Bird of the Month: Black-crowned Night Heron
Written by Andy McCormick
Andy McCormick, Volunteer and Former Board President of Eastside Audubon
Black-crowned Night Heron
Scientific Name: Nycticorax nycticorax
Length: 25”
Wingspan: 44”
Weight: 1.9 lb (870 g)
AOU Alpha Code: BCNH
A denizen of the evening and night when we hear its loud WOK call, the Black-crowned Night Heron is the most common heron in the world.
LARGE, RED EYES FOR NIGHT VISION
The Black-crowned Night Heron has large, light sensitive eyes that allow it to feed in low light situations, at a time of day when competition from other herons is reduced (Bell and Kennedy). Like many other herons the Black-crowned will stay motionless in a suitable fishing area and wait for prey to swim by. Its diet is made up primarily of fish, but it will also indulge in frogs, snakes, crustaceans, mussels, and clams. Because of its fish-oriented diet this heron has been exposed to accumulated contaminants and it has played a role as an indicator-species of environmental quality (Hothem, et al).
WATERFRONT SLOUCHER
The Black-crowned Night Heron is a stocky, black, and gray bird with a slouching appearance similar to a guy hanging out on a street corner (Dunne). Its neck is almost always retracted even in flight, and it leans forward when foraging while wading in water. It seldom looks like it is standing erect. During the day it will roost communally while tucked away in shrubs and trees.
The night heron was also known as a “night raven” from its genus name Nycticorax, from the Greek, nux, nuktos, night, and korax, raven. The name is repeated for the species epithet. It is thought that Carl Linnaeus used the name because of the heron’s ravenlike croaking call at night (Holloway).
COLONIAL NESTER
Colonies of Black-crowned Night Herons are usually found on islands, in swamps, and other areas around water. Other than seeking separation from mammalian predators, the herons do not seem particular about habitat and a wide variety of trees and shrubs are used for nesting. Once established, colonies can grow to as many as 500 nesting pairs and can be used for 30 – 50 years (Hothem, et al).
Black-crowned Night Herons build a platform nest of sticks. Males bring materials and females form the nest. Typically, 3-5 pale green or bluish eggs are deposited and incubation is shared between the parents. Young hatch in about three weeks. They begin moving around the nest in another two weeks and by the third week they have left the nest. Parents continue to feed the young for another few weeks.
A FRIEND OF SCIENCE
Black-crowned Night Herons are high on the food chain, widely distributed, and colonial nesters, all of which makes them excellent environmental indicators (Hothem, at al). Researchers have documented concentrations of organochlorines, such as DDT and PCB, in Black-crowned Night Herons, so far, however, at levels below that which will cause disruption of populations. In some areas these herons may be doing too well, and managers have had to reduce the population of some colonies that have created a nuisance because of noise, odor from guano, death of trees as leaves become coated with guano, and threats to air traffic.
The North American population of Black-crowned Night Herons is considered stable, but regional losses have been reported. The primary threat to this species is habitat destruction due to draining of wetlands for development and expansion of agriculture, and human disturbance on islands and other nesting sites (Hothem, et al). Management plans to improve some local breeding areas including some in urban areas has allowed some colonies to recover and expand, demonstrating the value of local habitat restoration programs.
Photo credit Mick Thompson.
References available upon request from amccormick@eastsideaudubon.org.