Great Blue Heron
General: The Great Blue Heron (Ardea herodias) is the largest and most common heron in North America. These wading birds are skilled fishers, thriving in a variety of geographic locations and climates. This species is protected by the United States Migratory Bird Treaty Act despite healthy numbers and stable populations, listed as Least Concern on the IUCN Red List.
Description: As the largest of the North American herons, the Great Blue Heron may reach an adult size of 4.5’ tall but only 4.5 to 7.5 lbs, due in part to their hollow bones. Their rounded wings span 5.5’ to 6.5’ in width. Males reach an adult size that is approximately 10% greater than females1. In addition to their overall size, Great Blue Herons may be identified by their long necks, long, tapered yellow bills and long, thin legs that are dull green in color. Their legs terminate in narrow, wide-set toes that allow these birds to walk on soft ground. Great Blue Herons have an overall dull blue-gray coloration, with white, black and brown streaking along the neck. Their white faces and white head-caps are accented with black eye-stripes that merge with black plumes on the back of the heads. Additional features include a shaggy grey ruff on the back of the neck, short tails, and tan feathers on the thighs. Juveniles are similar in color, but lack the plumes and shaggy feathers of adults. A juvenile may also be identified by its dark crown and mottled neck2. There may be as many as seven subspecies of Great Blue Herons, distinguished by size, color, and geographic location.
During flight, a Great Blue Heron will fold its neck and slowly beat its wings, reaching speeds of 20 to 30 mph3.
On average, a Great Blue Heron will live to 15 years of age in the wild. The oldest known Great Blue Heron lived to be 23 years old. Like many species, Great Blue Herons experience a high mortality rate in the first year of life, losing over half of juveniles to predation and starvation. Great Blue Herons are generally a solitary species and typically forage along. However, this species does nest in single-species colonies that may contain up to several hundred nests. Great Blue Herons are most active in the morning and at dusk to maximize fishing success. During the day they are inactive, sleeping with single-species flocks of up to 100 individuals. Great Blue Herons are a territorial species and have been known to be aggressively defensive.
Great Blue Herons are capable of producing seven distinct noises but relative to other species, they are fairly quiet. Sounds are made in response to disturbances or threats and to greet other herons. They also use physical gestures to communicate during courtship4.
Habitat: Great Blue Herons live in a variety of temperate and tropical habitats located in close proximity to water, often seen wading in marshes, sheltered bays and inlets, streams, ponds, swamps, wet meadows, along saltwater coastlines and at the edges of rivers and lakes. They may be found in fresh, salt or brackish water. East coast populations typically avoid shores, preferring to live inland5. Great Blue Herons tend to locate their nesting colonies away from human disturbances, in quiet areas including mature forests and islands.
Within their habitats Great Blue Herons are efficient at controlling insect and fish populations. These habitats place eggs and chicks at risk of predation by crows, ravens, eagles, bears, cultures, hawks, and raccoons. Adults may fall prey to larger predators. If a juvenile or adult is killed in close proximity to a colony, the colony will be abandoned. Great Blue Herons face other threats within their habitats, include collisions with wires, and loss of habitat due to land development and forestry.
Location: Great Blue Herons inhabit nearctic and neotropical regions. During the spring and summer, breeding colonies may be found across North and Central America, the Caribbean and the Galapagos Islands. Populations living in extreme northern climates may migrate south in the winter to Central and South America in search of food supplies. These migratory populations do not breed in their winter habitats. However, this species is highly adaptable (more so than other species of herons) and populations have been known to winter in environments as far north as British Columbia, the Alaskan coast, and New England.
Diet: Great Blue Herons are carnivorous. They usually hunt alone, seeking a variable diet of fish (making up the majority of their diet,) small mammals, insects, crustaceans, and reptiles, such as frogs and salamanders. They wade slowly or stand still, waiting for their prey to come within reach of their long necks and bills. They attack fast, grab their prey with their bills, and swallow their meals whole, causing some herons to choke to death if the prey is too large for their slender throats. In deep water environments, Great Blue Herons exhibit a variety of methods to locate and catch food. They may hover above the water, swim, or dive below the surface in pursuit of food.
Reproduction: Great Blue Herons form mating pairs that last for the duration of one breeding season. Northern populations breed between March and May and southern populations breed between November and April. Each season new pairs will form. Great Blue Herons nest in single-species breeding colonies containing from several to several-hundred breeding pairs. Isolated breeding and nesting is very rare for this species. Nesting begins in February when males choose a site and begin an elaborate display of courtship including flight, stretching, twig shaking, and physical shows. Great Blue Herons prefer to nest in tall trees but will also select locations in shrubs or on the ground as long as there is a nearby source of water. Colonies are usually situated in undisturbed wetlands, far from human activity and roads, at elevations up to 4900’6. Males collect the materials for the nest, constructed of sticks and lined with bark, pine needles, and small sticks. Females lay anywhere from two to seven pale blue-green eggs that are incubated for an average of 28 days by both parents. Females in northern environments tend to lay more eggs. In the event that a nest is destroyed or abandoned (adults may abandon a nest due to human intrusion or extreme noise,) a female may lay a second clutch. Both parents feed chicks by regurgitating food, showing preference for the largest chick. After two months the chicks reach fledging age, able to fly and survive on their own. However, fledglings will continue to return to the nest to be fed by their parents for several additional weeks. Male chicks generally experience faster growth rates, reaching a fledgling size up to 13% larger than females7. Great Blue Herons reach sexual maturity at 22 months old.
Notes of Interest: A subspecies of the Great Blue Heron living in southern Florida and the Caribbean is often mistakenly called a Great White Heron because of a color mutation resulting in pure white plumage8.
In 1999, Great Blue Heron colonies in Seattle, WA experienced a 40% abandonment rate in the middle of the breeding season. Experts now believe this exodus resulted from an increased presence of Bald Eagles in the area, known to harass herons and feed on their young. Crows may have also contributed, known to feed on nests after Bald Eagles.
In recent years, breeding colonies in Washington State were once again impacted by threats. Colonies that numbered in the hundreds were replaced with colonies containing only 30 to 40 nests. In addition to the presence of predators, forestry, land development, and the associated noises are believed to have contributed to this decline9.