In the spring, many birds are the most vocal. Although many birds make noises, it is the Passerines that are the most vocally advanced. They typically can learn many songs and some continue to learn throughout their life.
The male Northern Cardinal is a Passerine bird of North American that produces a beautiful song. Sometimes it will also just be chirping as well. I one pictured above was singing and I managed to record its song. Note in the middle you can hear a whistle or two of a Red-winged Blackbird as well, but it is the repeating sequence of notes that is the cardinal:
Try and listen for it next time you are out in the forest.