Great blue heron

Name and taxonomy

Carl Linnaeus described the great blue heron in the 18th century. The scientific name uses two old words for a heron. Ardea is Latin and herodias comes from Ancient Greek. Five subspecies are recognised. They differ only slightly in size and in the tone of the plumage.

The Florida subspecies occidentalis also has an all white form. That form is the great white heron, and it lives mainly near salt water. It differs in bill shape, in plume length and in the total lack of pigment. People long treated it as a separate species, and the argument is not settled. Birds between the two forms are called Würdemann's heron. A Würdemann's bird has a blue grey body and a white head.

Evolution and relatives

The great blue heron belongs to the genus Ardea, the typical large herons. It forms a superspecies with the grey heron of Eurasia and the cocoi heron of South America. The grey heron fills the same role in the Old World. That bird is smaller, at 90 to 98 centimetres, and lacks the brown tones. The cocoi heron has a white neck and a solid black crown.

Only the goliath heron and the white-bellied heron are larger. A great blue heron weighs about twice as much as a great egret. A large goliath heron weighs roughly twice as much again. Like all herons, it carries 20 or 21 neck bones of a modified shape. That neck folds into a tight S, which cranes cannot do.

Appearance

The bird measures 91 to 137 centimetres from head to tail. It weighs 1.82 to 3.6 kilograms. Males in British Columbia averaged 2.48 kilograms and females 2.11 kilograms. The tail is short for the body, at 15.2 to 19.5 centimetres. A walking heron takes a stride of about 22 centimetres, almost in a straight line.

The flight feathers are slaty grey with a faint blue cast. The thighs are red brown, and a red brown and black stripe runs up each flank. The neck is rusty grey, streaked black and white down the front. The face is nearly white, and a pair of dark plumes runs back from above the eye. Long plume feathers hang from the lower neck.

The bill is a dull yellow for most of the year. It turns orange for a short time as the breeding season opens. The lower legs are grey and also flush orange at that time. Males are slightly larger than females, and nothing else separates the sexes. Young birds are duller, with a dark crown, no plumes and a weak flank pattern.

Habitat and range

The heron breeds north to Alaska and the southern provinces of Canada. In winter the range runs south through Florida, Mexico and the Caribbean. Birds reach Colombia and Venezuela regularly. GBIF records the species in 63 countries. Vagrant birds have reached England, Greenland, Hawaii and also the Azores.

Almost any wetland in the range will do. Herons use fresh and salt marshes, mangrove swamps, flooded meadows and lake edges. Some birds nest at elevations up to 1,500 metres. The species does well in built up places, so long as the water holds fish. Herons rarely move far from water, though they sometimes fly over dry land.

Birds east of the Rocky Mountains and in the north are migratory. They winter on the coasts of the southern United States and further south. From the southern states down, and along the lower Pacific coast, birds stay all year. A heron can sit out a cold northern winter where flowing water stays open. A frozen lake would starve a bird that lives on fish.

Diet and feeding

Fish are the main food. The heron prefers fish of about 10 to 20 centimetres, but it can take much larger ones. Recorded prey includes fish 60 centimetres long and weighing around 900 grams. In Nova Scotia flounder made up 98 per cent of the diet. Sticklebacks, sculpins, gunnels and perch dominate in British Columbia.

The bird also eats frogs, toads, salamanders, small turtles, snakes and lizards. It has swallowed water snakes 105 centimetres long. Crayfish, shrimp, crabs, dragonflies and grasshoppers add to the diet. Shrews, rats, ground squirrels and moles are taken on land. One study in Idaho found voles made up 24 to 40 per cent of the food. Herons occasionally take small birds, including grebes and rails.

The usual method is to wade slowly and then stab with the bill. Most feeding happens in water less than 50 centimetres deep, or right at the edge. Dawn and dusk are the busiest times, though the bird also feeds at night. Long legs let it work deeper water than smaller herons can reach. Prey is located by sight and swallowed whole. A heron can choke on prey that is too large.

Behaviour and social life

The great blue heron hunts alone and defends its fishing spot. Loose flocks sometimes gather where fish are shoaling. A group finds a shoal more easily than a single bird. At night birds may roost together in flocks of more than a hundred. The species is territorial and defends the nest hard.

Hunting styles vary more than the slow walk suggests. A heron will stand still, probe, peck, run, hover or dive head first. It also drops onto the water feet first and can swim on the surface. Some herons hunt mice in dry ground far from water. The neck stays folded in flight, which tells a heron from a crane at a distance.

Communication and senses

The great blue heron is quiet compared with other herons. A disturbed bird in flight gives a soft kraak. A bird disturbed near the nest gives a harsh fraunk that can last about 20 seconds. A short ar greets another heron. Researchers count up to seven different calls.

Birds also clap the two halves of the bill together. Courtship uses stretched necks, raised crests and fluffed neck plumes. A male may circle the nest in flight or shake a twig at a female. The heron finds its food by sight, so it hunts best in daylight. Like other herons, it carries powder down, which helps clean and waterproof the plumage.

Reproduction and life cycle

Great blue herons nest in colonies, called heronries. A colony may hold from five to 500 nests, and about 160 is average. Heronries sit within about 4 to 5 kilometres of good feeding water. Sites are hard to reach on foot, such as islands or high branches. Where trees are absent, birds nest on the ground, on cacti or on artificial platforms.

Northern birds breed from March to May, and southern birds from November to April. Males arrive first and take a nest, then court from it. Pairs are monogamous within one season, and most birds take a new mate the next year. The nest is a bulky platform of sticks, about 50 centimetres across when new. Years of reuse can widen it beyond 120 centimetres. Nests in trees usually sit 9 to 21 metres above the ground.

The female lays three to six pale blue eggs, one every two days. Each egg weighs 61 to 80 grams. Both parents incubate for about 27 days, and the male sits for about half the day. Chicks hatch over several days rather than together. The first chick handles food better and often grows faster than the rest. One brood is raised each year.

Both parents feed the chicks by bringing food back up from the crop. A parent feeding chicks eats up to four times as much as one that is only incubating. At 45 days old a chick weighs 86 per cent of an adult. First flight comes after about 55 days in Alberta and about 80 days in California. Young birds return to be fed for about three more weeks. Herons can breed at about 22 months old.

Predators and threats

A full grown heron is a hard target, because of its size and its dagger of a bill. Bald eagles attack the species at every stage, from egg to adult. Golden eagles and great horned owls take adults less often. In one recorded fight a heron fatally wounded a golden eagle and then died of its own injuries. A bobcat once killed an adult heron, and a young Harris's hawk killed a subadult.

Turkey vultures, ravens and crows take eggs and small chicks. Red-tailed hawks, black bears and raccoons take larger nestlings. More than two thirds of young herons die before their first birthday. A colony sometimes empties after a predator kills a bird inside it. Collisions with power lines kill many birds each year.

Population and conservation

The IUCN lists the great blue heron as Least Concern. The population is widespread and increasing. The species needs no special protection at present. In the United States the Migratory Bird Treaty Act covers it. Habitat destruction is the largest threat to the species.

Human disturbance at a heronry causes most breeding failures. Repeated visits early in the season make birds abandon eggs or chicks. Some colonies still thrive beside busy places. Stanley Park in Vancouver has held as many as 183 nests near its main entrance. The heron sits at the top of its food web, so contaminants build up in its body. That makes it a useful measure of the health of a wetland.

Relationship with humans

Great blue herons live comfortably alongside people where water holds fish. They visit garden ponds and take expensive ornamental fish. Netting over a pond keeps them out. Some pond owners set out a decoy heron, because the birds avoid an occupied spot. The bird has no other notable economic effect on people.

Egg collectors and hunters once disturbed colonies. Recreation near a heronry is now the main source of trouble. A heron is probably the easiest large bird to watch, because it stands still for long periods. Photographers and birdwatchers may cause a colony to fail, so they should keep well back. A cornered bird may defend itself with the bill.

In culture and myth

In ancient Egyptian religion the Bennu was a god of the sun, creation and rebirth. New Kingdom artists drew it as a big grey heron carrying a crest of two feathers. Its title says that it came into being by itself. Texts say it flew over the water that existed before creation and called out from a rock. The Bennu appears on funerary amulets to mean rebirth, and Greek writers tied the story to the phoenix.

In English tradition a group of herons is called a siege. Heronshaw is an old English word for the same bird. A corrupted form of that word appears in Hamlet. In Russian civic heraldry the arms of Priozersk carry a grey heron. People in North America sometimes call this bird a crane, which is wrong.

John James Audubon painted the great blue heron for Birds of America. The plate was engraved and coloured in London. The original watercolour is held in New York. An orange version of the bird has been the badge of a minor league baseball team since 1996. The great white heron appears in the badge of a football club in Miami.