The fossil history of birds is less well-studied than that of mammals. This is hardly to say that it has been ignored, and we do know a reasonable amount about it. But a key problem with bird fossils is that, in order to fly, birds have light and fragile bones. These don't tend to survive the fossilisation process intact (bats don't do much better) and birds also lack the great standby of palaeontologists specialising in mammals - teeth.
Of course, a notable exception to this are the flightless birds, which can be enormous, and tend to have reasonably solid bones even when they aren't. It's a lot easier to find a reasonably intact skeleton of a terror bird, or even a penguin or fossil ostrich, than it is of something that flies. There's a reason that the previous posts in this April series have all focussed on the flightless sort.
But most birds do fly and are, let's be honest, well known for it. And, despite the difficulties, we do have a number of fossils of flying birds of various kinds - all the way back to Archaeopteryx - so that their evolutionary history is very far from being a blank book.