Bio Facts: Heron, Green
Butorides striatus virescens
Southern Canada to northern South America; its western limits end in the arid regions from North Dakota down into the Great Plains, although some sedentary populations live on the west coast. The east coast bounds with them.
Forested water margins of both salt and fresh water. Favorite habitats are mangrove-lined shores and estuaries, and dense woody vegetation fringed ponds, rivers and lakes.
Height: approximately 16 to 22 inches (40 – 56 cm); body - stocky build and legs that are relatively short compared to its elongated body. Adults have a glossy greenish-black cap and back, wings that are black grading into green and/or blue on the edges, and under parts that are gray. Its bill is dark with a long, sharp point and its legs are orange. The female adult tends to be smaller with duller and lighter plumage versus the male, especially during breeding. Young herons coloration is different. The neck and chest are striped with white and shades of brown. The back is also brown with white and beige spots.
In the wild, they eat invertebrates such as leeches, earthworms, dragonflies, damselflies, water bugs, grasshoppers and crayfish, fish such as minnows, sunfish, catfish, perch, and eels, and other vertebrates such as rodents, lizards, frogs, tadpoles and snakes. In the Zoo they are fed fish, mealworms, bird of prey diet, and galliforme diet, the latter two being scientifically, commercially prepared diet for this genus of birds.
The green backed heron is a carnivorous species, mainly eating fish and invertebrates. It is an opportunistic forager with a broad prey selection, depending on the availability of species present. They exploit a superabundant food resource. Typically, prey is captured with a darting stroke of head and neck, lunging the body towards the victim. Herons generally grasp and occasionally spear prey with their beaks.
Among North American diurnal herons, those that feed in the daytime, green backed herons exhibit the fewest kinds of feeding behaviors (nocturnal herons have even fewer). It is only 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. It is known to stand still for long periods of time before changing sites. Standing is often interspersed with slow walking in a crouched posture within water or along bordering vegetation. While stalking prey, slow foot movements can cause possible victims to scurry, which helps for sight location. 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).
The green backed heron’s mating system involves seasonal monogamous relationships. Courtship displays are typically sequenced. 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 cricked, legs are dangled and wing beats 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.
Non-aerial displays are interspersed with display flights. In the snap display, the male perches, then points his body, head and neck down until his bill tip is at or below the level of his feet and then snaps his mandibles together producing a click while feathers are erect. The stretch display involves the male pointing his bill straight up, stretching his neck, and then bending it backwards until the head almost touches his 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 back, eyes bulging and iris possibly turning from yellow to deep orange while emitting an “aaroo-aaroo” sound. The male performs this stretch display before the female is allowed to enter the eventual nest area.
The female performs a less intense stretch display silently after the male, which confirms the pair bond. At this time, the male stops flight and snap displays. They engage 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. After the last egg is laid, copulation ceases and incubation begins. It takes 19 to 21 days for the eggs to hatch. A normal clutch is 2 to 4 eggs, laid in 2-day intervals. Both adults feed and brood chicks, though at less frequent intervals as the chicks become more independent. 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 is normally done solitary, though it also can be in loose aggregations or in colonies. Nest building is a cooperative effort, with the male participating more in protection versus actual construction. Nest placement can be from ground level up to 32 to 66 feet (10 to 20 m), depending on plant height and thickness; branches in trees are favorable. There is no attempt at nest sanitation, though chicks void over the edge of nest once they’re able to walk.
The green backed heron’s heavy bill enables it to capture large prey. Color patterns of its plumage match closely to the coloration of its habitats. Feeding can take place at any time, day or night. A feeding technique of the green backed heron that is rare, as compared to other herons, is the capture of prey using bait. This is a prime example of the tool-making capacity of birds. It is hypothesized that it was learned from observing humans. A variety of baits and lures are used, such as crusts of bread, mayflies, and feathers of an immature bird. The heron’s technique is to put the bait on the water surface and, while still, wait for prey to attack the bait. Once attacked, it captures the prey. Success rates have been highest when live bait was used versus inanimate or dead bait. Juvenile herons are poorer at baiting prey than adults.
The two most similar groups of birds are the bitterns and night herons, but the adult green backed heron is not commonly mistaken for other species. Plumage differences (brown color overall for the bitterns, and striking black-and-white patterns for the night herons) make mistaken identities uncommon.
Economic importance is minimal with negative effects on fishing.
There are 30 subspecies commonly recognized throughout the world from Australia to Africa to the US, Thailand and Canada as well as Argentina and Chile. This subspecies is our native version.
The origin of the genus name is not certain, but it is probably derived from the Latin “butio” (bittern) and the Greek “eidos” (resembling). The subspecies name is the Latin “virescens” (becoming green), describing the color on the top of the head.
No populations are considered threatened or endangered. Increasing land use changes, wetland drainage, and water quality problems have caused problems, and could cause additional problems for this species in the future.
Human activities have had their effect on heron populations. Historically, the green backed heron was often shot for food, although this has stopped because it has been found to be impractical. In the past, fish hatcheries subject herons to predatory-control programs, regulating the number of individuals in a heron population, which still continues today at some hatcheries. 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 effected areas. Eggshell thinning has been recorded in comparison with pre-1947 egg samples, but it is not considered deleterious. Increased recreational and industrial use of river channels leads to the 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.