Walter Jetz of Yale University and his colleagues have put together the definitive molecular map of birds. They collected fossil and DNA data to determine how each of the 9,993 extant (living) species of birds is related to every other type of bird. They were able to see how both time and geographical location affected bird evolution.
Interestingly, there has been more rapid speciation in birds in the western hemisphere than in the eastern hemisphere. In contrast, speciation did not vary much between northern and southern hemispheres. Australia, Southeast Asia, Africa and Madagascar experienced counterintuitively low rates of diversification. Overall, the diversification rate has increased over the past 50 million years, contrary to what researchers expected to find.
The best part of this study is this lovely cladogram, courtesy of the University of Sheffield.
W. Jetz, G.H. Thomas, J.B. Joy, K. Hartmann, & A. O. Mooers (2012). The global diversity of birds in space and time Nature : doi:10.1038/nature11631