Saying that birds are dinosaurs is not the same as saying that all dinosaur are birds. Current thinking is that 'dinosaur' encompasses two main radiations of animals, and one of those had an off-shoot which led to birds; so the bird clade is nested inside the dinosaur clade. Hence, birds are dinosaurs, in the same way that birds are reptiles, if we're trying to use the terms in a cladistic fashion.
Evolution includes adaption of the type you're talking about, Romy. :] Evolution is more than just speciation; it's the process, driven by selective pressures, by which populations change in gene frequency over time.