Technically it is not underexposed much...you do have some highlights touching the right edge of the histogram so even more would have resulted in blown areas. The problem is the light coming from above has rendered most of the bird shaded. Either some fill-flash at time of capture, or some additional post-processing work is what the image needs. Here I lifted the midtones by quite a bit, and then selectively dodged areas of the breast and undertail. I also added back some darks to the wings afterwards...
P.S. A bit tight on the comp and the perch is not ideral, but I like the the criss-crossing angle of the pose/perch.