Great Blue Herons, egrets, and bitterns are medium- to large-winged birds with long necks and spear-like bills. Nearly all species feed primarily on aquatic animal life, e.g., fish, frogs, crayfish, and insects. They are common along the margins of most freshwater and saltwater bodies and wetlands. Their long legs, necks, and bills are adapted for wading in shallow water and stabbing prey.
Most species build their nests in trees near their foraging habitat, and many nest colonially. Members of this group range in size from the least bittern 28–36 cm bill tip to tail tip to the great blue heron 106–132 cm tall. The sexes are similar in size and appearance.
The great blue heron (Ardeaherodias) is the largest member of the group in North America and feeds primarily on aquatic animals. It is widely distributed in both saltwater and freshwater environments. There are the following subspecies in the United States and Canada:
1. h. wardi (Kansas and Oklahoma across the Mississippi River to Florida).
2. h. herodias (remainder of the North and Central American range).
3. h. fannini (Pacific coast of North America from Alaska to Washington).
4. h. occidentalis (extreme south of Florida)
5. h. occidentalis (the great white heron) is an all-white color morph that was formerly considered a separate species.
Males average slightly heavier in weight than females. Northern continental herons are somewhat smaller than those found in the south, which determined a relationship between age and body weight for nestling great blue herons where BW equals body weight in grams and A equals age in days.
Great Blue Heron Habitat
Great blue herons inhabit a variety of freshwater and marine areas, including freshwater lakes and rivers, brackish marshes, lagoons, mangroves, and coastal wetlands, particularly where small fish are plentiful in shallow areas. They are often seen on tidal flats and sandbars and occasionally forage in wet meadows, pastures, and other terrestrial habitats.
Great Blue Heron Nest
Great blue herons tend to nest in dense colonies, or heronries. The location of the heronry is generally close to foraging grounds, and tall trees are preferred over shorter trees or bushes for nest sites. They may also nest on the ground, on rock ledges, or on sea cliffs.
Great Blue Heron Food Habits
Fish are the preferred prey, but great blues also eat amphibians, reptiles, crustaceans, insects, birds, and mammals. When fishing, they mainly use two foraging techniques: standing still and waiting for fish to swim within a striking distance of 2–3 Great Blue Heron slow wading to catch more sedentary prey (such as flounder and sculpin). To fish, they require shallow waters (up to 0.5 m) with a firm substrate.
Fish up to about 20 cm in length were dominant in the diet of herons foraging in southwestern Lake Erie, and 95 percent of fish consumed by great blues in a Wisconsin population were less than 25 cm in length. Great blues sometimes forage in wet meadows and pastures in pursuit of lizards, small mammals, and large insects.
In northern areas, small mammals such as meadow voles may be an important part of the diet early in the breeding season, possibly because some aquatic foraging areas may still be partially frozen when the herons arrive. Consumption of larger prey (fish, frogs, and rodents) is often followed by drinks of water; terrestrial prey such as voles are usually dunked in water before they are swallowed.
Adult herons tend to deliver the same type and size of food to their nestlings that they consume themselves, but they deliver it well digested for young nestlings and less well digested as the nestlings grow. Adults tend to feed alone; although they may feed in single- or mixed-species flocks where there are large concentrations of prey, fledglings are frequently seen foraging together.
Adults undergo a complete molt in the late summer and fall and a partial molt of the contour feathers in the late winter and early spring. Young herons attain full adult plumage in the summer or fall molt at the end of their second year.
Great Blue Heron Migration
In the northern part of its range, most great blues are migratory, some moving to the southern Atlantic and the Gulf States to overwinter with the resident populations of herons, others continuing to Cuba and Central and South America. Most migrating herons leave their breeding grounds by October or November and return between February and April.
Breeding Activities and Social Organization
The male great blue heron selects the site for the breeding territory, and nests generally consist of a stick platform over 1 m in diameter. Great blues often use a nest for more than a year, expanding it with each use. Mean clutch sizes range from three to five in general; clutch size tends to increase with latitude.
Only one brood is raised per year; however, if a clutch is destroyed, great blues may lay a replacement clutch, usually with fewer eggs than the initial clutch. Both parents incubate and feed the young. During the breeding season, great blues are monogamous and colonial, with from a few to hundreds of pairs nesting in the same area or heronry. Colonies may be 2-4 Great Blue Herons and include other species, such as great egrets or double-crested cormorants.
Great Blue Heron Range
Breeding colonies are generally close to foraging grounds the distance between heronries and possible feeding areas in Minnesota lakes ranges from 0 to 4.2 km, averaging 1.8 km. Another study found that most heronries along the North Carolina coast were located near inlets with large concentrations of fish, an average of 7 to 8 km away.
Fifteen to 20 km is the distance that great blue herons regularly travel between foraging areas and colonies. In the northern portion of their range, great blue herons often build nests in tall trees over dry land, whereas in the southern part of their range, they usually nest in swamp trees, including mangroves.
Each breeding pair defends a small territory around the nest, the size of which depends on the local habitat and the birds’ stage of reproduction. Herons in some areas also defend feeding territories. In other areas, great blues appear to be opportunistic foragers, lacking strict fidelity to feeding sites. A study shows that herons often returned to the same general areas, but different individuals often used the same areas at different times.
Population density. Because great blues nest colonially, local population density (i.e., colony density, colony size, and the number of colonies) varies with the availability of suitable nesting habitat as well as foraging habitat. On islands in coastal Maine, Gibbs and others found a significant correlation between colony size and the area of tidal and intertidal wetlands within 20 km of the colonies, which was the longest distance herons in the study colonies traveled on foraging trips.
In western Oregon, the size of heronries ranges from 32 to 161 active nests, and the area enclosed by peripheral nest trees within the colonies ranges from 0.08 to 1.21 ha. Population dynamics. Most nestling loss is a result of starvation, although some losses to predation do occur. A study of 243 nests in a coastal California colony shows that 65 percent of the chicks fledged, 20 percent starved, 7 percent were taken by predators, and 7 percent were lost to other causes. Estimates of the number of young fledged each year by breeding pairs range from 0.85 to 3.1.
Based on banding studies, about two-thirds of the fledglings do not survive more than one year, although they may survive better in protected wildlife refuges. Values for later years indicate that about one-third to one-fifth of the 2-year-old and older birds are lost each year.
The great egret (Casmerodiusalbus) is almost the same size (96 cm in length) as the great blue heron and is found over a limited range in the breeding season, including areas on the central and eastern United States and the east and west coasts. It winters in coastal areas of the United States and in 2–5 Great Blue Heron, Mexico, and farther south. The great egret’s habitat preferences are like those of the great blue heron.
The snowy egret (Egretta thula), one of the medium-sized herons (51 to 69 cm), shuffles its feet to stir up benthic aquatic prey. It is found mostly in freshwater and saltwater marshes but also sometimes follows cattle and other livestock, as does the cattle egret. It breeds in parts of the western, southeastern, and east coasts of the United States and winters along both coasts of the southern United States and farther south.
The cattle egret (Bubulcusibis) is seen in agricultural pastures and fields, where it follows livestock to pick up insects disturbed by grazing. An Old-World species, it was introduced into South America and reached Florida in the 1950s. It reached California by the 1960s and has been continuing to expand its range.
The green-backed heron (Butorides striatus), one of the smaller herons (41 to 56 cm), breeds over most of the United States except for the northwest and southern Midwest. It has a winter range like that of the snowy egret and seems to prefer water bodies with woodland cover.
The tricolored heron (Egretta tricolor) (formerly known as the Louisiana heron) is common in salt marshes and mangrove swamps of the east and gulf coasts, but it is rare inland.
The little blue heron (Egretta caerulea) is common in freshwater ponds, lakes, marshes, and coastal saltwater wetlands of the Gulf Coast States. Juveniles are easily confused with juvenile snowy egrets. This species hunts by walking slowly in shallow waters, and its diet typically includes fish, amphibians, crayfish, and insects.
The black-crowned night heron (Nycticorax nycticorax), characterized by a heavy body, a short, thick neck, and short legs (64 cm), is a common heron of freshwater swamps and tidal marshes, roosting by day in trees. It typically feeds by night, predominantly on aquatic species, fish, amphibians, and insects. This heron is extremely widespread, occurring in North and South America, Eurasia, and Africa. It breeds over much of the United States and parts of central Canada and winters along both coasts of the United States and farther south.
The yellow-crowned night heron (Nyctanassa violacea) (61 cm) is similar to the black-crowned but is more restricted in its range to the southeastern United States. It roosts in trees in wet woods, swamps, and low coastal shrubs.
The American bittern (Botaurus lentiginosus), another of the medium-sized herons (58 to 70 cm), is a relatively common but elusive inhabitant of freshwater and brackish marshes and reedy lakes. It is a solitary feeder of 2–6 Great Blue Herons and consumes fish, crayfish, reptiles, amphibians, insects, and even small mammals. Its breeding range includes most of Canada and the United States, although much of the southern United States is inhabited only during the winter.
The least bittern (Ixobrychusexilis), the smallest of the North American herons (33 cm), is also an elusive inhabitant of reedy areas. Its breeding range is restricted largely to the eastern half of the United States.