The
great blue heron (
Ardea herodias) is a large wading
bird in the
heron family
Ardeidae, common near the shores of
open water and in
wetlands over most of
North America and
Central America as well as the
Caribbean and the
Galápagos Islands. It is a rare vagrant to Europe, with records from Spain, the
Azores,
England and the Netherlands. An all-white population found only in the
Caribbean and southern Florida was once treated as a separate species
and known as the
great white heron.
Contents
Taxonomy
The great blue heron was one of the many species originally described by
Carolus Linnaeus in his 18th century work,
Systema Naturae.
[2]
The great blue heron is replaced in the
Old World by the very similar
grey heron (
Ardea cinerea),
which differs in being somewhat smaller (90–98 cm (35–39 in)), with a
pale gray neck and legs, lacking the browner colors that great blue
heron has there. It forms a
superspecies with this and also with the
cocoi heron from
South America, which differs in having more extensive black on the head, and a white
breast and neck.
There are five
subspecies:
[3]
- Ardea herodias herodias Linnaeus, 1758. Most of North America, except as below.
- Ardea herodias fannini Chapman, 1901. The Pacific Northwest from southern Alaska south to Washington; coastal.
- Ardea herodias wardi Ridgway, 1882. Kansas and Oklahoma to northern Florida. Sightings in southeastern Georgia have occurred.
- Ardea herodias occidentalis Audubon, 1835. Southern Florida, Caribbean islands. Formerly known as a separate species, the great white heron.
- Ardea herodias cognata Bangs, 1903. Galápagos Islands.
Description
It is the largest North American
heron and, among all extant herons, it is surpassed only by the
Goliath heron (
Ardea goliath) and the
white-bellied heron (
Ardea insignis).
It has head-to-tail length of 91–137 cm (36–54 in), a wingspan of
167–201 cm (66–79 in), a height of 115–138 cm (45–54 in), and a weight
of 1.82–3.6 kg (4.0–7.9 lb).
[4][5][6][7] In
British Columbia, adult males averaged 2.48 kg (5.5 lb) and adult females 2.11 kg (4.7 lb).
[8] In
Nova Scotia and
New England, adult herons of both sexes averaged 2.23 kg (4.9 lb),
[9] while in
Oregon both sexes averaged 2.09 kg (4.6 lb)
[10] Thus, great blue herons are roughly twice as heavy as
great egrets (
Ardea alba), although only slightly taller than them, but can themselves weigh about half as much as a large
Goliath heron.
[11] Notable features of great blue herons include slaty
flight feathers,
red-brown thighs, and a paired red-brown and black stripe up the
flanks; the neck is rusty-gray, with black and white streaking down the
front; the head is paler, with a nearly white face, and a pair of black
plumes running from just above the eye to the back of the head. The
feathers on the lower neck are long and plume-like; it also has plumes
on the lower back at the start of the breeding season. The bill is dull
yellowish, becoming orange briefly at the start of the breeding season,
and the lower legs gray, also becoming orangey at the start of the
breeding season. Immature birds are duller in color, with a dull
blackish-gray crown, and the flank pattern only weakly defined; they
have no plumes, and the bill is dull gray-yellow.
[3][12][13] Among standard measurements, the wing chord is 43–49.2 cm (16.9–19.4 in), the tail is 15.2–19.5 cm (6.0–7.7 in), the
culmen is 12.3–15.2 cm (4.8–6.0 in) and the tarsus is 15.7–21 cm (6.2–8.3 in).
[14][15]
The heron stride is around 22 cm (8.7 in), almost in a straight line.
Two of the three front toes are generally closer together. In a track
the front toes as well as the back often show the small talons.
[16]
The subspecies differ only slightly in size and plumage tone, with the exception of subspecies
occidentalis, which as well as normal colored birds, also has a distinct white
morph, known as the
great white heron (not to be confused with the
great egret, for which "great white heron" was once a common name). It is found only in south
Florida and some parts of the
Caribbean.
The great white heron differs from other great blues in bill
morphology, head plume length, and in having a total lack of pigment in
its plumage. It averages somewhat larger than the sympatric race
Ardea herodias wardi and may be the largest race in the species. In a survey of
A. h. occidentalis in
Florida,
males were found to average 3.02 kg (6.7 lb) and females average
2.57 kg (5.7 lb), with a range for both sexes of 2 to 3.39 kg (4.4 to
7.5 lb).
[4]
This is mainly found near salt water, and was long thought to be a
separate species. Birds intermediate between the normal morph and the
white morph are known as
Würdemann's heron; these birds resemble a "normal" great blue with a white head.
The theory that great white heron may be a separate species (
A. occidentalis) from great blue heron has again been given some support by
David Sibley.
[17]
Voice
Their call is a harsh croak. The heron is most vocal during the
breeding season, but will call occasionally at any time of the year in
territorial disputes or if disturbed.
Nonvocal sounds include a loud bill snap, which males use to attract a
female or to defend a nest site and which females use in response to
bachelor males or within breeding pairs.
[18] The bill snap may be analogous the territorial song of passerines.
[18] Bill clappering, the rapid chattering of the tips of the bill, is very common between paired herons.
[18]
Similar species
The "great white heron" could be confused with
great egret but is larger, with yellow legs as opposed to the great egret's black legs. The
reddish egret (
Egretta rufescens) and
little blue heron (
Egretta caerulea)
could be mistaken for the great blue heron, but are much smaller, and
lack white on the head and yellow in the bill. In the southern reaches
of its range, the great blue sometimes overlaps in range with the
closely related and similarly sized
cocoi heron (
Ardea cocoi).
The cocoi is distinguished by a striking white neck and solid black
crown, but the duller juveniles are more easily confused. More
superficially similar is the slightly smaller
grey heron,
which may sometimes vagrate to the Northern coasts of North America.
The grey heron (which occupies the same ecological niche in
Eurasia
as the great blue heron) has very similar plumage but has a solidly
soft-gray neck. Erroneously, the great blue heron is sometimes referred
to as a "crane".
Distribution and habitat
Flying with nesting material in
Illinois, USA
The great blue heron is found throughout most of North America, as far north as
Alaska and the southern
Canadian provinces. The range extends south through
Florida,
Mexico and the
Caribbean to
South America. Birds east of the
Rocky Mountains in the northern part of their range are
migratory and winter in
Central America or northern
South America. From the
southern United States southwards, and on the
Pacific coast, they are year-round residents.
[3]
However their hardiness is such that individuals often remain through
cold northern winters, as well, so long as fish-bearing waters remain
unfrozen (which may be the case only in flowing water such as streams,
creeks and rivers).
The great blue heron can adapt to almost any wetland habitat in its range. They may be found in numbers in fresh and saltwater
marshes,
mangrove swamps, flooded meadows, lake edges, or
shorelines.
They are quite adaptable and may be seen in heavily developed areas as
long as they hold bodies of water bearing fish. Great blue herons rarely
venture far from bodies of water but are occasionally seen flying over
upland areas. They usually nest in trees or bushes near water's edge,
often on island (which minimizes the potential for predation) or
partially isolated spots.
[19]
It has been recorded as a
vagrant in England,
[20] Greenland,
Hawaii, and the
Azores.
[3]
Behavior
Diet
Eating a small fish, the main prey.
On a slow-flying glide
The primary food for great blue heron is small
fish, though it is also known to opportunistically feed on a wide range of
shrimp,
crabs, aquatic
insects,
rodents and other small
mammals,
amphibians,
reptiles, and small
birds. Primary prey is variable based on availability and abundance. In
Nova Scotia, 98% of the diet was
flounders.
[9] In
British Columbia, the primary prey species are
sticklebacks,
gunnels,
sculpins and
perch.
[21] Californian herons were found to live mostly on
sculpin,
bass,
perch,
flounder and top
smelt.
[22] Non-piscivore prey is rarely quantitatively important, though one study in
Idaho showed that from 24 to 40% of the diet was made up of
voles.
[23]
Herons locate their food by sight and usually swallow it whole. Herons have been known to choke on prey that is too large.
[24][25] It is generally a
solitary
feeder. Individuals usually forage while standing in water but will
also feed in fields or drop from the air, or a perch, into water. Mice
are occasionally predated in upland areas far from the species' typical
aquatic environment.
[19] Occasionally loose feeding flocks form and may be beneficial since they are able to locate schools of fish more easily.
[19]
As large wading birds, great blue herons are capable of feeding in
deeper waters and thus are able to harvest from niche areas not open to
most other heron species. Typically, the great blue heron feeds in
shallow waters, usually less than 50 cm (20 in) deep,
[19]
or at the water's edge during both the night and the day, but
especially around dawn and dusk. The most commonly employed hunting
technique of the species is wading slowly with its long legs through
shallow water and quickly spearing fish or
frogs
with its long, sharp bill. Although usually ponderous in movements, the
great blue heron is adaptable in its fishing methods. Feeding behaviors
variably have consisted of standing in one place, probing, pecking,
walking at slow speeds, moving quickly, flying short distances and
alighting, hovering over water and picking up prey, diving headfirst
into the water, alighting on water feet-first, jumping from perches
feet-first, and swimming or floating on the surface of the water.
[19]
Breeding
At the nest
This species usually breeds in
colonies,
in trees close to lakes or other wetlands. Adults generally return to
the colony site after winter from December (in warmer climes such as
California and
Florida) to March (in cooler areas such as
Canada).
Usually colonies include only great blue herons though sometimes they
nest alongside other species of herons. These groups are called
heronry (a more specific term than "
rookery").
The size of these colonies may be large, ranging between 5–500 nests
per colony, with an average of approximately 160 nests per colony.
Heronry are usually relatively close, usually within 4 to 5 km (2.5 to
3.1 mi), to ideal feeding spots.
[19]
Heronry sites are usually difficult to reach on foot (e.g., islands,
trees in swamps, high branches, etc.) in order to protect from potential
mammalian predators. Trees of any type are used when available. When
not, herons may nest on the ground, sagebrush, cacti, channel markers,
artificial platforms, beaver mounds and duck blinds. Other waterbirds
(especially smaller
herons) and, occasionally, even fish and mammal-eating raptors may nest amongst colonies.
[26][27]
Although nests are often reused for many years and herons are socially
monogamous within a single breeding season, individuals usually choose
new mates each year.
[18] Males arrive at colonies first and settle on nests, where they court females; most males choose a different nest each year.
[18] Great blue herons build a bulky stick
nest.
Nests are usually around 50 cm (20 in) across when first constructed,
but can grow to more than 120 cm (47 in) in width and 90 cm (35 in) deep
with repeated use and additional construction.
[28] If the nest is abandoned or destroyed, the female may lay a replacement
clutch.
Reproduction
is negatively affected by human disturbance, particularly during the
beginning of nesting. Repeated human intrusion into nesting areas often
results in nest failure, with abandonment of eggs or chicks.
The female lays three to six pale blue
eggs.
Eggs can measure from 50.7 to 76.5 mm (2.00 to 3.01 in) in length and
29 to 50.5 mm (1.14 to 1.99 in) in width, though the smallest eggs in
the above sample may have been consider "runt eggs" too small to produce
viable young. Egg weigh range from 61 to 80 g (2.2 to 2.8 oz).
[29] One
brood is raised each year. First broods are laid generally from March to April.
[30][31] Eggs are usually laid at 2 day intervals,
incubated for around 27 days and hatch asynchronously over a period of several days.
[18]
Males incubate for about 10.5 hours of each day while females usually
incubate for the remainder of each day and the night, with eggs left
without incubation for about 6 minutes of each hour.
[18]
The first chick to hatch usually becomes more experienced in food
handling and aggressive interactions with siblings, and so often grows
more quickly than the other chicks.
[32] Both parents feed the young at the nest by
regurgitating
food. Parent birds have been shown to consume up to four times as much
food when they are feeding young chicks (about 4300 kJ/day) than when
laying or incubating eggs (about 1200 kJ/day).
[18] By the time they are 45 days old, the young weigh 86% of the adult's mass.
[33]
After about 55 days at the northern edge of the range (Alberta) and 80
days at the southern edge of the range (California), young herons take
their first flight.
[18]
They will return to the nest to be fed for about another 3 weeks,
following adults back from foraging grounds and are likely to gradually
disperse away from their original nest over the course of the ensuing
winter.
[18]
Young herons are not as successful at fish capture as adults, as strike
rates are similar but capture rates about half that of adults during
the first 2 months post-fledging.
[18]
Predation
Predators of eggs and nestlings include
turkey vultures (
Cathartes aura),
common ravens (
Corvus corax) and
American crows (
Corvus brachyrhynchos).
Red-tailed hawks (
Buteo jamaicensis),
American black bears (
Ursus americanus) and
raccoons (
Procyon lotor) are known to take larger nestlings or fledglings and, in the latter predator, many eggs.
[8][34][35][36]
Adult herons, due to their size, have few natural predators, but a few
of the larger avian predators have been known to kill both young and
adults, including
bald eagles (
Haliaeetus leucocephalus) (the only predator known to attack great blue herons at every stage of their life-cycle from in the egg to adulthood),
golden eagles (
Aquila chrysaetos) and, less frequently,
great horned owls (
Bubo virginianus) and
Harris's hawks (
Parabuteo unicinctus).
[37][38][39][40][41] An occasional adult or, more likely, an unsteady fledgling may be predaceously snatched by an
American alligator (
Alligator mississippiensis) or an
American crocodile (
Crocodylus acutus).
Using their considerable size and dagger-like bill, a full-grown heron
can be a formidable foe to a predator. In one instance, during an act of
attempted predation by a golden eagle, a heron was able to mortally
wound the eagle although itself succumbed to injures sustained in the
fight.
[42]
When predation on an adult or chick occurs at a breeding colony, the
colony can be abandoned by the other birds, but this does not always
occur. The primary source of disturbance and breeding failures at
heronries is human activities, mostly through human recreation or
habitat destruction, as well as by egg-collectors and hunters.
[21][43]
In art
John James Audubon illustrates the great blue heron in
Birds of America,
Second Edition (published, London 1827–38) as Plate 161. The image was
engraved and colored by Robert Havell's, London workshops. The original
watercolor by Audubon was purchased by the
New-York Historical Society where it remains to this day (January 2009).
Gallery
-
Wading at Grande Lakes Audubon Cooperative Wildlife Sanctuary Orlando, FL
-
-
Profile of the head of a great blue heron. The sharp bill is useful for spearing fish and frogs
