It is strange when you see a Great Blue Heron panting. This behavior is called Gular Fluttering and is a cooling behavior in which birds rapidly flap membranes in the throat to increase evaporation.
I was at the Sequoyah National Wildlife Refuge in Oklahoma this past Sunday photographing a Great Blue Heron. This Heron has been in the same location for several days catching shad and other types of fish. I usually drive to this area before sunrise and park and watch this bird as the sun rises. This Heron doesn't care much about the vehicles on the road and I usually can get some good photos.
After the Great Blue Heron spends some time feeding it will perch on a log in the water and begin this Gular Fluttering if it is a hot morning. Photos just are not able to show this very well, so I decided to try shooting a video.
I do see other birds doing this especially Owls. The Great Blue Herons seem to demonstrate it better.