Great Blue Herons forage in marine coastal environments and in freshwater habitats, but nest on islands or in wooded swamps, where few mammals or snakes can prey on them. The birds sometimes nest alone, but often do so in colonies consisting of a dozen to several hundred pairs. Scientists do not know precisely how herons choose whether to be sociable. It seems that advantages to colonial behaviour include better defence of nests and greater chance of discovering mobile schools of fish: once one heron finds a good foraging spot, others may follow it to the same location.

The Great Blue Heron is generally silent, but it does have a repertoire of noises. It gives a frawnk sound at breeding colonies when alarmed, a gooo call at the end of one of its courtship displays, an occasional ee call when flying, and sometimes a series of clucks when foraging. The heron also utters a roh-roh-roh sound when it approaches the nest, perhaps to alert its mate to its arrival.

Part of the males’ courtship displays are loud bill snaps. Females snap bills when they are approaching unmated males and after they have formed a breeding pair. It is also common for paired birds to engage in a rapid side-to-side tapping of each other’s bill tips.

Adult Great Blue Herons have few natural enemies. Eagles occasionally attack them, and crows, ravens, gulls, birds of prey and raccoons prey upon the eggs and young; mortality of the young is high, but often for reasons other than predation.

Heavy rains and cold weather at the time of hatching take a substantial toll. Also, when food is scarce, the weakest young birds often do without and waste away. Pesticides are suspected of causing reproductive failures and deaths, although data obtained up to this time suggest that toxic chemicals have not caused any decline in overall population levels.