Little Blue Heron

Egretta caerulea (Linnaeus)

Ardea caerulea Linnaeus, 1758. Syst. Nat. ed. 10, 1, p.143: North America (=Carolina).

Other names: Blue Heron, Blue Gaulin in English; Garceta azul, Garza azul, Garza blanca, Garza pinta, in Spanish; Aigrette bleue, Petit héron bleu, Crabier bleu, Crabier noir, Aigrette bleue, Petit Héron Bleu in French; Blauriher in German; Garça-azul, Garça-morena in Portuguese.

Description

The adult Little Blue Heron is an all-dark medium heron, usually seen stalking with great deliberation in shallow water. Uniquely among dark herons, the juvenile is all white.

Adult: The Little Blue Heron has a grey blue body and wings. Its head is purple maroon. The distal 1/3 of the bill is black, slate grey towards the head, slightly down turned. The lores are dull green. Irises are yellow. The chin, throat and even upper neck may have some white. Neck has variable maroon cast, with a few slate blue lanceolate plumes. Legs and feet are grey green to green to grey (Rodgers 1980a).

In breeding, the head and neck turn red brown. The crest develops a tuft of long brown red lanceolate plumes. The base of the bill is cobalt blue, the tip remaining black. Iris is grey green. Lores are turquoise blue. Lanceolate plumes on the neck are red brown. Long lanceolate plumes on the back, some extending 10-15 cm beyond the tail, are slate blue. Legs and feet turn black.

Variation: The sexes are alike in color and plumage. Males are larger than females. No geographic variation is recognized (Parkes 1955), although a Mexican population is characterized as having a proportion of juveniles with chestnut grey top of head and wings (Dickerman and Parkes 1968). A great deal of individual variation occurs. The red coloration on adults is variable in extent and in color from chestnut to brown. Birds with slate bodies may have a variable amount of white on chin, and may have white heads and necks. Birds with white bodies may have grey crowns and wings, and may have white plumes. Other birds have plumes tending toward filamentous rather than lanceolate.

Juvenile: The juvenile Little Blue Heron is entirely white until into the second year. The bill is pale green blue at the base with the tip black. Iris is yellow to white. Lores are pale cobalt. Tips of the white flight feathers are dusky. Legs and feet green yellow to light grey. Molt is complicated and probably individually variable but well described (Rodgers and Smith 1995a). As white feathers are replaced by dark, the white plumage is followed by a “piebald” or “calico” intermediate stage.

Chick: Chicks are covered by white grey to brown grey down, with a crest of long hair like, tawny down. The down turns white within one week. Bill is pink grey at base, pink blue outward, and turning pinker in first week Chicks can be identified up close by their dusky wing tips. Iris is light grey. Skin, including leg and foot, is pink grey to grey.

Voice: The Little Blue Heron is generally a quiet bird; its vocalizations have been well documented (Rodgers and Smith 1995a). “Scah” and “Sken” calls are the threat and aggressive calls, given during alert, defense or attack. The “Gwa” call is the flight call. A repeated “Gerr” call is the disturbance call. “Unh” call is used in the Stretch display. The “Ehoo” call, rendered “Eh, oo, ah, eh, eh”, is the Greeting Ceremony call. The nestling begging call is a repeated “Hee”. Bill Snap is part of the Stretch.

Weights and measurements: Length: 64-74 cm. Weight: 296-410 g.

Field characters

The Little Blue Heron is identified by its solid blue or white color, green to green grey legs (except in breeding), down curved bill, and feeding method. It is distinguished from the dark Reddish Egret by being smaller and thinner, having down curved bill, and feeding slowly. It is distinguished from the Green Heron by lack of dark crown, larger size, relatively long legs and neck. It is distinguished from the Tricolor Heron by its dark belly. The immature Little Blue Heron is distinguished from the Snowy Egret (and white Little Egret) by its grey and black (not all black) down curved bill, by blue grey (not yellow in Snowy) lores, by dusky green grey (not black) legs in non-breeding, by its green (not yellow) feet in all seasons, by dusky tipped primaries (usually not visible), and by its slow feeding behavior. It is distinguished from the white Reddish Egret by being smaller, thinner, slighter, with green (not blue) legs and feet, grey (not flesh) bill base, and by feeding slowly. It is distinguished from the Great White Egret and white Great Blue Heron by being much smaller.

Systematics

The Little Blue Heron is one of the medium herons, and appears from morphological and biochemical evidence to be closely related to the Snowy Egret. Subspecific variation has not been identified (Parkes 1955).

Range and status

The Little Blue Heron occurs through much of the Western Hemisphere in North America, Central America, West Indies, and South America.

Breeding range: The normal breeding range in North America is from south Maine, along the Atlantic coast (more common south of New Jersey) along the Gulf Coast through east Mexico, nesting in a few locations south to Belize. It nests inland regularly in the south east United States and up the Mississippi River Valley to Illinois, Indiana, west inland to Kansas, Oklahoma, and Texas. In the west it nests patchily in central and south California and Arizona, coastal Baja California, west coast of Mexico, inland in Jalisco and Guanajuato. It nests in Central America, west South America to Lima, Peru, and central and east South America south Brazil.

It also nests sporadically patchily outside this range in central North America - Arizona, New Mexico, Colorado (Ryder et al. 1989), South Dakota, North Dakota (Naugle et al. 1996), Iowa (Silcock 1984), Minnesota. Throughout its range, the Little Blue Heron occurs in breeding season in places where breeding has not been documented, including inland United States, West Indies, and Central America.

Nonbreeding range: In North America, the range during the nonbreeding season is along the coast from New Jersey (more commonly Virginia), south throughout peninsular Florida, Bermuda, and West Indies to Lesser Antilles, Trinidad, and Netherlands Antilles, along the Gulf of Mexico in the United States and Mexico, in south east California, Baja California, Mexico, through Central America to Panama, to Peru, Brazil, Uruguay, and north Argentina (Contreras 1993, Morales 2000).

Migration: The northern populations of the Little Blue Heron are migratory, wintering as far as north South America (Dusi 1967, Mikuska et al. 1998). In North America, eastern nesting birds move south in September and October along the coast to Florida, to West Indies and South America. Birds nesting inland migrate south to the Gulf coast and follow the coast to Mexico, Central America, to north South America some crossing between Yucatan and Cuba, the other Greater Antilles, Bahamas, and north South America (Colombia, Venezuela, Guyanas). Spring return migration is March–April. Evidence suggests migrations occur in South America as well (Morales 2000).

Prior to migration, post breeding dispersal is a typical feature of this species, at least in North America. Birds move in all directions, but especially northward before beginning southward migration. The Little Blue Heron is the one of the most dispersive of North American herons, with a mean dispersal distance of over 1,000 km (Melvin et al. 1999). Sightings outside the usual range are common. Birds occur across south Canada from Newfoundland west to south east Saskatchewan, south through the central United States, except the Rocky Mountains and Great Basin, and on the Pacific coast north to British Columbia (Innes 1993). In the south of its range, accidentals occur south of the breeding range in Argentina (Contreras 1993). Accidental sightings have occurred in Hawaii, west Greenland, the Azores, and an exceptional observation in South Africa (Bennett 1992, Brooke and Parkin 1995).

Status: It is thought that the Little Blue Heron did not suffer directly from the plume trade as other species did and so its population was not reduced at the beginning of the 20^th Century. Populations in south east United States were estimated to be about 135,000 birds in the 1970-80’s (Butler et al. 2000). This species is hard to census, however, and there are few data on trends. As far as is known, there have been no significant range changes this Century. There is some indication of local decreases in the north east United States. Population and range are increasing in Baja California (Massey and Palacios 1994). The population appears to be stable overall in both North and South America (Butler et al. 2000, Morales 2000).

Habitat

The Little Blue Heron feeds in many types of shallowly flooded habitats. It uses freshwater and estuarine marshes, tidal flats, salt marshes, mangrove swamps, coastal lagoons, rivers, streams, ponds, lakes reservoirs, canals, ditches, aquacultural facilities, rice fields, and flooded grasslands. It particularly feeds in or on emergent or floating vegetation, such as rush (Eleocharis), grass (Panicum), or cord grass (Spartina). The essential habitat requirements include vegetated areas of shallow water for foraging and isolated forest patches for nesting. It is generally a bird of the lowlands, but occurs to 3,000 m, even at 3,700 m in the Andes.

Foraging

The foraging ecology and food habits of this species are well known (Meanley 1955, Jenni 1969, Recher and Recher 1969, Mock 1974, Kushlan 1976a, 1978c, Willard 1977, Domby and McFarlane 1978, Rodgers 1975, 1982, 1983, Caldwell 1981, Scott and Powell 1982, Burchsted 1983, Niethammer and Kaiser 1983, Burchsted 1983, Kent 1983, Bancroft et al. 1994, Parsons 1995, Miranda and Collazo 1997). The Little Blue Heron typically feed diurnally, alone or in small groups, less frequently in large groups. When feeding with flocks, it often is toward the outside of the group. Commensal feeding is common using beaters as diverse as ibises and manatee, and it very typically follows Snowy Egrets (Kushlan 1978c, Caldwell 1981, Scott and Powell 1982).

It hunts usually by Walking very Slowly through shallow waters while carefully watching for potential prey. In one study, it spent 73% of its time in slow feeding (Willard 1977). It proceeds in a slow, deliberate, methodical way, Walking then Peering Over and continuing to Walk. It will stop and use Standing, but not as frequently as Walking. It frequently uses its feet to stir up prey by Foot Raking. Less frequently it uses more active behaviors such as Walking Quickly, Running, Hopping, Wing Flicking, and Open Wing Feeding. In a variation of Open Wing feeding, called Whirling, the heron will spin around, head under its wing open looking in the water (Burchsted 1983). It occasionally uses aerial feeding techniques, such as Hovering over deeper water (Mock 1974, Rodgers 1975).

It often defends its individual feeding area, not only from other Little Blue Herons, but other species such as terns. It uses Forward, Supplanting Runs and supplanting Flights. It robs prey and it robbed by other species when it catches a prey item with high handling time (Clark et al. 1989).

Several species of herons have distinct juvenile plumage, but none starts white and ends dark, the two major color schemes of the typical herons. The selective advantage remains unclear. Juveniles in some circumstances forage less effectively than do adults, but not always. White birds are attacked more by hawks but less by other egrets and are allowed to feed closer to Snowy Egrets, can feed in brighter situations, and have reduced heat stress (Mock 1980, Ellis 1980, Caldwell 1981, 1986). The most likely explanation is that white and dark birds choose to feed in different habitats by different methods and with a differing degree of sociality, but these ideas need to be definitively studied.

The Little Blue Heron’s diverse diet typically includes fishes, amphibians, and insects, the proportions of which differ among feeding areas. Fish predominate in coastal east North America; in other areas it appears to be a frog specialist, in other places crabs or shrimp (Miranda and Collazo 1997, Smith 1997). Invertebrates are important in the diet, especially crayfish, shrimp, and crabs but also isopods, worms, crabs, beetles, flies, dragonflies, bugs. Prey taken typically are bottom dwelling or slow moving species.

Breeding

The breeding biology of the Little Blue Heron is fairly well understood (Meanley 1955, Maxwell and Kale 1995, Rodgers 1980a, b, c, d, 1987, Werschkul 1979, 1982, Frederick and Collopy 1989a, c, Smith 1995a, Smith and Collopy 1995, Naugle et al. 1996, Miranda and Collazo 1997). Time of nesting varies with peak nesting in May in north east United States, April in south east United States, February–August in Trinidad and July–October in Venezuela (Rodgers and Smith 1995a). In an area, Little Blue Heron breeding chronology is consistent from year to year, despite differing environmental conditions (Rodgers 1987).

The Little Blue Heron nests in trees and bushes, especially on islands. Nest is placed variably from 0.5 to 12 m high. Plants used include cypress, mangrove, pine (Pinus), button bush (Cephalanthus), willow (Salix), cat tail (Typha), bamboo, cactus, and many more (Rodgers and Smith 1995a). The type of plant, and to some extent the vegetative structure appears to be relatively unimportant to this species. More important, probably, is the protection offered by the site. It will use artificial islands, such as those created from dredge spoil.

The Little Blue Heron is a gregarious, colonial breeder, usually nesting in intraspecfic groups and with other waterbird species. There is some suggestion that nesting numbers are adversely affected by the numbers of co-nesting Cattle Egrets (Dusi 1968, Burger 1978a, b, Dusi and Dusi 1995).

The nest is a platform with a slight depression, built of sticks and other material reflecting local availability, as the male does not wander far from the nest to collect material. The nest is often small, 30-45 cm wide and 21 cm deep. The male starts the nest, collects most of the nest material and presents it to the sitting female.

The male selects a display site, which it defends using Bill Thrusts and the Forward, which has a Bill Snap component. It advertises from the site, which initially is quite large, contracting as courtship progresses. This leaves space for new birds to set up territories. As a result, Little Blue Heron colonies can be asynchronous within a neighborhood, very different from the Snowy Egret for example.

The courtship behavior of the Little Blue Heron is relatively simple compared to other egrets. The principal male advertising display is the Stretch, first given soon after the territory is occupied. The bird erects head, back and neck plumes, orients the bill upward to the vertical, lunges upward, gives a Bill Snap, drops down with drooping wings, and then rises up, extends the bill, and gives the soft “Unh” call, which may not be easily heard.

Paired displaying birds also use Standing, Bill Clappering, Wing Preen displays. Bill Clappering is appeasement and pair strengthening display. Circle Flights are rare. A Snap display was not seen in the definitive study (Rodgers 1980a). After pairing, they engage in mutual rubbing, allopreening, and Bill Clappering (called bill-nibbling by Rodgers 1980a), the latter sounding like a rattling chatter. The Greeting Ceremony is used in nest relief. The “Ehoo”call is given at a distance of 5-10 m as the arriving birds walks to the nest.

Copulation usually takes place on the nest, and promiscuous mating behavior is characteristic of the species. These extra-marital copulations occur when the male intrudes onto a neighboring female’s nest site, usually when her mate is away. A female at the nest may mate with several males within a short period of time, for example 16 with 3 males in 7 h. Extrapair copulations may be forced. Males have to protect their reproductive investment by guarding their nest and mate. From pairing until the final egg is laid, the male does not feed and is seldom away from the nest site for longer than five minutes, and then only to collect nest material.

The plumage distinction between juvenile and adult Little Blue Herons suggests that adult plumage is required for successful breeding. Rodgers (1978a), however, found that small numbers of subadults, mostly males, do attempt to breed.

The eggs are pale blue, 41-51 x 31-36 mm. The clutch is 3 or 4 eggs, range 1-6 eggs, laid over a period of several days, up to eight days for large clutches. Replacement clutches can follow within 2.5-4 weeks. Incubation usually begins with the second egg or later and lasts for 21-23 days. Both parents incubate. Rodgers (1978b), however, observed an incubating heron remove four eggs from its nest by carrying each crosswise in its mandibles, presumably to deposit them in a new nest.

Chicks hatch at 1 to 2 day intervals. Asynchronous hatching of eggs leads to brood reduction. The legs and feet of hatchlings grow rapidly, which allows them to be ambulatory by the time they are 13 days old. At this time both parents can leave the nest to forage. Both parents feed the young. For the first five days food is regurgitated onto the nest floor. Thereafter young take the regurgitated food by grasping the adult’s bill crosswise. After 21 days the young are independent of parental care except for feeding, which continues until they are 50 days old

The nesting success of Little Blue Herons is somewhat variable, but overall fairly high. Survivorship is 73% from egg to 2 weeks. Overall survivorship to independence is about 50%, high for medium egrets. 1-3 young are produced per nest. Nestling mortality is usually the result of younger chicks starving, leading to a modest brood reduction. Predation is relatively low and random in occurrence (Rodgers and Smith 1995a).

Population dynamics

Given its maturation is visible; it is known that Little Blue Herons tend to nest early. Most start when they are approaching their second birthday. However, first year birds do attempt to nest, in one colony site, up to 60% of the displaying birds but fewer than 2% of the nesting herons were juveniles. Nothing is known about adult mortality. Longevity is nearly 14 years (Clapp et al. 1982).

Conservation

This species, even if declining locally, is not of global conservation concern. Killing of Little Blue Herons at fish farms, legally or illegally, may impact local populations if not managed from a population viability perspective. Contaminant burdens have not proven to be excessive, although there may be heavy metal effects on reproduction (Spahn and Sherry 1999). Probably the most crucial conservation concern is the protection and management of nesting and feeding sites. Degradation and destruction of feeding habitat, particularly wetlands, on both breeding and non-breeding grounds, is occurring throughout the range. Important colonies need to be protected with buffer zones to inhibit disturbance. Dredge material islands should be designed and managed to enhance their use as colony sites.

Research needs

New methods for monitoring nesting Little Blue Herons need to be developed and monitoring programmes established to assess breeding population size and trend, especially in its North American range. Range-wide surveys should be conducted to identify important feeding, nesting, and wintering sites. Studies of the wintering biology of the species should be undertaken. The survival of postfledging juveniles and adults needs to be better studied in order to understand the demography of the species. Populational effects of nesting competition between Little Blue Herons and Cattle Egrets should be re-examined. The adaptive and ecological consequences of age-related plumage dimorphism need definitive examination. Patterns of subspecific population structure should be determined using biochemical methods.

Overview

The Little Blue Heron is a specialist in shallow water stalking, and it benefits from being alone where it can walk very slowly along taking care not to scare its potential prey, or by feeding commensally following another animal capturing the prey it disturbs. Its engagement in feeding aggregations apparently is an extension of its commensal feeding. Although its has a large repertoire of active feeding techniques, it hardly uses them. The fundamental adaptive strategy of this species appears to be minimizing energy use, which may account for its slow movement, its use of commensals, its subsistence on small sized prey, and its surprisingly low metabolic rate. An energy conservation approach to life allows it to catch fewer, smaller, more diverse prey than an active forager like a Snowy Egret. The resulting diet is both globally broad but regionally and temporally distinctive. Fish, crayfish, or frogs can each dominate the diet in different places or at different times. It chooses its diet to match the situation, so that in each area or each ecological situation, it tends to be locally specialized, concentrating on a particular subset of available prey, likely those that are most efficiently caught by its slow, deliberate feeding approach.