The phylogenetic tree, or tree of life, for class aves is a work in progress. Scientists are still making discoveries that affect the relationships between animals and the ideas on how they evolved. Here are a few phylogenetic trees that show the generally accepted view on birds and their evolution, as well as a geological timescale. Since these trees can be a bit overwhelming, here is a verbal description of what's going on as well. It is believed that crocodiles and birds diverged from each other in the early Triassic Period, with fossils of Xilousuchus sapingensis from close to 25o million years ago supporting this hypothesis. The next major evolutionary event was the finding of Archaeopteryx, dating back 150 million years to the late Jurassic Period. The evolution of birds is then unremarkable until the years surrounding the Cretaceous Extinction, dating back 65-70 million years. During this time, birds rapidly diverged into the subclasses and orders that we are familiar with today. The first divergence comes from the Palaeognathae clade, a group of flightless birds that includes kiwis, emus, ostriches, and moas. Next, Galliforme clade split off. These birds are the fowl, composed of organisms like the chicken, duck or goose. Next, the Strisores diverged; this group of birds includes hummingbirds and nighthawks. Then, the Columbaves clade branched off, forming a group that consists of species like pigeons, doves, and cuckoos. After that, the Gruiformes diverged, including birds such as cranes and rails. Next was the Aequorlitornithes, a clade of birds that includes puffins, flamingos, and other shorebirds. Finally, there are the Inopionaves, a clade that includes parrots, falcons, hoatzins, and woodpeckers. From these very general categorizations, birds have evolved and changed into many more distinct clades and species, but further detailing of this evolution is not necessary here.
Further Reading: The Most Comprehensive Avian Tree of Life
Further Reading: The Most Comprehensive Avian Tree of Life