For example, can you spot the chorus frog in this picture. You can click on the image to enlarge it.
Here's a closer look.
I spent about 45 min to an hour just waiting, listening and watching for these little fellas. They would chirp when I was a fair distance from them. As soon as I began moving in their general direction, they stopped. Finally I got close enough to where I thought if one started chirping, I could spot it. I really wanted to see it puffing it's throat out to call. I knew I was close, but for the life of me could not see it. Then finally it began to chirp.
Patience does have its rewards. Look at that air sack!
Here's a little sampling of how loud these little chirpers are.