The call of the gray treefrog (Hyla versicolor) is a short trill. In the audio clip below, you can hear the short trills of several gray treefrogs. Also in the background of the clip is the longer call of the American Toad (Bufo americanus)
The two photos below show the same male Gray Treefrog, and were taken moments apart. The top photo shows him in the middle of his advertisement call; his vocal sac is inflated and you can see ripples in the water caused by his trill vibrating through his body. The bottom photo shows him between calls. There is no disturbance in the water. His vocal sac is still partially inflated, but it is not nearly as full as during his call.
I liked this photo so much that I turned it into coloring page that you can print out and color in with your kids. You can find a printable copy on my
coloring pages site.