Butorides virescens

Geographic Range

Green herons have a wide range in North America, but are generally found near wetlands. They occur as far north as southern Canada and as far south as northern South America. They are found throughout the eastern United States as far west as North Dakota and the Great Plains states, although some sedentary populations occur on the west coast. During the breeding season they are found primarily in the eastern United States, with some populations in the Pacific Northwest as well. Non-breeding individuals are found in Mexico and Central America, Texas, southern New Mexico and Arizona, and the Caribbean islands. Small vagrant populations winter in Hawaii and the United Kingdom. Some populations migrate and others are sedentary populations. Sedentary populations occur along the east and west coasts of the United States and Central America. Most populations in North America, however, are migratory. After breeding they disperse southwards, in mid-September. Spring migration occurs from March to April, an earlier arrival than most other herons. (Davis and Kushlan, 1994; Hancock and Kushlan,1984)

• Biogeographic Regions
• nearctic nearctic
  • native native
• oceanic islands oceanic islands
  • native native

Habitat

Green herons live along forested water margins. Their general distribution is limited by the availability of wetlands. They frequent both salt and fresh water, showing great flexibility in habitat choice. Favored habitats are mangrove-lined shores and estuaries, and dense, woody vegetation fringing ponds, rivers and lakes (Hancock and Kushlan, 1984).

• Habitat Regions
• temperate temperate
• tropical tropical
• terrestrial terrestrial

• Aquatic Biomes
• lakes and ponds
• rivers and streams
• coastal coastal

• Wetlands
• marsh marsh

• Other Habitat Features
• riparian riparian
• estuarine estuarine

Physical Description

Green herons are small and stocky, with legs that are relatively short, compared to other herons. Body length ranges from 41 to 46 cm. Adults have a glossy greenish-black cap and back, wings that are black grading into green and/or blue on the edges, and underparts that are gray. The bill is dark with a long, sharp point and the legs are orange. Female adults tend to be smaller, with duller and lighter plumage than that seen in males, particularly in the breeding season.

Coloration of immature herons is different. The neck and chest are striped with white and shades of brown. The back is also brown with white and beige spots. The coloration of immature and adult birds is quite cryptic in dense vegetation. (Hancock and Kushlan, 1984; Davis and Kushlan, 1994)

• Other Physical Features
• endothermic endothermic
• homoiothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• male larger
• male more colorful

Reproduction

Green herons are seasonally monogamous. Courtship displays are stereotyped. They begin with Flying Around displays resembling natural flight, but oriented to breeding sites with skow calls. From here, courtship becomes aggressive. Pursuit Flight, Circle Flight and Forward displays are used, where a rasping 'raah-raah' call exposes the red mouth-lining. Crooked-Neck Flight displays are more aggressive, where the neck is partially flexed, legs are dangled, and wingbeats are audible. Much like the Crooked-Neck Display, the Flap Flight Display shows the greatest intensity of flight displays. Here, the male lurches through the air with exaggerated flapping producing a whoom-whoom-whoom sound in a crooked-neck posture with crest, neck, and scapular feathers erect and often giving a roo-roo call before landing.

Nonaerial displays are interspersed with display flights. In the Snap Display, the male perches, then points body, head and neck down until bill tip is at or below the level of his feet and then snaps his mandibles together, producing a click while also erecting his feathers. The Stretch Display involves the male pointing his bill straight up, stretching his neck, and then bending it backwards until the head almost touches its back with interscapular plumes erect and fanned. In this posture, he sways his neck and head from side to side with crest, breast, and flank feathers sleeked back, eyes bulging, and iris possibly turning from yellow to deep orange while emitting an aaroo-aaroo sound.

Males perform this Stretch Display before a female is allowed to enter the eventual nest area. The female then performs a less intense Stretch silently after the male, which confirms the pair-bond. At this time, the male stops Flight and Snap displays. The pair then engages in mutual bill-snapping and feather nibbling, though those behaviors are reduced soon thereafter. Copulation occurs on the nest platform during the nest-building stage. It lasts about ten seconds with several hours between copulations. (Davis and Kushlan, 1994; Hancock and Kushlan, 1984)

• Mating System
• monogamous monogamous

After the last egg is laid, copulations cease and incubation persists for 19-21 days. A normal clutch is 2 to 4 eggs, laid in 2-day intervals. Fledging occurs when chicks are 16 to 17 days old, and independence is gained between 30 and 35 days.

Nesting takes place in forest and swamp patches, over water or in plants near water. Nesting pairs normally nest alone, but loose aggregations of mated pairs can form. Nest building is a cooperative effort, with the male participating more in protection versus actual construction. Nest placement can be from ground level to 20 meters, depending on plant height and thickness; branches in trees are favored. There is no attempt at nest sanitation, though chicks void over the edge of the nest once they're able to walk.

• Key Reproductive Features
• iteroparous iteroparous
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• oviparous oviparous

Both adults feed and brood chicks, though at less frequent intervals as the chicks become more independent.

• Parental Investment
• altricial altricial
• male parental care male parental care
• female parental care female parental care

Lifespan/Longevity

The oldest know wild green heron was a banded bird that was captured when it was almost 8 years old. There is very little information on lifespan in these birds.

Behavior

Green herons are shy birds so are rarely observed, although they may be quite common. They are active during the day. They have a characteristic slow, deliberate walk and in flight they have slow and steady wingbeats. They may also swim on occasion in pursuit of prey. Green herons are territorial and will aggressively defend both foraging and nesting territories from conspecifics.

• Key Behaviors
• flies
• natatorial natatorial
• diurnal diurnal
• motile motile
• migratory migratory
• sedentary sedentary
• solitary solitary
• territorial territorial

Communication and Perception

Green herons use their keen vision, hearing, and sense of touch to perceive their environment. They have especially acute vision that helps them to capture prey. Green herons have an elaborate set of calls and body postures that they use for communicating with other green herons. Examples are their elaborate courtship displays, warning calls when a predator is detected, and territorial displays.

• Communication Channels
• visual visual
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

Green herons are carnivorous, mainly eating fish and invertebrates. They are opportunistic foragers with a broad prey base, depending on the availability of species present. They exploit superabundant food resources, such as breeding frogs. Their invertebrate diet includes: leeches, earthworms, dragonflies, damselflies, waterbugs, grasshoppers, and crayfish. Some of the many fish eaten are: minnows, sunfish, catfish, perch, eels, and, in urban areas, goldfish. Other vertebrates eaten are rodents, lizards, frogs, tadpoles, and snakes.

Their heavy bill enables them to capture large prey. Feeding can take place at any time, day or night. Typically, prey is captured with a darting stroke of the head and neck, lunging the body towards the victim and either grabbing or impaling the prey.

Among North American diurnal herons, green herons exhibit the fewest kinds of feeding behaviors. They are known to use 15 of the 36 heron feeding behaviors. The most common feeding technique is to stand in a crouched position, horizontal to the water surface, with neck and head retracted. They stand still for long periods of time before changing sites. Standing is often interspersed with slow walking in a crouched posture in the water or bordering vegetation. Herons use their feet to cause potential prey to move and then capture them. They may also dive from perches head first into deep water, becoming submerged, although this isn't a very efficient method. The greatest capture success is in shallowest water (0-10 cm) and the poorest success is in deeper water (20-30 cm).

Green herons are one of the few tool-using birds. They use a variety of baits and lures, such as crusts of bread, mayflies, and feathers. They then put the bait on the water surface and wait for prey to attack the bait. They stand motionless near the bait until a small fish or other animal approaches and then grab the prey. Success rates have been highest when live bait was used versus inanimate or dead bait. Juvenile herons are not as adept at baiting prey (Davis and Kushlan, 1994; Hancock and Kushlan, 1984; Hancock, 1999).

• Primary Diet
• carnivore carnivore
  • piscivore piscivore
  • insectivore insectivore
  • eats non-insect arthropods

• Animal Foods
• mammals
• amphibians
• reptiles
• fish
• insects
• terrestrial non-insect arthropods
• terrestrial worms
• aquatic or marine worms

Predation

Snakes, crows, and common grackles are known to eat green heron eggs. Raccoons eat nestlings. Adult birds may be preyed on by large birds of prey. Green herons remain vigilant to protect themselves from predators. They also utter warning calls when predators approach.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Green herons are important predators of fish and invertebrates in the aquatic ecosystems where they live.

Economic Importance for Humans: Positive

Green herons are enjoyed by birdwatchers.

Economic Importance for Humans: Negative

Economic importance is minimal with negative effects on fishing (Davis and Kushlan, 1994).

Conservation Status

Human activities have had their effect on the heron. Historically, green herons were sometimes hunted for food. Fish hatcheries kill green herons to regulate their populations and prevent them eating their young fish. Pesticides have been found to accumulate in heron tissues, but no evidence of general reproductive failure has been observed, despite some localized effects to herons in highly polluted areas. Eggshell thinning has been recorded in comparison with pre-1947 egg samples, but it is not considered lethal. Increased recreational and industrial use of river channels leads to decreased use by green herons. Yet this has not led to decreased use of backwater habitats, for example, ponds. The main concern for the heron has been conservation and management of wetlands as a whole, versus the drainage and development that depletes habitat in which they live. No populations are considered threatened or endangered (Davis and Kushlan, 1994).

Additional Links

Encyclopedia of Life

BioKIDS Critter Catalog

Contributors

Tanya Dewey (author), Animal Diversity Web.

Josh Butzbaugh (author), Natural Resources And Environment, Terry Root (editor), University of Michigan-Ann Arbor.

References

Davis, W., J. Kushlan. 1994. The Green Heron. The Birds of North America .

Hancock, J., J. Kushlan. 1984. The Herons Handbook . New York City, NY: Harper and Row Publishers.

Hancock, J. 1999. Herons and Egrets of the World . London, UK: Academic Press.