The Green River Formation of Wyoming recently disclosed a tiny new bird fossil that's really inspiring. The early Eocene is a great age for mammals and birds. In the Caenozoic, the earliest birds diverged from their ancestors with wing shapes adapting to the animals' diverse niches. Their bony remains are not enough for such shape determination, so finding feathers was essential.
The tree-swifts, swifts and hummingbirds are the only living groups of a large assembly of Apodidae with the well-known flight habits of these birds. The chimney swift, Chaetum pelagicus, looks similar to the fossil but this ancestor had none of the modern swift's well known abilities in flight.
Eocypselus rowei was a complete fossil with a poorly preserved skull. The short round beak is its most obvious feature. What separates the species from modern relatives is its long digits and legs, which are adapted for something rarely achieved by swifts- perching on a branch! The Apodids, especially the swifts, are by definition, almost legless! They even sleep on the wing.
By comparison, the humerus of this bird resembles swifts, although the less elongated wing shape is very different from the swift. Early hummingbirds had short stout wings, obviously a little later than this early specimen of the group.
Daniel T. Ksepka of North Carolina State University in Raleigh and several colleagues in US universities and Lance Grande of the Field Museum of Natural History in Chicago published their paper, "Fossil evidence of wing shape in a stem relative of swifts and hummingbirds," in the Proceedings of the Royal Society B: Biological Sciences.