It is an aggressive call generally given in territorial confrontations by male loons. Each male has an individual yodel. Only the male yodels, so that is one way to tell males from females. The hoot is a soft,one-note call loons close quarters. The hoot is hardly heard by humans.
Chicks also have a call.
It is usually used when they' re begging for food. They have been around about 25 million years. Loons are 36 inches long. Their backs and wings are black with white spots. The head and neck are shinging black and green, and the neck has white lines and the eyes are red.