Yellow-crowned night heron
The yellow-crowned night heron is one of two species of night heron in the genus Nyctanassa. Unlike the black-crowned night heron, which has a worldwide distribution, the yellow-crowned is restricted to the Americas. It is known as the bihoreau violacé in French and the pedrete corona clara or yaboa común in some Spanish-speaking countries.
Taxonomy
The yellow-crowned night heron was formally described by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his Systema Naturae. He placed it with herons, cranes and egrets in the genus Ardea and coined the binomial name Ardea violacea. Linnaeus based his account on the "crested bittern" that had been described in 1729–1732 by the English naturalist Mark Catesby in the first volume of his The Natural History of Carolina, Florida and the Bahama Islands. Linnaeus specified the type locality as North America, but this has been restricted to the Carolinas following Catesby. The yellow-crowned night heron is now placed together with the extinct Bermuda night heron in the genus Nyctanassa that was introduced in 1887 by the Norwegian born zoologist Leonhard Stejneger. The genus name combines the Ancient Greek nux meaning "night" with anassa meaning "queen" or "lady"; the specific epithet is from Latin violaceus meaning "violet-coloured". The related heron, the Bermuda night heron, was endemic to Bermuda, but probably became extinct following human colonization.Being a heron, the yellow-crowned night heron is related to egrets and bitterns, and, more distantly, to pelicans and ibises. The night herons are usually considered to have separated from the day herons.
Various classifications recognize five subspecies, but little is known as to how much they are integrated together and how much their geographic range varies. Their body size decreases from north to south, and the shape of their bills varies geographically, depending on the size of the crustaceans they eat in different regions.
Subspecies
Five subspecies are recognised:- N. v. violacea – central, east USA to east Mexico and east Costa Rica
- N. v. bancrofti Huey, 1927 – west Mexico to west Nicaragua, Socorro Island and West Indies
- N. v. caliginis Wetmore, 1946 – Panama and west Colombia to Peru
- N. v. cayennensis – Panama and northeast Colombia to northeast Brazil
- N. v. pauper – Galápagos Islands
Distribution and migration
Winter and year-round range
It winters where the climate allows for year-round crab activity: tropical and subtropical regions, southern Florida, the Gulf Coast, and the eastern Texas coast. It is also found in Mexico, Central America, the Galápagos Islands, the Caribbean and northern South America where generally resident.Breeding range
From 1925 to 1960, the yellow-crowned night heron spread northward for reasons still not clear. In addition to its winter and year-round range, it can be found in the south-east inland of the United States during breeding season, and additional isolated breeding colonies have been recorded even farther inland, all the way to the northern border of the United States.Migration
Different subspecies and populations have different migratory behaviour. Subspecies and populations that are insular or live in warmer areas are confirmed to be sedentary. The tropical subspecies and populations migrate, but to an extent that is still unclear for lack of data. The migratory behaviour of the yellow-crowned night heron has changed with its expansion north: one subspecies migrates to the northern limits of its range, moving north and west after breeding. Birds observed outside of the normal range are usually first-years or strays.Migration occurs in mid-March in lower latitude, and from mid-March to April in higher latitude.
Description
General shape
The yellow-crowned night heron is a rather stocky wading bird, ranging from and from, the females being a little smaller than the males. The yellow-crowned night heron has a wingspan ranging from. The neck, slim when extended, exposes its large head, with a large and heavy bill.Colors
The body and back are a smooth grey-blue, with a black scaled pattern on the wings. The long legs are yellow and turn coral, pink or red during courtship. The most characteristic part of the yellow-crowned night heron is the head: black and glossy, with white cheeks and a pale yellow crown going from the bill, between the eyes and to the back of the head, giving the bird its common name. Such colours make the face appear striped in a horizontal black-white-black-white pattern. Long, thin, white feathers grow to the back of the crown during mating season. The bill, also black, is thick and deeply set under the eyes which are dark orange or red.Flight
Like all herons, the yellow-crowned night heron flies with long, slow purposeful wing beats. It can be found gliding over water with its legs easily visible, extended straight below the tail, unlike the black-crowned night heron, whose legs can barely be seen in flight.Juveniles and immatures
It takes about three years for yellow-crowned night herons to acquire the full physical appearance of adults. Before that, the young birds show signs of immaturity, such as a brownish body, an overall greyish head, drab colors and spots and streaks on their plumage.Although the adults are easy to tell apart, juvenile yellow-crowned night heron can look very similar to juvenile black-crowned night heron. Yellow-crowned juveniles tend to stand straighter and have heavier bills and longer legs, and their spots and streaks are finer than those of the black-crowned.
Habitat
The yellow-crowned night heron looks for shallow water to live in: marshes, wooded swamps, and lakeshores for inland populations, and thickets, mangroves and cliff-bound coasts for coastal populations. It can also be found in areas that do not always have enough water, but that get flooded on a regular basis. Its habitat is closely linked to that of the crustaceans that make for most of its diet, and it tolerates fresh water, brackish water and saltwater.Another important habitat factor is nesting sites. The yellow-crowned night heron needs bushes or trees to build nests, although it will use rock ledges where vegetation is unavailable.
Unlike the black-crowned night heron, the yellow-crowned does not mind living near humans and can be found in wooded neighborhoods, nesting on rooftops and driveways. Such cohabitation may not go smoothly and can create conflicts with humans.
Behavior
Vocalizations
The most common call of the yellow-crowned night heron is a loud, sharp and quick squawk that the bird gives shortly after taking off or uses as an alarm call or an aggression call. The young beg for food with a soft chu-chu-chu call that becomes louder as the chicks grow older and more demanding. A different range of vocalizations plays an important part in courtship and pair formation. Males and females use a yup-yup call during the greeting ceremony, and a huh! call when they form a pair among other nesting birds. The male concludes his courtship display with a whoop which may or may not lead to the pair actually forming.Feeding
The yellow-crowned night heron eats mainly crustaceans as well as insects, some fish or worms. Crabs preyed upon include fiddler crabs, marsh crabs, blue crabs and green crabs. It can also feed on amphibians, molluscs, lizards, snakes, small rodents and small birds. The geographic location of the heron is closely related to the prey it may find, and the size and shape of its bill allows it to hunt for prey of specific size.The yellow-crowned night heron may forage any time of the day and night, although it prefers the night to feed the young. It becomes very aggressive if another individual approaches when foraging, but will tolerate a safe distance of about ten meters. It selects prey visually, either stalking it or standing and waiting for it to come within reach. When hunting crayfish, the heron stands at the entrance of the burrow, always facing the sun so its shadow is not cast over the entrance of the burrow, which would alert the crawfish. It can also choose an alternate pattern of walking slowly towards prey with its body bent and its head retracted, then standing and waiting before walking slowly again, sneaking up effectively on unsuspecting crabs. Sometimes it can be seen running in shallow water after prey, but it will not follow one in deep water.
Once close enough, it lunges with its bill. Small prey are swallowed whole while larger prey, it will try to dismember in order to eat the body first and the legs last, or to jab it straight through the body. It will also carry crabs, molluscs or fish away from the water to prevent them from escaping.
Any indigestible material, such as crab shells, is ejected in a pellet, and it is quite common to find shells and pellets scattered around foraging and nesting areas.
Breeding
Courtship and nest-building
Like many other aspects of its life, the yellow-crowned night heron's breeding season depends closely on the emergence of crabs in the spring; the crab cycle itself depends on temperatures. As such, the breeding season of the yellow-crowned night heron varies geographically, typically between March and May. In some tropical locations, it can breed all year.It is still unclear exactly how, and for how long, the yellow-crowned night heron forms a mating pair. Some herons arrive at breeding grounds already in pairs, meaning they either found a mate during their migration to the breeding grounds, or they have been with this mate for some time. Others form their pair or renew it upon reaching the breeding grounds.
The breeding grounds are chosen near water and host loose, rather small colonies of reproducing herons, unlike those of most wading birds that welcome large colonies. Colonies often start small with only a pair nesting, then grow over the years and can last over 20 years.
About 9–10 days after arriving at the breeding grounds, the pair builds a nest. The male usually chooses an emplacement and starts to build the nest for the female. Eventually, both birds decide where to build their nest, sometimes starting several nests before settling down. At first, the male brings material for the female to build the nest, then both perform both roles. Sometimes they will steal material from a nearby nest being built. They can be seen repairing their nest well into the breeding season. Trees and bushes are the preferred location for nests, the herons will usually build in high branches away from the trunk. A pair can use the same nest for years, enlarging it every season; the first nest is usually just large enough to hold the eggs. Nest-building is not the result of successful courtship, but rather an active part of the courtship.