Feast your eyes on the first complete evolutionary tree for all known modern bird species. It's exhaustive, colorful, and beautiful to behold - a little like an avicular Hillis Plot. But for all that this diagram tells us about birds' evolutionary histories, what's really interesting is what it says about how birds continue to evolve today.
It's Okay to be Smart's Joe Hanson explains:
It was thought that any given species would expand and diversify quickly into subspecies (like the many different kinds of honeybees), soon maxing out its environment and filling all the ecological "niches". Then, competition over limited resources would thin that down to the few most adaptable species. This tree says the opposite, that birds are continuing to diversify even today, and fast.
The center of this tree, anchoring branches built using fossil and DNA sequence data, reaches back nearly 50 million years, to the earliest days of birds branching off of dinosaurs.