Loons

Common loons have four different calls, each used for a different purpose.

1. Wail: Loons use this long, mournful cry (something like a wolf howl) to call to a faraway mate or chick. If you hear a wail, listen carefully: You will almost always hear a response from the other bird.
2. Tremolo: This long trill sounds like a crazy laugh. (This may be why "loony" can mean crazy.) It's an alarm call used when predators or people come too close. It's also the only call loons make while flying. 
3. Yodel: Male loons make this wild, shrieking call to defend their territories against intruders. Each male's yodel is a little different, so scientists can use them to tell one bird from another.
4. Hoot: This is how loons communicate with each other when they're close together. It's a quiet, single note call, not heard very often.  

Want to hear these loon calls for yourself? Visit this Web site, scroll down and click on Common Loon:
http://naturesound.com/stokes/samples.html

Photo by USFWS