Great blue herons fish in both the night and the day, with most of their activity occurring around dawn and dusk. Herons use their long legs to wade in shallow water and their sharp "spearlike" bills to catch their food. Great blue herons' diet consists of mainly fish, but also includes frogs, salamanders, lizards, snakes, birds, shrimps, crabs, crayfish , dragonflies, grasshoppers, and many other aquatic insects. Herons locate their food by sight and usually swallow it whole. Herons have been known to choke on prey that is too large.
The Great Blue Heron feeds mostly in calm freshwater and along seacoasts. Occasionally, it finds its food in surf and in fields. Its main food is small fish less than half the length of its bill, or under 65 mm long. On occasion, it also eats shellfish, insects, rodents, amphibians (mostly frogs), reptiles, and small birds.
The Great Blue Heron has two principal fishing techniques. The first consists of standing motionless, its neck extended at an angle of about 45 degrees to the water’s surface. Only the head and eyes move to locate the prey. If no fish comes within range after a few minutes, the heron gradually moves a short distance away and takes up a similar position. When a potential meal comes close enough, the heron slowly folds its neck back and moves one leg in the direction of the prey. Suddenly, its entire body unbends, its head plunges into the water, it catches the prey in its bill, and it swallows it outside the water, using a deft movement of the head to drop the prey headfirst into its gullet.
Using the second technique, the heron slowly wades around in about 15 to 25 cm of water until it drives a fish out from a hiding place. The heron then stops and slowly stretches its neck. When the prey is within range, the bird uncoils its body and thrusts its head into the water in pursuit. When it has eaten the catch, the heron resumes its walk. Should the bird fail to find sufficient fish in an area, it flies a short distance away and resumes fishing.
When captured prey is too large to be gulped down immediately or has dangerous spines, the heron drops the prey back into the water and grabs hold of it repeatedly and violently with its beak until the catch is dazed or the spines snap. Then it can be swallowed more easily. Sometimes two fish are caught simultaneously.
Other techniques are observed, but more rarely: for example, Great Blue Herons in flight sometimes dive underwater to catch fish; others hover over the water and submerge their heads to catch fish; and some swim in deep water and feed on fish found near the surface.