Cliff swallows

I was reminded of the cliff swallows, small migratory birds that live part of the year in San Juan Capistrano in California, and famously all leave at once by the thousands to fly to Goya, in Corrientes province, Argentina. Festivals in both cities accompany both the arrival and the departure of the massive flock of migratory birds. It has also brought the two cities together after a fashion; although legally cities can't enter into treaties, but the two cities have signed a so called "declaration of brotherhood." Exactly what that means other than feel-good PR I don't know (and I suspect nothing) but I was struck by the image; how many fantasy settings have little details like that? Little details like that really make settings come alive and feel more "verisimilitudinistic"--i.e., they better give off the illusion of reality.

I like to make darker, more pulpish, more savage settings, filled with alien life a little more like Barsoom than like California or Corrientes (which, although I've never been there myself, is by all accounts a pleasant enough place---I've had friends who lived there.) So migrating swallows need to be replaced with hairless, carnivorous bat-things, like this bad-boy originally painted for the Peter Jackson remake of King Kong. They could migrate, couldn't they? Sure, why not?