|Great blue heron|
Temporal range: Pleistocene-present
|Dark form, near Tarpon Springs, Florida|
|White form, possibly A. h. occidentalis, in Cayo Guillermo National Park, Cuba|
Present during migration
The great blue heron (Ardea herodias) is a large wading bird in the heron family Ardeidae, common near the shores of open water and in wetlands over most of North America and Central America, as well as the Caribbean and the Galápagos Islands. It is a rare vagrant to coastal Spain, the Azores, and areas of far southern Europe. An all-white population found in south Florida and the Florida Keys is known as the great white heron. Debate exists about whether this represents a white color morph of the great blue heron, a subspecies of it, or an entirely separate species. The status of white individuals known to occur elsewhere in the Caribbean, and very rarely elsewhere in eastern North America, is unclear.
The great blue heron was one of the many species originally described by Carl Linnaeus in his 18th-century work, Systema Naturae. The scientific name comes from Latin ardea, and Ancient Greek ? (er?dios), both meaning "heron".
The great blue heron is replaced in the Old World by the very similar grey heron (Ardea cinerea), which differs in being somewhat smaller (90-98 cm (35-39 in)), with a pale gray neck and legs, lacking the browner colors that the great blue heron has there. It forms a superspecies with this and also with the cocoi heron from South America, which differs in having more extensive black on the head and a white breast and neck.
The great blue heron is the largest heron native to North America. Among all extant herons, it is surpassed in size only by the goliath heron (Ardea goliath) and the white-bellied heron (Ardea insignis). It exhibits a minor degree of sexual dimorphism; with males slightly larger than females, but otherwise the sexes are not easily outwardly distinguishable. It has head-to-tail length of 91-137 cm (36-54 in), a wingspan of 167-201 cm (66-79 in), a height of 115-138 cm (45-54 in), and a weight of 1.82-3.6 kg (4.0-7.9 lb). In British Columbia, adult males averaged 2.48 kg (5.5 lb) and adult females 2.11 kg (4.7 lb). In Nova Scotia and New England, adult herons of both sexes averaged 2.23 kg (4.9 lb), while in Oregon, both sexes averaged 2.09 kg (4.6 lb) Thus, great blue herons are roughly twice as heavy as great egrets (Ardea alba), although only slightly taller than them, but they can weigh about half as much as a large goliath heron.
Notable features of great blue herons include slaty (gray with a slight azure blue) flight feathers, red-brown thighs, and a paired red-brown and black stripe up the flanks; the neck is rusty-gray, with black and white streaking down the front; the head is paler, with a nearly white face, and a pair of black or slate plumes runs from just above the eye to the back of the head. The feathers on the lower neck are long and plume-like; it also has plumes on the lower back at the start of the breeding season. The bill is dull yellowish, becoming orange briefly at the start of the breeding season, and the lower legs are gray, also becoming orangey at the start of the breeding season. Immature birds are duller in color, with a dull blackish-gray crown, and the flank pattern is only weakly defined; they have no plumes, and the bill is dull gray-yellow. Among standard measurements, the wing chord is 43-49.2 cm (16.9-19.4 in), the tail is 15.2-19.5 cm (6.0-7.7 in), the culmen is 12.3-15.2 cm (4.8-6.0 in), and the tarsus is 15.7-21 cm (6.2-8.3 in). The heron's stride is around 22 cm (8.7 in), almost in a straight line. Two of the three front toes are generally closer together. In a track, the front toes, as well as the back, often show the small talons.
The subspecies differ only slightly in size and plumage tone, with the exception of A. h. occidentalis, native to South Florida, which also has a distinct white morph, known as the great white heron (not to be confused with the great egret, for which "great white heron" was once a common name). The great white heron differs from other great blues in bill morphology, head plume length, and in having a total lack of pigment in its plumage. It averages somewhat larger than the sympatric race A. h. wardi and may be the largest race in the species. In a survey of A. h. occidentalis in Florida, males were found to average 3.02 kg (6.7 lb) and females average 2.57 kg (5.7 lb), with a range for both sexes of 2 to 3.39 kg (4.4 to 7.5 lb). This is mainly found near salt water, and was long thought to be a separate species. Birds intermediate between the normal morph and the white morph are known as Würdemann's heron; these birds resemble a "normal" great blue with a white head.
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. In the southern reaches of its range, the great blue 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 vagrate to 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.
The great blue heron is found throughout most of North America, as far north as Alaska and the southern Canadian provinces in the summer. In winter, the range extends south through Florida, Mexico, and the Caribbean to South America. Birds east of the Rocky Mountains in the northern part of their range are migratory and winter in the coastal areas of the Southern United States, Central America, or northern South America. From the Southern United States southwards, and on the lower Pacific coast, they are year-round residents. However, their hardiness is such that individuals often remain through cold northern winters, as well, so long as fish-bearing waters remain unfrozen (which may be the case only in flowing water such as streams, creeks, and rivers).
The great blue heron can adapt to almost any wetland habitat in its range. It may be found in numbers in fresh and saltwater marshes, mangrove swamps, flooded meadows, lake edges, or shorelines. It is quite adaptable and may be seen in heavily developed areas as long as they hold bodies of fish-bearing water.
Great blue herons rarely venture far from bodies of water, but are occasionally seen flying over upland areas. They usually nest in trees or bushes near water's edge, often on islands (which minimizes the potential for predation) or partially isolated spots.
The primary food for great blue heron is small fish. It is also known to opportunistically feed on a wide range of shrimp, crabs, aquatic insects, rodents, and other small mammals, amphibians, reptiles, and birds, especially ducklings. Primary prey is variable based on availability and abundance. In Nova Scotia, 98% of the diet was flounders. In British Columbia, the primary prey species are sticklebacks, gunnels, sculpins, and perch. California herons were found to live mostly on sculpin, bass, perch, flounder, and top smelt. Non-piscine prey is rarely quantitatively important, though one study in Idaho showed that from 24 to 40% of the diet was made up of voles.
Herons locate their food by sight and usually swallow it whole. They have been known to choke on prey that is too large. It is generally a solitary feeder. Individuals usually forage while standing in water, but also feed in fields or drop from the air, or a perch, into water. Mice are occasionally preyed on in upland areas far from the species' typical aquatic environments. Occasionally, loose feeding flocks form and may be beneficial since they are able to locate schools of fish more easily.
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, 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 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.
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. 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.
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. Males arrive at colonies first and settle on nests, where they court females; most males choose a different nest each year. 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. 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. The park's colony has had as many as 183 nests.
The female lays 3 to 6 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). One brood is raised each year. First broods are laid generally from March to April. Eggs are usually laid at two-day intervals, incubated around 27 days, and hatch asynchronously over a period of several days. 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.
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. 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). By the time they are 45 days old, the young weigh 86% of the adult's mass. 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. 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. 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.
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. Adult herons, due to their size, have few natural predators, but a few of the larger avian predators have been known to kill both young and adults, including bald eagles (Haliaeetus leucocephalus) (the only predator known to attack great blue herons at every stage of their lifecycle from in the egg to adulthood), golden eagles (Aquila chrysaetos), and less frequently, great horned owls (Bubo virginianus) and Harris's hawks (Parabuteo unicinctus).
An occasional adult heron, or more likely, an unsteady fledgling, may be snatched by an American alligator (Alligator mississippiensis) or an American crocodile (Crocodylus acutus). 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. 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.
John James Audubon illustrates the great blue heron in Birds of America, Second Edition (published, London 1827-38) as Plate 161. The image was engraved and colored by Robert Havell's London workshops. The original watercolor by Audubon was purchased by the New-York Historical Society.
Wading at Grande Lakes Audubon Cooperative Wildlife Sanctuary, Orlando, FL
Eating a common snapping turtle (Chelydra serpentina) hatchling