I was taking some pictures over at the Baskett Slough Wildlife Refuge near where I live. I noticed a little hawk sitting on the wire above. When I stopped my car and hopped out for a better view, he looked suspicious and chirruped down at me. I'm pretty good at imitating bird calls, so I whistled back.
At first he seemed terribly confused--I think it might have been because, though I sounded like a bird, I sported no discernable feathers.