Boat-billed Heron

Cochlearius cochlearius (Linnaeus)

Cancroma cochlearia Linnaeus, 1766, Syst. Nat. ed. 12, 1, p. 233: Cayenne (Guiana)

Cochlearius cochlearius zeledoni (Ridgway), 1885: Mazatlan (Sinaloa), Mexico; Cochlearius cochlearius panamensis Griscon, 1926: Corozal, Canal Zone;

Cochlearius cochlearius phillipsi Dickerman, 1973: Atasta, Campeche, Mexico; Cochlearius cochlearius ridwayi Dickerman, 1973: Coyoles, Yoro, Honduras.

Other names: Martinete Cucharón, Pato Pico de Barco, Garza Bruja, Huairavo, Boca d’agua in Spanish; Coá, Arapapá in Portuguese; Savacou huppé in French; Kahnschnabel in German.

Description

The Boat-billed Heron is a stocky, medium sized mostly black and white and sometimes buff heron, with a huge black bill.

Adult: The Boat-billed Heron’s head and the crest of long, lanceolate occipital plumes are black. The broad, thick, flat - entirely outlandish - bill is black with a yellow wash at the base of the lower bill. The forehead and cheeks are white. The eyes are huge and bulge out. The irises are dusky or brown. The lores and orbital skin are grey with a green yellow spot on the eyelid. The gular pouch, visible only when distended, is variably dusky sulfur yellow, yellow and grey, and pink. The upper back is black, the rest of the back and upper wings are grey. The under parts are a rich rufous, and the flanks and leg feathering are black. The mantle is buff brown merging into vinaceous on the sides of the neck. The legs are green. During breeding the mouth lining, lores and gular area become black. The peak of the crest plumes is achieved prior to nesting.

Variation: The sexes are alike, but males have longer occipital plumes. The five recognized subspecies differ in size and plumage color (Dickerman 1973, Dickerman et al. 1982). Cochlearius is the palest form, with light grey upper parts, and pure white breast and neck. Panamensis is the darkest form, olive grey above and darker lavender below; the face, throat and upper breast are suffused with olive brown. The remaining subspecies are paler, the upper parts are lavender grey; the throat and breast are buff brown. They differ slightly from one another in size and color intensity on the underside. Phillipsi is the largest. Ridgwayi is a little bigger than zeledoni and also darker below, though not as dark as panamensis. Specimens from western Costa Rica are considered to intergrade between ridgwayi and panamensis. Specimens from northern Guatemala intergrade between ridgwayi and phillipsi.

Juvenile: Juveniles have a dull rufous back and wings, and are dingy white below with a pinkish or buff wash. They have a shorter crest than the adult. They move through six plumages from hatching to adult (Dickerman et al. 1982). The juvenile cochlearius has a tawny brown back. Juveniles of the other races are browner than adults, but lighter than cochlearius.

Chick: The downy chick is pale grey above, dull white to pale grey below and has a blackish crown lacking erectile tufts. In its color pattern and lack of a crest, it differs from other nestling herons (Dickerman 1971). It has greenish-yellow skin. The chick also has a triangular short beak, broad at the base and tapering to a blunt point.

Voice: This is a vociferous heron. Its raucous jeering laughing call is commonly heard along the mangroves and forests I which it occurs and nests. This “An” call and its variations are characteristic of the species. But they are varied in style and probably content. The An call is a repeated vocalization, rendered “an, an, an, an”, or “oh, oh, an-an”, or “Ha, go, go, go, go” is the disturbance, threat and general presence call used in the colony and on the feeding grounds. This single call is highly variable and used on many occasions. The “an, an, an, an” has been called the Short Chant. When the “an, an, an, an” notes rise it indicates enhanced agitation. A more complex “An” call is called The “Long Chant”, starting with several notes, then a series of paired notes. This can elicit a chorus from birds in the colony. It is the antagonistic, territorial call used in Greeting Display, Supplanting Flight, when fleeing an attack, during disturbance, and in chorus. A variation is also the flight call, used in flying back and forth to roost.

A faint “Ump” call, made with the bill closed in the throat is a contact call. “Pop” (also called “Clap-clap”, is the sound made by the snapping of their substantial bills together. The Pop call is usually doubled and may be joined by an An call. They give a “High Cry” when fleeing other herons.

Weights and measurements: Length: 45-51 cm. Weight: males 680-770 g, females 503-726 g.

Field characters

The Boat-billed Heron is identified by its broad, thick bill, its moderate size, black cap, white forehead and cheeks, and grey back and wings. It flies with slow wing beats, which may be deep or shallow. It cannot be confused with any other bird.

Systematics

The systematic relationship of the Boat-billed Heron is one of the best-studied questions of heron biology (Cracraft 1967, Dickerman and Juarez 1971, Dickerman 1973, Payne and Risley 1976, Sheldon 1987, Sheldon et al. 1995, McCracken and Sheldon 1998). The conclusions have ranged from being placed in its own family to being classified as a night heron. Molecular studies have now brought clarity to the situation. The Boat-billed Heron is a heron but constitutes as sister group to other heron groups (Sheldon et al. 1995, K. McCracken pers. comm.). Thus it is properly its own subfamily, Cochleariinae. This understanding is coherent with morphological and behavioral evidence.

Recognizing the ancient lineage and distinctiveness of the Boat-billed Heron suggest that the relationships among the several recognizable populations of Boat-billed Herons deserve additional study. The northern and South American birds are sometimes separated as different species, and the several forms may be more distantly related than is now appreciated.

Range and status

The Boat-billed Heron is a bird of South and Central America.

Breeding range: The Boat-billed Heron occurs coastal north central Mexico south to Peru, Bolivia and northeast Argentina. The race zeledoni (formerly including all Boat Bills from Mexico to the Nicaraguan border) is restricted to the Mexican west coast from Sinaloa to Guerrero. Ridgwayi occurs in Chiapas (Mexico), coastal Guatemala, El Salvador and west Honduras. In northern Guatemala ridgwayi intergrades with phillipsi. Phillipsi occurs on the Caribbean slopes of Mexico from the latitude of Cuidad Victoria to the tip of the Yucatan peninsula and south to Belize, and likely but not proven in Nicaragua. Panamensis and ridgwayi intergrade in west Costa Rica. Panamensis occurs through coastal Costa Rice through most of lowland Panama. Cochlearius occurs throughout most of the South American range except the north west coast and reaches east Panama.

Migration: The Boat-billed Heron is rather sedentary. Nonbreeding records occur outside the breeding range. Panamensis occurs along the Pacific coast of South America down to Lima, Peru, suggestive of a migration or regular dispersal. It occurs occasionally in northern Argentina, south Brazil (Rio Grande do Sul). Local movements to tidal areas in nonbreeding season are reported.

Status: The species is widely distributed and not uncommon where it occurs.

Habitat

The Boat-billed Heron uses wooded or mangrove fringes of fresh water creeks, lakes and marshes with thick bushes or trees overhanging water, which it requires for daytime roosting and nesting. It is a low land species, occurring to 650 m.

Foraging

Despite its great bill, this species for the most part feeds like a typical heron, using both usual and specialised feeding techniques. It feeds by Standing, sometimes for many minutes, usually in Crouched posture. It Walks Slowly, with very slow deliberate steps on the ground or along branches and roots. It proceeds in a very Crouched posture, described as hunch-backed (Mock 1976b). It sometimes uses Neck Swaying, a rhythmic movement of the body but not the head, and Peering Over when observing food items. It also feeds by Walking Quickly and by Running, frantically dashing about, stopping, and Stabbing. It moves short distances by hopping up and flapping once or twice (Mock 1976b). Using these behaviors it appears to feed visually (Biderman and Dickerman 1978).

It captures prey both by Stabbing and by full-bodied Lunging. They also Probe and use their large bill in Ploughing (Wetmore 1965, Willard 1979). In the latter, the individual wades along bill partially submerged, thrusting forward and scooping with each step, thus using the wide bill to catch sedentary prey in mud and leaf litter. Probing and Ploughing are non-visual techniques.

It feeds nocturnally and crepuscularly, as suggested by its huge eyes. It is easily disturbed while foraging, and readily flies off calling. During the day it perches in dense trees and bushes, and also retreats there when disturbed. Its resting posture is characteristic, and being used during the day, is more often observed than its foraging. At roost, its head appears to rest on its shoulders, bill forward and slightly down. When sleeping, the bill falls further and rests on its breast. Or it may lean its head to one side and rest it under the wing (Sick 1993). It spends its time at roost, sleeping, resting, preening .It preens by both nibbling and stropping in a typical heron fashion but also uses the hook on the end of its bill. It occasionally feeds during the day. It feeds alone, flying from the communal roost to independent, probably defended, feeding areas. Occasionally in tidal pools and other situations, it will feed in loose aggregations.

The diet is broad and includes insects, shrimps, fish, amphibians and small mammals. Crustaceans are particularly important to the diet. Shrimp include Panaeus, Macrobrachium. Fish include mullet (Mugil), sleeper (Dormitador), snook (Centropomis), and catfish (Ariidae).

Breeding

The breeding biology of the Boat-billed Heron has been the subject of several studies (Juarez and Dickerman 1972, Dickerman and Juarez 1971, Mock 1976b, Alvarado 1992, Hernández and Fernández 1999). The breeding season is variable and may be dependent on a combination of factors such as the rains and food availability. It is in the rainy season in most places but can be in the dry season in coastal habitats. They are nesting in April–August in Mexico, June–June in Honduras, June in Honduras, February–April in coastal east Mexico and Panama, August in Surinam, June–October in Trinidad, November in Brazil. These seasons need to be clarified throughout the range of the species.

The Boat-billed Heron nests in trees and bushes, especially in mangroves. They use flooded forests, swamps, and trees along and overhanging water such as streams and pools. The heron nests solitarily or in small groups of about a few to a dozen pairs. They also form parts of mixed heronries, and seem particularly to nest with Striated or Green Herons. In mixed colonies Boat-bills often nest with each other but away from the main colony.

Nests are a flat platform made of sticks, with leafy branches, 30-35 cm wide, and 10-15 cm thick (Hernández and Fernández 1999). They are often small, although old ones are typically reoccupied or enlarged or bigger nests of other species taken over. In one site 55% were reused nests (Hernández and Fernández 1999). They are usually placed 0.5-10 m above the ground on thick supportive interior branches. Contrasted with other herons, the Boat-billed Heron’s nest is found in the denser and more closed vegetation.

The courtship displays of the Boat-billed Heron differ somewhat from those of other herons, mostly in taking advantage of the display opportunities of the great bill, and perhaps being constrained by the mechanical limitations of that bill. Mock (1976b) found few that he considered to be homologous with other known heron displays. It is a highly vocal heron in its courtship behavior. Boat-billed Herons appear to arrive at the nesting colony already paired. As a result males do not perform the typical advertising displays from nesting territories before pair-formation. But they do show a series of behaviors in and near the nest site (Mock 1976b), most of which are threats of various sorts.

Crest Raising is particularly common, and is used when the bird is disturbed and also in other displays. The degree of crest raising is variable but at full expansion it forms a black fan, contrasting sharply with the white forehead. As a display, the heron may also extend its neck up and forward in a slight arch. Crest Raising is a disturbance and greeting display.

Tall Rocking consists of a heron extending its neck up and forwards, raising its crest, and rocking slowly from side to side. It is a mild threat. Twig Shaking is also a threat. In a striking Wings Out display, given usually after landing, the heron extends both wings out to the side. The Pop Display features the “Pop” call. The male flexes its feet slightly, snaps the head up with the bill pointing down, lowers the rump making the body vertical, erects its crest half way, flicks the out wings out, and gives the loud resonating “Pop” call by snapping the bill, sometimes with an “An” call. Bill snaps are also used in territorial defense. In the Forward (also called the Single Clap display) the bird walks towards another, extends its neck up and forward, erects its crest and snaps its bill once. The display also has a shrieking “An” call (also called an An-snap), which is more aggressive.

Between members of a pair, both Contact and Non-contact Bill Clappering are the most common displays between members of a pair and often follow a Pop Display. The paired birds spend time together in and around their display territory. The walk around together, appearing to examine sites. Bill Clappering occurs as the birds re-meet during the walk. The most intriguing display is the Bill Duel (Alvarado 1992, Sick 1993). The birds face each other or the male is behind the female, and they touch bills at the tips. When the female’s bill is within the males, he closes it and they begin a series of rhythmic movements, for several minutes before separating.

The eggs are pale blue to green. They usually spotted with cinnamon on the larger end, sometimes forming a ring. The spotting is usual but not invariably reported, likely because the eggs fade to white or blue white with a calcareous wash during in incubation or in egg collections. In this way the eggs appear to be more like an ibis or tiger heron than a typical heron (Sick 1993). Eggs average 49.5 x 33.0 in Mexico, 49.7 x 35.9 mm in Trinidad, 50.3 x 35.3 mm in Surinam (see Dickerman 1971). The clutch is usually 3 eggs, range is 1-4 eggs. Incubation is 26 days, range of 25-27 days. Incubation is shared by both parents and begins with the first egg.

The young are at first fed entirely at night. The adult is aggressive in defending the young from all intruders, a behavior not typical of herons. The defense is with a Forward, in which the bird stretches its neck forward, crest raised, wings half open, calling, and bill snapping. Chicks develop quickly and can take to branches with great agility (Sick 1993). Nesting success is little studies. Hernández and Fernández (1999) determined that it was relatively low. They inferred predation by grackles (Quiscalus). Winds of tropical storms also cause chick mortality.

Population dynamics

Nothing is known about the population biology of the species.

Conservation

The species is widespread and found in suitable habitat throughout its range. There is little information available on population sizes and status. In areas counts are 200-300 pairs. Overall it is common in places but rarely abundant (Morales 2000). It is particularly common in parts of Mexico, Costa Rica, Belize, Nicaragua, and Suriname. It is uncommon in Panama.

Research needs

Despite the interest and the studies, we still do not really understand much about the biology and ecology of this unique species. The foraging ecology of this species is not yet understood in its entirety. The role of its bill in feeding needs to be more thoroughly examined to determine its function within an annual cycle. It seems likely that there is more to this heron’s use of its large bill than is presently known. The calls and songs of this species should be explicated and functions determined, again with the involvement of the bill in mind. The demography of this species is unknown and should be examined through banding studies. An interesting result of Boat-billed Heron being understood as occupying an old and distinctive line in heron evolution is the possibility of there being unrecognized differentiation among Boat-billed Heron populations. It may be that these populations are more distinct than is now appreciated. So it would be worthwhile to further examine not only their systematic relationships but whether there are differences in their behaviour and ecology as well.

Overview

The Boat-billed Heron is a heron of a different sort, evolutionarily, behaviourly and anatomically. At least one feeding behaviour, Scoop feeding, makes use of its grandest feature. Its courtship behaviour is very different from typical herons, as is its vocal repertoire of songs and calls. It is basically a solitary to modestly aggregative, nocturnal, shallow water wading species. Its broad range, and local commonness shows it continues to be a successful species, one that suggests the potential for diversity within the family of herons.