A tiny heron, furtive and surpassingly well camouflaged, the Least Bittern is one of the most difficult North American marsh birds to spot. Despite its inconspicuousness, however, the species can be rather common within appropriate habitat in its breeding range.
This is one of the smallest herons in the world, with perhaps only the Dwarf Bittern and the Black-backed Bittern averaging smaller in length. This tiny bittern can measure from 28 to 36 cm (11 to 14 in) in length, and the wingspan ranges from 41 to 46 cm (16 to 18 in). Body mass is from 51 to 102 g (1.8 to 3.6 oz), with most birds between 73 and 95 g (2.6 and 3.4 oz), making this perhaps the lightest of all herons. This bird’s underparts and throat are white with light brown streaks. Its face and the sides of the neck are light brown; it has yellow eyes and a yellow bill. The adult male is glossy greenish black on the back and crown; the adult female is glossy brown on these parts. They show light brown parts on the wings in flight.
This Least Bittern is often an elusive bird. They spend much time straddling reeds. When alarmed, the Least Bittern freezes in place with its bill pointing up, turns its front and both eyes toward the source of alarm, and sometimes sways to resemble wind-blown marsh vegetation. This is perhaps a predator-avoidance behavior, since its small size makes the bittern vulnerable to many potential predators. Thanks to its habit of perching among the reeds, the Least Bittern can feed from the surface of water that would be too deep for the wading strategy of other herons.
The nest is a well-concealed platform built from cattails and other marsh vegetation. The female lays 4 or 5 eggs in extreme cases from 2 to 7. The eggs are pale blue or green. Both parents feed the young by regurgitating food. A second brood is often produced in a season.
References: Wikipedia, Cornell Lab of Ornithology