survived attacks significantly more than small bats. This may explain why large species of bats flew more frequently in daylight relative to their populations than small bats. In the laboratory pipistrelle Pipistrellus pipistrellus and brown long-eared bats Plecotus auritus avoided flying in the light. In the wild, bats did not avoid flying on days which were particularly hot or sunny, suggesting that in Britain overheating does not inhibit daylight flights. Heavier pipistrelle bats avoided the light more than lighter pipistrelle bats. This may reflect lower manoevrability and hence greater susceptibility of heavy bats (within a species) to predation, and may explain the reduced frequency of day-light bats during late June and early July when they are heavily pregnant. -from Author