Complex sentence: They are superficially similar to swallows, but are actually not closely related to passerine species at all; swifts are in the separate order Apodiformes, which they share with hummingbirds.
Split into simpler sentences: They are superficially similar to swallows, but are not closely related to passerine species. Swifts are in the separate order Apodiformes, which they share with hummingbirds.