Nombre en español: Avetorillo Bicolor
Nombre en inglés: Least Bittern
Nombre científico: Ixobrychus exilis
El avetorillo panamericano o huairavillo de dorso negro (Ixobrychus exilis) es una especie de ave pelecaniforme de la familia Ardeidae. Su área de distribución cubre la mayor parte del continente americano.
Se distinguen las siguientes subespecies:
- Ixobrychus exilis bogotensis Chapman, 1914 en Colombia
- Ixobrychus exilis erythromelas (Vieillot, 1817) de Panamá hasta Paraguay
- Ixobrychus exilis exilis (Gmelin, 1789) en Norteamérica, América Central y las Antillas
- Ixobrychus exilis limoncochae D. W. Norton, 1965 en Ecuador
- Ixobrychus exilis peruvianus Bond, 1955 en Perú, con registros de algunos ejemplares en Chile que probablemente sean de esta subespecie.
- Ixobrychus exilis pullus Van Rossem, 1930 en el noroeste de México
The least bittern (Ixobrychus exilis) is a small heron, the smallest member of the family Ardeidae found in the Americas.
The least bittern is one of the smallest herons in the world, with perhaps only the dwarf bittern and the black-backed bittern averaging smaller in length. It can measure from 28 to 36 cm (11 to 14 in) in length, and the wingspan ranges from 41 to 46 cm (16 to 18 in). Body mass is from 51 to 102 g (1.8 to 3.6 oz), with most least bitterns weighing between 73 and 95 g (2.6 and 3.4 oz), making this perhaps the lightest of all herons. A recent manual of avian body masses cites another species in this genus, the stripe-backed bittern, as having a mean body mass slightly lower than the least bittern, which is credited with a mean mass of 86.3 g (3.04 oz).
The bird’s underparts and throat are white with light brown streaks. Its face and the sides of the neck are light brown; it has yellow eyes and a yellow bill. The adult male is glossy greenish-black on the back and crown; the adult female is glossy brown on these parts. They show light brown parts on the wings in flight.
The least bittern is an elusive bird. They spend much time straddling reeds. When alarmed, the least bittern freezes in place with its bill pointing up, turns its front and both eyes toward the source of alarm, and sometimes sways to resemble wind-blown marsh vegetation. This is perhaps a predator-avoidance behaviour, since its small size makes the bittern vulnerable to many potential predators. Thanks to its habit of perching among the reeds, the least bittern can feed from the surface of water that would be too deep for the wading strategy of other herons. The least bittern and much larger and different-looking American bittern often occupy the same wetlands but may have relatively little interaction because of differences in foraging habits, preferred prey, and timing of breeding cycles. The least bittern arrives on its breeding grounds about a month after the American bittern and leaves one or two months earlier. John James Audubon noted that a young captive least bittern was able to walk with ease between two books standing 4 cm (1.6 in) apart. When dead, the bird’s body measured 5.7 cm (2.2 in) across, indicating that it could compress its breadth to an extraordinary degree.
These birds nest in large marshes with dense vegetation from southern Canada to northern Argentina. The nest is a well-concealed platform built from cattails and other marsh vegetation. The female lays four or five eggs, in extreme cases from two to seven. The eggs are pale blue or green. Both parents feed the young by regurgitating food. A second brood is often produced in a season.
These birds migrate from the northern parts of their range in winter for the southernmost coasts of the United States and areas further south, travelling at night.
They mainly eat fish, frogs, crustaceans and insects, which they capture with quick jabs of their bill while climbing through marsh plants.
The numbers of these birds have declined in some areas due to loss of habitat. They are still fairly common but are more often heard than seen. They prefer to escape on foot and hide than to take flight. These birds make cooing and clucking sounds, usually in early morning or near dusk.
Taxonomy and nomenclature
The least bittern was originally described in 1789 by J. F. Gmelin based on specimens from Jamaica.
The least bittern forms a superspecies with the little bittern and yellow bittern.
There are five widely recognised subspecies.
- I. e. exilis (Gmelin, JF, 1789): in north and Central America and the Caribbean
- I. e. pullus van Rossem, 1930: in north-west Mexico
- I. e. erythromelas (Vieillot, 1817): in eastern Panama and around eastern coasts of South America south to Paraguay
- I. e. bogotensis Chapman, 1914: in Colombia
- I. e. peruvianus Bond, 1955: in Peru
Birds from Ecuador are sometimes assigned to a sixth subspecies, I. e. limoncochae Norton, DW, 1965. North American birds were formerly divided into two subspecies, eastern (I. e. exilis) and western (I. e. hesperis), but this is no longer believed to be a valid distinction.
Cory’s least bittern
A dark rufous morph, I. e. neoxenus, termed «Cory’s bittern» or «Cory’s least bittern» was originally described by Charles Cory as a separate species in 1885 from a specimen collected on or near the Caloosahatchee River, near Lake Okeechobee, in southwest Florida. Cory stated that the specimen was «without doubt perfectly distinct from any other known species». Further specimens followed over the next decades from Florida, Michigan, Illinois, Wisconsin, Ohio, and Ontario.
Initially, Cory’s least bittern was accepted as a valid species, with Elliott Coues and Richard Bowdler Sharpe both including it in published species lists. As early as 1892, however, doubts were raised about the validity of Cory’s least bittern as a separate species. Nonetheless, in 1896 Frank Chapman wrote a detailed paper supporting its retention as a valid species. Outram Bangs later argued, in 1915, that this view was wrong and proposed that Cory’s should become a junior synonym of least bittern. This view eventually prevailed, with the American Ornithologists’ Union removing the species from their list of North American birds in 1923, although others held dissenting views until at least 1928.
Cory’s least bittern was once fairly common, but it is now exceptionally rare, with only five sightings since 1950. More than 50% of the historical records are from the Toronto region of Ontario. Initially known only from the North American subspecies exilis, it was first recorded in the South America subspecies erthyromelas in 1967.
The bird has a large range and a large total population, and the International Union for Conservation of Nature has assessed its conservation status as being of «Least Concern». The least bittern is protected under the Migratory Bird Treaty Act of 1918.