I'd say that if you're writing a fic, anything goes. I've created a whole race of whale relatives for one fic and a birdless ecology for another. Insects filled the void. They would do that here on Earth if something took away the birds. I'm thinking back to First Commandment where the writers had the extreme UV kil all the daytime animal life above ground, so no birdies...for me, it just begged the question: just what did the natives hunt for food then? But seriously, for a story you can make a creature up and have some vet examine it. The trick is making the reaction of the vet be believeable... maybe go find one and ask what one would do when presented with an unfamiliar animal?