Great blue heron

The "great white heron" could be confused with the great egret (Ardea alba), but is larger, with yellow legs as opposed to the great egret's black legs. The reddish egret (Egretta rufescens) and little blue heron (Egretta caerulea) could be mistaken for the great blue heron, but are much smaller, and lack white on the head and yellow in the bill. At the southernmost extent of its range (e.g., Colombia and Panama), the great blue heron sometimes overlaps in range with the closely related and similarly sized cocoi heron (A. cocoi). The cocoi is distinguished by a striking white neck and solid black crown, but the duller juveniles are more easily confused. More superficially similar is the slightly smaller grey heron, which may sometimes appear as vagrants on the northern coasts of North America. The grey heron (which occupies the same ecological niche in Eurasia as the great blue heron) has very similar plumage, but has a solidly soft-gray neck. Erroneously, the great blue heron is sometimes referred to as a "crane". A heron is differentiated from a crane in flight. The crane's neck is straight and the heron's is always curved.

Eating a small fish, the main prey

On a slow-flying glide

The primary food for the great blue heron is fish. While they can prey on various sizes of fish from small fingerlings to large adult fish, measuring 60 cm (24 in) in length and weighing around 900 g (2.0 lb), small to medium-sized fish around 10–20 cm (3.9–7.9 in) are usually preferred.[22][23] Primary prey fish is variable based on availability and abundance. In Nova Scotia, 98% of the diet was flounder.[12] In British Columbia, the primary prey species are sticklebacks, gunnels, sculpins, and perch.[24] California herons were found to live mostly on sculpin, bass, perch, flounder, and top smelt.[25][26][27]

Besides fish, it is also known to feed on a wide range of prey opportunistically. Amphibians such as leopard frogs, American bullfrogs, toads and salamanders are readily taken, as well as reptiles such as small turtles, snakes and lizards.[6][28][29] They can take on sizeable snakes, including water snakes 105 cm (41 in) in length.[22][30][31] Aquatic crustaceans (such as crayfish, shrimp and crabs), grasshoppers, dragonflies and aquatic insects are taken as supplementary prey.[6][28][29] They also prey on small mammals including shrews, rats, ground squirrels, and moles.[6] One study in Idaho showed that from 24 to 40% of the diet was made up of voles. Remains of muskrats (Ondatra zibethicus) and long-tailed weasels (Mustela frenata) was also found in pellets during the study.[32] There are reports that great blue heron prey on both young and adults of eastern cottontails (Sylvilagus floridanus).[33][34] Though not often, birds such as black rails (Laterallus jamaicensis), phalaropes, American dippers (Cinclus mexicanus), pied-billed grebes (Podilymbus podiceps) and chicks of marsh terns (Chlidonias) are also taken.[6][35][36]

Herons locate their food by sight and usually swallow it whole. They have been known to choke on prey that is too large.[37][38] It is generally a solitary feeder. Individuals usually forage while standing in water, but also feed in fields or drop from the air, or perch, into water. Mice are occasionally preyed on in upland areas far from the species' typical aquatic environments.[20] Occasionally, loose feeding flocks form and may be beneficial since they are able to locate schools of fish more easily.[20]

As large wading birds, great blue herons are capable of feeding in deeper waters, thus are able to harvest from niche areas not open to most other heron species. Typically, the great blue heron feeds in shallow waters, usually less than 50 cm (20 in) deep,[20] or at the water's edge during both the night and the day, but especially around dawn and dusk. The most commonly employed hunting technique of the species is wading slowly with its long legs through shallow water and quickly spearing fish or frogs with its long, sharp bill. Although usually ponderous in movements, the great blue heron is adaptable in its fishing methods. Feeding behaviors variably have consisted of standing in one place, probing, pecking, walking at slow speeds, moving quickly, flying short distances and alighting, hovering over the water and picking up prey, diving headfirst into the water, alighting on water feet-first, jumping from perches feet-first, and swimming or floating on the surface of the water.[20]

At the nest

This species usually breeds in colonies, in trees close to lakes or other wetlands. Adults generally return to the colony site after winter from December (in warmer climes such as California and Florida) to March (in cooler areas such as Canada). Usually, colonies include only great blue herons, though sometimes they nest alongside other species of herons. These groups are called a heronry (a more specific term than "rookery"). The size of these colonies may be large, ranging between five and 500 nests per colony, with an average around 160 nests per colony. A heronry is usually relatively close, usually within 4 to 5 km (2.5 to 3.1 mi), to ideal feeding spots.[20] Heronry sites are usually difficult to reach on foot (e.g., islands, trees in swamps, high branches, etc.) to protect from potential mammalian predators. Trees of any type are used when available. When not, herons may nest on the ground, sagebrush, cacti, channel markers, artificial platforms, beaver mounds, and duck blinds. Other waterbirds (especially smaller herons) and, occasionally, even fish and mammal-eating raptors may nest amongst colonies.[39][40]

Although nests are often reused for many years and herons are socially monogamous within a single breeding season, individuals usually choose new mates each year.[41] Males arrive at colonies first and settle on nests, where they court females; most males choose a different nest each year.[41] Great blue herons build a bulky stick nest. Nests are usually around 50 cm (20 in) across when first constructed, but can grow to more than 120 cm (47 in) in width and 90 cm (35 in) deep with repeated use and additional construction.[42] If the nest is abandoned or destroyed, the female may lay a replacement clutch. Reproduction is negatively affected by human disturbance, particularly during the beginning of nesting. Repeated human intrusion into nesting areas often results in nest failure, with abandonment of eggs or chicks. However, Vancouver B.C. Canada's Stanley Park has had a healthy colony for some years right near its main entrance and tennis courts adjacent to English Bay and not far from Lost Lagoon.[43] The park's colony has had as many as 183 nests.[44]

The female lays three to six pale blue eggs, which can measure from 50.7 to 76.5 mm (2.00 to 3.01 in) in length and 29 to 50.5 mm (1.14 to 1.99 in) in width, though the smallest eggs in the above sample may have been considered "runt eggs" too small to produce viable young. Egg weights range from 61 to 80 g (2.2 to 2.8 oz).[45] One brood is raised each year. First broods are laid generally from March to April.[46][47] Eggs are usually laid at two-day intervals, incubated around 27 days, and hatch asynchronously over a period of several days.[41] Males incubate for about 10.5 hours of each day, while females usually incubate for the remainder of each day and the night, with eggs left without incubation for about 6 minutes of each hour.[41]

Juvenile

The first chick to hatch usually becomes more experienced in food handling and aggressive interactions with siblings, so often grows more quickly than the other chicks.[28] Both parents feed the young at the nest by regurgitating food. Parent birds have been shown to consume up to four times as much food when they are feeding young chicks (about 4300 kJ/day) than when laying or incubating eggs (about 1200 kJ/day).[41] By the time they are 45 days old, the young weigh 86% of the adult's mass.[48] After about 55 days at the northern edge of the range (Alberta) and 80 days at the southern edge of the range (California), young herons take their first flight.[41] They return to the nest to be fed for about another 3 weeks, following adults back from foraging grounds, and are likely to gradually disperse away from their original nest over the course of the ensuing winter.[41] Young herons are not as successful at fish capture as adults, as strike rates are similar, but capture rates are about half that of adults during the first 2 months after fledging.[41]

Predators of eggs and nestlings include turkey vultures (Cathartes aura), common ravens (Corvus corax), and American crows (Corvus brachyrhynchos). Red-tailed hawks (Buteo jamaicensis), American black bears (Ursus americanus), and raccoons (Procyon lotor) are known to take larger nestlings or fledglings, and in the latter predator, many eggs.[11][49][50][51] In exceptional case, a young Harris's hawk (Parabuteo unicinctus) killed a subadult great-blue heron.[52] Adult herons have few natural predators and are rarely preyed upon due to their large size and sharp beak, but bald eagles (Haliaeetus leucocephalus) are known to attack great blue herons at every stage of their lifecycle from in the egg to adulthood.[22][53][54] And less frequently, golden eagles (Aquila chrysaetos) and great horned owls (Bubo virginianus) are known to take adults.[55][56] There is a single report that a large bobcat (Lynx rufus) managed to subdue and kill an adult great blue heron.[57] Using its considerable size and dagger-like bill, a full-grown heron can be a formidable foe to a predator. In one instance, during an act of attempted predation by a golden eagle, a heron was able to mortally wound the eagle, although it succumbed to injuries sustained in the fight.[58] When predation on an adult or chick occurs at a breeding colony, the colony can sometimes be abandoned by the other birds. The primary source of disturbance and breeding failures at heronries is human activities, mostly through human recreation or habitat destruction, as well as by egg-collectors and hunters.[24][59]