We haven’t done any models to test these questions. We know, however, that body mass does not entail a substantial problem for flight in the bats, at least not for commuting flight. Ibáñez et al. 2003 cites the case of a giant noctule which had a body mass of 72 g when captured, and around 18 h later, after being kept in a bag, its body mass had decreased to 61 g (a difference of 11 g). Note that mean body mass of the population is less than 50 g.
More of a problem is probably the killing, manipulation and consumption of the bird prey. Ibáñez et al. 2003 cites that aerial-hawking bats usually consume prey of up to 5% their own body mass (Fenton 1990, see Ibáñez et al. 2003), while they estimate that the birds consumed by N. lasiopterus could be up to 30% of the bat’s weight. Based on this prey/predator mass relationship, they hypothesize that Ia io could also be a predator of migratory birds. Interestingly, this was later confirmed by Thabah et al. 2007.
The main problem that an aerial-hawking bat has while killing an aerial prey is that the bat’s wings and tail are used for caging the prey in the process of hunting, and cannot be employed for active flight. Therefore the whole process needs be fast so as not to compromise flight. We hypothesized that bats would need to fly and hunt the birds very high to have time to manipulate a difficult prey while avoiding a collision with the ground. Our radar data show that this is true, and bats perform search flights over 500 m above the ground.