As discussed indirectly, more and more evidence is leaning toward the bipedal, carnivorous dinosaurs as the origin of the aven group. In fact, birds may have been a close relative to the Velociraptor. A few skeptics still regard crocodilomorphs, basal archosaurs, or even a variety of non-archosaurian diapsids, as the ancestors. These theories are not as well supported with facts, as they are with the need to hold on to old ideas.

It has been shown in the recent fossils found in the Liaoning Province of northeastern China that the Late Jurassic birds are larger than any of the known Early Cretaceous flying birds. This indicates that those birds may have undergone a size reduction at about the same time that they developed improved powers of flight. This may also be related to the development of a keeled sternum and the ability to land on and take off from level surfaces. Much of the characteristics of birds were not truly developed and effective in their function of flying until mid to late Cretaceous. This is seen in the newly-found group Iberomesornis In fact this is the same time when we get the explosion/diversification of the aven group.