We can only speculate about these interesting questions with our limited current knowledge. We will try however to bring up a few ideas.
Let’s start with the evolutionary history of N. lasiopterus. Not so much is known about this, but a recent phylogeny of Nyctalus by Salgueiro et al. (2001, Genetica 130:169-181) reports a difference of 6% in cyt b between N. lasiopterus and N. noctula. They do not mention when the two species separated. If we consider a rate of divergence of 4% per one million years, we would conclude that the two species diverged more than one million years ago (this is just our rough estimation).
Bird migration, or at least the ability to migrate, is believed to be an ancient character originated very early in avian history (Zink, 2002, J Avian Biol 33:433-436). Migration is also very flexible and bird migratory routes in the Mediterranean have probably changed periodically together with climate and habitat changes. We do not know how exactly bird migratory routes looked liked at the time and place of giant noctule’s origin (which we can also not date). Some extant songbird species that now migrate across the Iberian Peninsula appear to be very old, for example the separation between Sylvia boris, Sylvia atricapilla and Sylvia communis seems to date back to 5-6 MY ago (Covas and Blondel 1997, “Biogeography and history of the Mediterranean bird fauna, Ibis 140:395-407). Thus, when N. lasiopterus originated, many extant migratory songbirds were probably already present in the area, experiencing dynamic migrations whose routes shifted together with their environment.
The relationship between the large body mass of giant noctules and bird predation is obvious in the sense that large size is required to overpower a large prey. But this last question is also related to the first question on the relationship between size and dependence on the bird resource. We can speculate, for example, that N. lasiopterus evolved from N. noctula in an area with high density of bird migration. Note that some bird feathers have also appeared circumstantially in the feces of N. noctula!! It is also not so rare for bats to pursue prey too large for them to capture. Some unusually large N. noctula could have occasionally succeeded in hunting a bird, and with time, larger sizes in these bats could have been selected because of higher fitness benefits of bats taking advantage of bird migration, ending in the origin of N. lasiopterus-aviator. This original species might have then been able to colonize other areas with lower density of migrants, with more or less success (e.g. Nyctalus aviator and/or certain populations of N. lasiopterus adapting to areas with very low migratory bird densities?). Unfortunately, we can only speculate at this point, and many other scenarios could also have been possible.