SPECIES

Great Blue Heron Ardea herodias

Ross G. Vennesland and Robert W. Butler
Version: 1.0 — Published March 4, 2020
Text last updated April 28, 2011

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Identification

Largest heron in North America, standing about 160 cm tall, 97 to 137 cm in length, with a mass of 2.1 to 2.5 kg. Sexes similar; females average smaller. Legs and neck long. Adults have long body and occipital plumes, shorter in immatures, absent in juveniles. Middle toe has a small comb (pectinate); wings are long and rounded; bill is long and tapered, tail is short. In flight, folds its neck in a S-shape and extends its legs along the body axis. It flies with deep, slow wingbeats.

Herodius (blue) group: Upperparts gray, fore-neck is streaked with white, black, and rust-brown. Head is mostly white with a wide blue stripe running from above the eyes to the back of the head. Its eyes are yellow, and the bill is yellowish. Its legs are brownish or greenish. Adults have distinctly white crowns, whereas juveniles are solid gray. Juveniles attain some white in the crown through the preformative molt over the course of the first winter, and by the second fall their crowns are mixed gray and white. By the third fall, adult plumage is largely attained, and birds are not safely ageable except in the hand through examination of wing molt patterns.

Occidentalis (white) group: resembles Herodias group except its plumage generally is wholly white (some individuals have a few dark feathers). Juveniles lack plumes shown by adults, but are otherwise similar; not safely aged after the first winter, except through in-hand examination of wing molt patterns.

'Wurdemann's Heron' is a hybrid between the white and blue morph, and is highly variable overall. Most have grayish bodies and extensively white heads and necks.

Similar Species

The Great Blue Heron is unlikely to be confused with any other regularly occurring North American bird. It resembles the Grey Heron (Ardea cinerea) of Europe and Africa, which is found occasionally as a vagrant in the West Indies (American Ornithologists' Union 1983), except that Great Blue Herons have gray feathers with a violaceous tinge on the back and sides of the neck, and chestnut feathers on the thighs. Adult Grey Herons are slightly smaller (90–98 cm long), with a pale gray neck and white feathers on the thighs (Hancock and Kushlan 1984). Juveniles Greys lack the rusty fringed upperwing coverts of the Great Blue Heron, and have grayish thighs.

Plumages

Great Blue Herons have 10 functional primaries, 18-19 secondaries (including 3 tertials), and 12 rectrices; herons are diastataxic (see Bostwick and Brady 2002) indicating that a secondary has been lost evolutionarily between what we now term s4 and s5. Wings are broad and moderately rounded, tail is short and squared, and middle toe is pectinated. Ornamental plumes develop on the hind crown (occipital plumes), breast (pectoral plumes), and scapular area (scapular plumes). Geographic variation in appearance moderate. The following plumage descriptions pertain to the widespread North American subspecies A. h. wardi; see below and Systematics: Geographic Variation for specifics on taxonomy of distinct white Florida subspecies (or morph) "occidentalis" as well as plumage variation in up to three other recognized subspecies in North America and the Galapagos Islands. No confirmed geographic or sex-specific variation in molt strategies reported, although variation is to be expected in tropical populations due to variable environmental constraints and day-length regimes. White individuals ("occidentalis") likely require less energy to replace melanin-free feathers than dark individuals, which may lead to more extensive preformative and prebasic molts, but study is needed (Pyle 2008).

Following based primarily on plumage descriptions in Bent 1926, Roberts 1955, Palmer 1962a, Oberholser 1974, Cramp and Simmons 1977, Voisin (1991), and Kushlan and Hancock (2005); see Baker (1993, for Grey Heron) and Pyle (2008) for specific age-related criteria. Sexes show similar appearances in all plumages although maximum length of ornamental plumes appears to average longer in males than in females in breeding birds (Pyle and Howell 2004). Definitive plumage typically assumed following Third or Fourth Prebasic Molt. White-plumaged birds of Florida ("occidentalis") entirely white in all plumages, and some intermediates between white and dark-plumaged birds (sometimes referred to as Wurdemann's Herons, "A. h. wurdemanni") encountered in southeastern United States. See Systematics: Geographic Variation for more details.

Natal Down

Present Apr-Jul (Bent 1926) describes downy chick as having white (terminally) to grayish (basally) down, up to an inch long on head and back, softer and more scantily covering flanks, and absent on throat.

Juvenile (First Basic) Plumage

Present Jun-Oct. Overall tinged brownish and with looser feathers compared to adults; plumage otherwise similar to Definitive Basic except crown gray; back feathers and wing coverts with rust-brown edging, tipping, or shaft streaks; occipital, scapular, and pectoral plumes similar in structure to other feathers (i.e., not ornamental as in later plumages).

Formative Plumage

"Basic I" or "First Basic" plumage of previous authors. Present Sep-Aug. Similar to Juvenile Plumage except crown, head, and some back feathers replaced, grayer, without rust fringing; crown may have 1 cm2 white patch at 8 mo; ornamental occipital, scapular, and pectoral plumes (as in Definitive Plumage) can develop but average shorter and weaker than definitive feathers. Crown color, lack of molt patterns in wing, and retained juvenile wing coverts with rust tips or shaft streaks distinguishes Formative Plumage from subsequent Basic Plumages.

Second Basic Plumage

Present Sep-Aug. Similar to Definitive Basic Plumage but white of medial crown variably streaked with dusky; a few juvenile wing coverts often retained, faded brownish, with remnants of rust (faded to buff) tips or shaft streaks often present; 4-7 juvenile outer primaries and corresponding primary coverts and 6-12 juvenile secondaries (among s2-s4 and s6-s13) retained, brownish, and narrow, contrasting with fresher, grayer, and broader replaced secondaries and inner primaries.

Third Basic Plumage

Present Sep-Aug. Similar to Definitive Basic Plumage but white of medial crown sometimes lightly streaked with dusky; 1-2 juvenile outer primaries and corresponding primary coverts and 1-4 juvenile secondaries (among s4 and s7-s10) occasionally retained, very brownish, and narrow, contrasting with fresher, grayer, and broader replaced secondaries and inner primaries, the latter present in generations (see Pyle 2006, 2008). An unknown proportion of individuals in Third Basic Plumage have white crowns, have replaced all juvenile flight feathers, and are indistinguishable from individuals in Definitive Basic Plumage.

Definitive Basic Plumage

Present Sep-Aug. Wings, back, and sides of neck slaty gray; crown black on sides and white medially; face whitish; primaries darker than rest of wing; bend of folded wing black and cinnamon to rufous; underparts off-white to white, the front of neck streaked black, white, and rusty, the sides and flanks black, and the femoral feathering cinnamon to rusty. At height of mating season (Feb-Apr) when basic feathers fully developed, ornamental black, lanceolated, occipital plumes extend from side of crown up to 210 mm in length, grayish lanceolated scapular plumes extend over back up to 280 mm in length, and grayish to whitish, filamentous to lanceolated pectoral plumes extend below breast up to 300 mm in length (Pyle and Howell 2004). White-plumaged birds average shorter ornamental plumes (Pyle 2008). Primaries and secondaries can have one (rarely), two, or three generations of feathers in Staffelmauser patterns (Pyle 2006, 2008); these patterns (including lack of juvenile secondaries and outer primaries), lack of dusky streaks in white crown, and lack of rust-or buff-tipped body feathers and wing coverts distinguishes Definitive Basic Plumage from previous plumages.

Molts

Molt and plumage terminology follows Humphrey and Parkes (1959) as modified by Howell et al. (2003, 2004). Great Blue Heron exhibits a Complex Basic Strategy (cf. Howell et al. 2003, Howell 2010), including incomplete prebasic molts and a limited preformative molt but no prealternate molts (Bent 1926, Palmer 1962a, Oberholser 1974, Cramp and Simmons 1977 (for Grey Heron, A. cinerea), Pyle 2008; Fig. 3); see Pyle and Howell (2004) for evidence that prealternate molts, previously assigned to Great Blue Heron, are lacking in this species and other North American herons and egrets. Definitive molt cycle typically assumed following the Third Prebasic Molt.

Prejuvenile (First Prebasic) Molt

Complete, May-Aug, in the nest. No detailed information on timing or sequence of pennaceous feather irruption and development. Juvenile feathers push out down so that chicks are well feathered and are growing flight feathers by one-third grown (Bent 1926, Palmer 1962a).

Preformative Molt

"Prebasic I" or "First Prebasic" molt of previous authors. Limited to partial, Sep-Apr, primarily on non-breeding grounds. Includes most or all head and neck feathers, some back feathers and scapulars, and possibly a few proximal lesser and median upperwing secondary coverts in some individuals, but no primary coverts, primaries, secondaries, or rectrices. Formative ornamental occipital plumes can develop which are shorter than Definitive Basic plumes (cf. Pyle and Howell 2004). Molt protracted and proceeds caudally, such that larger ornamental breast feathers and scapulars often not replaced until Feb-Apr. This protracted and/or suspended over-winter has been reported to include a First Prealternate Molt but no follicles appear to be activated twice; thus only a single (Preformative) molt exists.

Second Prebasic Molt

Incomplete to complete, Mar-Nov. Similar in sequence to Definitive Prebasic Molt (below) but timing of flight-feather replacement earlier in timing due to lack of constraints related to breeding. Remiges exhibit Staffelmauser (stepwise) sequence and pattern of flight-feather replacement (Stresemann and Stresemann 1966; see Definitive Prebasic Molt); the outer 4-7 juvenile primaries (among p4-p10) and corresponding primary coverts and 6-12 juvenile secondaries (among s2-s4 and s6-s13) typically retained during the Second Prebasic Molt, to commence Staffelmauser (Pyle 2006, 2008).

Definitive Prebasic Molt

Incomplete, Apr-Dec, on or near breeding grounds. May rarely be complete, especially in white-plumaged individuals. Third Prebasic Molt may average earlier, particularly in non-breeding individuals. In breeding individuals some dropping of throat plumes may begin after eggs are laid Apr-May in British Columbia (RWB), but majority of molt occurs following commencement of flight-feather replacement in Jun. Slow growth of ornamental plumes has been mistaken for a Definitive Prealternate Molt but no follicles appear to be activated twice within the molt cycle; thus, only a single (Prebasic) molt exists.

Primaries replaced distally (p1 to p10), secondaries replaced proximally from s1 and s5 and distally from the tertials, and rectrices probably replaced distally (r1 to r6) on each side of tail, with some variation in sequence of rectrix replacement possible; flight-feather sequence as in Grey Heron (Milstein et al. 1970). Molt pattern among primaries and secondaries exhibits Staffelmauser whereby incomplete molts result in series of commencement points within primaries and secondaries, beginning with termination points of previous prebasic molt and also initiating new series commencing at p1, s1, s5, and/or the tertials. Replacement thus typically proceeds in 2-3 (rarely 1 or 4) waves through the wing, resulting in 1-3 "sets" (generations) of feathers following completion of molt (Pyle 2006, 2008). One or two juvenile outer primaries (among p9-p10) and 1-4 secondaries (among s4 and s7-s10) rarely can be retained during Third Prebasic Molt. Staffelmauser appears to be a product of insufficient time to undergo a complete wing-feather molt but has adaptive benefits in producing multiple small gaps in the wing during molt, which retains wing integrity and ability to fly and forage (Tucker 1991, Shugart and Rohwer 1996, Pyle 2005).

Bare Parts

Breeding Season

In Florida, bright orange or red bills, bright lime-green lores, bright red legs and feet, yellow iris (Meyerriecks 1960a). In Minnesota and Texas, bills mostly yellow, bright cobalt-blue lores, reddish legs; irides deeper yellow (Mock 1976). British Columbia: bright yellow bills, blue-green lores, and greenish-yellow legs (RWB). Maryland: red bills, blue lores, reddish legs (Dolesh 1984). Color fades soon after egg laying begins. Legs and bills "reddish" in Nova Scotia (Quinney and Smith 1979).

Nonbreeding Season

Great Blue and Great White in Florida have dull yellow bills, pale grayish-blue lores, and yellow irides; legs and feet yellowish-green in former, brownish or greenish-black in latter (Meyerriecks 1960a). In British Columbia, bills dorsally yellowish-brown and ventrally dull yellow, lores pale grayish-blue, legs brownish-green, feet yellow-green, irides yellow (RWB).

Measurements

Adult male Great Blue Herons are significantly larger than adult females (Table 1). 70% of adults in British Columbia can be sexed with 95% confidence using length of exposed culmen (Simpson 1984c, Butler et al. 1990).

Little information is available on Occidentalis group herons, but measurements are likely similar to those of Herodias group herons (Palmer 1962a).

See also Systematics: geographic variation. Detailed measurements in Dickerman 2004a.

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Adult Great Blue Heron, Everglades NP, FL, January.

Adult Great Blue Herons have clean white crowns, whereas juveniles and immatures are duskier gray, becoming progressively whiter with age. ; photographer Marie Reed

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Immature Great Blue Heron, Carmel, CA, 23 September.

The crown of this bird is mixed grayish and white, indicating an immature bird. This bird is likely in its second fall (possibly third). By the third fall most are indistinguishable from adults. Great Blue Herons often sit in trees, which can be surprising given their long legs and ground-foraging behavior.

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Juvenile Great Blue Heron, Princeton WMA, IA, 23 July.

Note the overall dusky grayish plumage of this juvenile Great Blue, and the solidly dark gray crown. Over the course of the first winter, the crown feathers will begin to be replaced with whiter ones, and some short ornamental plumes will develop.; photographer Ernesto Scott

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Adult Great Blue Heron (white-morph), Captiva Is., FL.

The 'Great White Heron' is generally considered a color-morph of the Great Blue (A. h. occidentalis), though some authorities suggest it is a distinct species. Where the dark and white forms overlap in Florida, intermediate birds known as 'Wurdemann's Herons' can be found; they have the grayish bodies of a Great Blue Heron, but the white head and neck of the Great White Heron.; photographer William L. Newton

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Adult 'Wurdemann's Heron', Florida Panhandle, 7 March.

'Wurdemann's Herons' are hybrids between the blue and white morph of the Great Blue Heron. They are most common in south Florida, but can range throughout the state. The following is a link to this photographer's website: http://www.flickr.com/photos/boggybayou/.

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Fig. 3. Annual cycle of breeding, molt, and migration of Great Blue Herons in British Columbia.

Coastal populations are non-migratory. Thick lines equal peak activity, thin lines off peak.

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Juvenile Great Blue Herons at the nest, Texas, April.

These birds are nearly fledged, and will leave the nest within a few days.; photographer Rick and Nora Bowers

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Great Blue Heron swallowing a large fish, Florida.

Such large prey form a small part of this species' diet in most areas.; photographer Arthur Morris

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Adult Great Blue Heron, Placida, FL, February.

Unlike several other heron species, Great Blues are not very active when foraging. Instead they stand mostly motionless, creeping close to the water before stabbing at prey items with their long bill.; photographer Arthur Morris

Recommended Citation

Vennesland, R. G. and R. W. Butler (2020). Great Blue Heron (Ardea herodias), version 1.0. In Birds of the World (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.grbher3.01