Every summer my husband has this cardinal that he whistles back and forth too. They are so sweet to listen to - it is like they are communicating. Our cardinal and his mate have a nest in our japanese maple and they have 3 babies. We had to save the babies last week because the nest had a hole in the bottom. But we saved the babies and Craig was able to get them back in the nest. Here is a picture of our cardinal and my babe when they were singing to each other. Isn't he beautiful!!!