A counter used to count birds, rabbits, and similar animals (typically read as "-wa" after a number; reading may change to "-ba" or "-pa").
庭にスズメが三羽とまっている。
Three sparrows are perched in the garden.
うさぎを二羽飼っている。
I keep two rabbits.
湖に白鳥が一羽泳いでいた。
A single swan was swimming on the lake.