Common loons defend breeding territories on fresh water lakes in the northern US and Canada. While a great deal is known in general about their breeding biology it was the advent of banding that enabled identification of individual loons.

Cornell professor of neurobiology and behavior Charles Walcott and colleagues Walter Piper and Jay Mager have been studying a banded population of loons near Rhinelander, WI for the past 18 years. They have found that loons are quite faithful to the lakes on which they breed returning for an average of 5 years. Loons looking for a breeding site will either pick a vacant lake, replace a missing breeder or actively displace a pair member.

Female fights are relatively benign with the winner taking over the territory and the resident male, the loser moving to another lake in the vicinity. For males, fights are more serious; in 30% of such fights a male is killed. And if a male is killed it is always the resident, never the intruder.