JamieCristofaniRedkite catching some sun (feedback appreciated)
ZachariahLearn about levels or curves (slightly different tools) if you want to brighten the underside of the bird without it looking unnatural.
KevinFRKThose are the tools to play with (might also be called Gamma Adjustment), but I think in this particular case, because of those lovely areas where the sun is shining through, having the rest of the body dark (and the sky bright) works really rather well.