Great Blue Heron Ardea herodias
Version: 1.0 — Published March 4, 2020
Text last updated April 28, 2011
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At some northern latitudes, adult Great Blue Herons gather in flocks on the ground for several days in spring before entering colony sites (“gathering grounds” sensu Mock 1976). The species is not known to occupy gathering grounds in Texas or Michigan; in some regions, individuals roost at colony-sites year-round (Cottrille and Cottrille 1958, Mock 1976). The function of gathering grounds is unclear.
Little published information on dates of pair formation. Dates of the earliest adult Herodias group herons returning to colony-sites are as follows: California, Dec (Brandman 1976); Texas, Jan (Mock 1976); British Columbia, mid-Jan on the coast, late Mar at interior sites (Butler et al. 1986, RWB, Cannings et al. 1987, Vennesland 2000); Pennsylvania, mid-Feb (E. Bruckner pers. comm.); Ohio, mid-Feb to mid-Mar (Burkholder and Smith 1991); and Alberta, late Mar (Vermeer 1969d). Herons in some colonies are apt to depart from colony sites with little provocation at this early point in the nesting cycle (Vos et al. 1985, Butler 1995, Vennesland 2010). Herons may be inhibited from displaying to each other by high winds and low temperatures (Palmer 1962a). Courtship to egg laying occurs from early Jan to mid-Mar in California (Brandman 1976); and mid-Feb to early Apr in British Columbia (Vennesland 2000, RWB). Occidentalis group herons breed year-round, but most begin between Sep and Feb (Powell 1983).
First Brood Per Season
Figure 3. Dates for first eggs being laid by Great Blue Herons include: late Feb in Oregon (Henny and Bethers 1971) and northern California (Pratt 1970, Pratt and Winkler 1985); early Mar in southern California (Brandman 1976); 3rd week of Mar in Idaho (Collazo 1981); late March and April in Ohio (Burkholder and Smith 1991); Mar to early Apr in coastal British Columbia (Butler 1989, Vennesland 2000); mid-Apr in Nova Scotia (McAloney 1973, Quinney 1983); and late Apr in Alberta (Vermeer 1969d).
Second Brood Per Season
It is unclear how many late-laid clutches are first or second attempts for Great Blue Herons. In California, 2 pairs renested after fledging their first brood, and 5 pairs renested after the death of nestlings (Brandman 1976). In a study of 35 breeding colonies, Vennesland (2000) found that 25% of 208 failed nests were re-used by the same or a different pair, and that late nesting pairs produced fewer fledglings than early nesting pairs. One study found that most herons chose new mates after the failure of first nesting attempts (Simpson 1984c). In British Columbia, the nesting season is longer (about 4 months) than needed biologically (about 3 months; Vennesland 2000), providing time for re-nesting attempts. However, there likely is insufficient time to raise 2 broods at northern latitudes.
Great Blue Herons nest as single pairs, but mostly in colonies (Figure 4) that are typically < 500 nests (Butler 1997, Gibbs and Kinkel 1997, Kelly et al. 2007), but in at least one case > 1,000 pairs (Witt 2006). The number of nests has been positively related to the area of nearby foraging habitat in British Columbia (Butler 1991, Knight 2010), Oregon (Werschkul et al. 1977, Bayer and McMahon 1981), Maine (Gibbs et al. 1987b, Gibbs 1991), and Illinois (Gibbs and Kinkel 1997). In some regions, the location of colony sites has been best explained by the distribution of foraging habitats (coastal British Columbia - Butler 1991, Knight 2010; Maine - Gibbs 1991; and Illinois - Gibbs and Kinkel 1997), but not in the San Francisco Bay area (Kelly et al. 1993).
Colony sites are located on average between 2.3 to 6.5 km from their primary foraging location (Doyd and Flake 1985, Butler 1991, Custer and Galli 2002, Knight 2010). Other important factors determining nest site selection include buffering habitat to human activity (vegetation or water), lower road density and larger woodlands (Parker 1980, Gibbs and Kinkel 1997), but sometimes herons nest closer to humans than expected by chance, presumably to take advantage of foraging opportunities (Knight 2010).
Colony site selection is also predator-driven. Where mammalian predators are common, herons usually select nest sites that are difficult to reach (e.g., islands, trees in swamps, high branches, etc). On the coast of British Columbia, nests are placed so that they are difficult to identify from the air but are easy to identify from ground level, presumably because mammalian predators are rare but aerial ones are common (RGV). In some cases, colony locations are chosen in close proximity to Bald Eagle territories, likely to take advantage of the territoriality of the resident eagles (Jones 2010; see also Conservation: effects of predator activity).
The recovery of beaver (Castor canadensis) populations in the ne. USA and s. Canada may have benefited this heron by providing a patchwork of large and small swamps and wet meadows, potential nesting and foraging areas (e.g., N.Y. State; Andrle and Carroll 1988). Large colonies have been shown to persist at a location longer than small colonies (Kelly et al. 2007, RGV unpubl. data).
Within nesting sites, male herons typically arrive in colonies and settle on nests, where they court females (Brandman 1976, Mock 1976). Nest site fidelity is apparently weak, with one study reporting that 13 of 14 marked individuals chose different nests the following year (Simpson 1984c). However, fidelity to the choice of tree species within colonies can be strong (Kelsall and Simpson 1980). Some colonies have long histories of use: 71 yr in Stanley Park, British Columbia (RWB), 37 yr in Minnesota (M. Partch pers. comm.). Many colonies are abandoned after only a few years of use (RGV unpubl. data).
Nests (Herodias group) are found mostly in trees, up to 30 m or more above ground. Nesting sites in various locations, including lowland swamps, upland forests (hardwood or conifer), on islands, by forest-bordered lakes and ponds, and riparian woodlands (Kelly et al. 1993, Butler 1997, Gebauer and Moul 2001, Kelly et al. 2007). Where trees are not available, herons nest on the ground (usually only on predator-free islands; Taverner 1926, Gill and Mewaldt 1979, Peresbarbosa and Mellink 2001), in bullrushes, on shrubs (Behle 1958, Vermeer 1969d, Kelly et al. 1993, Vennesland 2000), cacti (Rosenberg et al. 1991), sagebrush (Blus et al. 1980), mangroves (generally islands; Powell 1983), duck blinds (Palmer 1962a, Kelly et al. 1993), channel markers (Henny and Kurtz 1978, Blus et al. 1980), and artificial nest platforms (Sandilands 1980). Great Blue Herons nest with other species of herons (Custer et al. 1980b), waterbirds, and even eagles, hawks, owls, and vultures (Mengel 1965b, Ryser 1985, Simpson et al. 1987, Vennesland 2000, Jones 2010). In Oregon colonies, nest density varied from 39 to 750 nests/ha (Bayer and McMahon 1981). Heron colony sites are located in all manner of habitats, from pristine wetlands and woodlands, to highly developed areas such as urban centers (Butler 1997, Kelly et al. 2007, Knight 2010).
Most Occidendalis group herons in Florida nest in small colonies on isolated mangrove islands (Powell and Powell 1986) with Herodias group herons, Roseate Spoonbills and Reddish Egrets also present (Powell et al. 1989).
Great Blue Herons build nests primary from sticks (Butler 1997), but when nesting on the ground can also use other materials such as Salt Grass (Peresbarbosa and Mellink 2001). Herons gather sticks and other nesting materials from the ground, nearby trees and shrubs, or from unguarded and abandoned nests (including nests of their predators such as Bald Eagles; RGV). Stick gathering is done primarily by males and placed on the nest primarily by females (Cottrille and Cottrille 1958, Palmer 1962a, Mock 1976). Nest building can take from 3 d to 2 wk, and completed nests consist of a platform of twigs with a saucer-shaped interior lined with pine needles, moss, reeds, dry grass, mangrove leaves, or small twigs (Cottrille and Cottrille 1958, Palmer 1962a, Pratt 1970). Four nests in British Columbia weighed 2,570, 2,600, 2,800 and 4,820 g (Butler 1989). Nesting material is generally added throughout the nesting period.
Vary greatly, from flimsy new platforms of sticks just 0.5 m diameter, to bulky older structures 0.9 to 1.2 m across (Andrle and Carroll 1998). Nests used over several seasons can be about 1 m deep (RGV).
No information available.
Maintenance Or Reuse Of Nests, Alternate Nests
Nests are often reused for many years (RGV); maintained throughout the nesting period with twigs gathered near the nest (Pratt 1970) primarily by the male (mostly when eggs are laid and hatched) and placed primarily by the female (Brandman 1976). Herons may build a new nest if an early attempt fails (Pratt 1970).
Bent 1926 reported the length of Great Blue Heron eggs ranging from 50.7 (runt?) to 76.5 mm, with breadth 29.0 (runt?) to 50.5 mm. Palmer (Palmer 1962a) reported lengths ranging from 61.38 to 65.57 mm, breadth 45.07 - 46.49 mm (for 5 subspecies). Butler (Butler 1989) reported similar dimensions (for 6 subspecies), with volume ranging from 6.3 to 7.0 cm3 and estimated fresh weight ranging from 67.7 to 79.9 g.
In Nova Scotia, fresh weight 70.4 g in 1977 (range 63.5–79.5, n = 27), 71.6 g in 1978 (range 61.0–78.0, n = 34; Quinney and Smith 1979).
Miniature (runt) eggs are rarely laid; one from British Columbia was 35 x 45 mm (RWB).
The average concentration of calcium in 8 Great Blue Heron eggshells from the Columbia River, WA, was 37.3% (Rickard and Schuler 1990). The average concentration of other elements (u g/g) in the same 8 eggs was 6.7 Zn, 170 Sr, 190 Ba, 48 Fe and 9.0 Cu.
Dull pale blue, fading slightly with age (see Harrison Harrison 1978a).
Smooth or slightly rough.
Onset Of Broodiness And Incubation In Relation To Laying
Both males and females incubate (Pratt 1970, Brandman 1976, Mock 1979). One study in California found that incubation began after the first egg was laid, setting up asynchronous hatching (Pratt 1970).
No information available.
Incubation bouts for Great Blue Herons are long. In one study, males were on the nest an average of 10.4 h/d, females 3.5 h/d (n = 7 nests; Brandman 1976). Females generally incubate at night. In another study, adults incubated eggs on average for 54 min/h (Pratt 1970). Eggs were rolled by an adult on average once every 2 h (Pratt 1970).
Hardiness Of Eggs
No information is available.
Preliminary Events And Vocalizations
Shell Breaking And Emergence
Chicks take ≤ 48 h to break completely free from their egg (Quinney 1982). Time between hatching of the first and last eggs in a clutch (when all eggs hatched) ranged from 2 to 6 d (mean = 3 d; McAloney 1973) and 4 to 8 d (Quinney and Smith 1979) in Nova Scotia, 5 to 8 d in California (Pratt 1970, Brandman 1976). Sometimes eggs hatch on the same day (Pratt 1970, Quinney 1982). In one Texas study, the first eggs hatched on average 1.6 d before the second eggs, which hatched 1.7 d before the third eggs, which in turn hatched 1.9 d before the fourth eggs (Mock 1978b).
Parental Assistance And Disposal Of Eggshells
Condition At Hatching
Semi-altricial; back, head, and sides of chicks are covered in pale gray down that is especially bushy on the crown (Harrison 1978a). Wings, bill, and legs are pinkish gray and mostly unfeathered. Eyes are bluish and open. Calling (sounds like tik-tik-tik) commences within minutes of leaving the egg.
Mean weight of chicks at hatching in 2 Nova Scotia colonies was 49.3 g (SD = 3.2, n = 5) and 51.8 g (SD = 7.1, n = 39; Quinney and Smith 1979). In British Columbia, 8 males at hatching weighed on average 54.2 g (SE = 1.6), 4 females 52.6 g (SE = 0.9) (Bennett et al. 1995).
Occidentalis group chicks are covered in white down at hatching.
Growth And Development
Mass increase. Hand reared and wild Great Blue Heron chicks have similar growth curves; males and females had a similar mass at hatching, but males were 13% heavier at fledging (Bennett et al. 1995). The maximum energy required by growing nestlings occurred between 26 and 41 d of age, and the total energy required for maintenance peaked at 30 to 39 d (Bennett et al. 1995).
Quinney (Quinney 1982) weighed and measured growth rates of chicks up to 30 d of age. Last hatching chicks grew slower than their earlier hatching nest mates. Mass increase for the chicks was nearly linear in first 30 d of growth; there was no measurable sexual dimorphism, at least before 28.5 d. Sixteen 45-d-old chicks weighed on average 86% of adult weight. The asymptote of growth curves for hand-reared chicks was 2179 g (female) and 2465 g (male) at 60 d of age (Bennett et al. 1995).
Growth of body parts. Culmen, tarsus, proximal wing length, wing chord and length of tenth primary all increase linearly. However, growth over the entire nestling period is probably best explained by a logistic growth curve (Quinney 1982, Bennett et al. 1995). Young chicks (about 2 wk age) have legs and feet disproportionately large compared to their body size, perhaps to aid in clinging to the nest (RGV).
Control of body temperature. Little information. Chicks observed gular pouch fluttering at 18 d of age (Pratt 1970). This behavior was positively correlated with ambient temperature (Dowd and Flake 1985a). Sunbathing has been observed at 27 d (Pratt 1970).
Behavior and locomotion. Described by Pratt (Pratt 1970) and Dowd and Flake (Dowd and Flake 1985a). Observations have been made of preening at 6 d of age, chicks staggering to their feet at 14 d and walking steadily at 21 d.
Brood reduction or siblicide, prevalent in many nests, has been hypothesized to be the outcome of aggression precipitated by size of prey items delivered to chicks by parents (Mock Mock 1985, Mock and Parker 1986, Mock 1987; see Parental Care below). Wing-flapping has been observed as early as the fourth week, and chicks have been observed making short hops into branches near their nest at 7 wk. Sustained flights have been observed at 60 d. Chicks appeared to recognize parents approaching the colony at 9 wk. Chicks departed their nests at a mean of 81 d of age (range 64–91 d), with chicks of late-nesting pairs departing at a mean of 67 d.
Both parents brood nestlings; begins immediately after hatching and lasts 3 to 4 wk (RWB).
Both parents feed chicks (see Behavior: self maintenance). Newly hatched chicks have been seen to peck at an adult's bill, the nest, and each other (Pratt 1970). When arriving to feed chicks, an adult stands on the rim of the nest and places/regurgitates food into the open bill of the chicks (Cottrille and Cottrille 1958). By the end of the second week, chicks can grasp the adult's bill momentarily, and by the end of the third week chicks can pull the adult's bill into the nest, a behavior they exhibit until they fledge (Pratt 1970). Occasionally, chicks have been seen to reach into an adult's open mouth (Cottrille and Cottrille 1958). Chicks will eat regurgitated food dropped by parents in the nest up to about 30 d after hatching, after which they take food directly from their parents (Mock 1987). Food remains are eaten by the parents. The largest chicks tend to get the most food (Mock Mock 1985, Mock and Parker 1986, Mock 1987).
In one study, aggression between nestmates was low and not related to brood size (Mock 1987), but in another aggression was the leading cause of nestling mortality (Sullivan and Payne 1988). Brood reduction apparently is not mediated by the hunger of nestmates, but by the size of prey items delivered by parents (Mock 1987). In this study, chicks fed small prey items competed to monopolize food boluses and fought with nest mates more frequently than chicks that were fed large prey items. This result may explain the different results outlined above looking at sibling aggression.
In a study looking at the feeding of chicks, two-day-old chicks were fed 10 times in 13 h, 6-d-old chicks were fed 6 times in 13 h, and fledglings at about 65 d were fed twice in 15 h (Pratt 1970). Dowd and Flake 1985b found that the number of feeding visits/h peaked at about 29 d after hatch, with the lowest rate several weeks before fledging.
Not a big part of their repertoire. Great Blue Herons generally throw eggshells from their nests (Brandman 1976). Feces, partly eaten prey, and dead chicks can remain in the nest or fall to the ground below. Chicks sometimes regurgitate into the nest, onto the ground or onto intruders below when disturbed (RWB).
Parental Carrying Of Young
This behavior is not known to occur.
This behavior is not known to occur.
Brood Parasitism by Other Species
This behavior is not known to occur.
Departure From The Nest
Mean ages of first flight for Great Blue Herons include: Alberta, 52.6 d (range 51–54 d, Vermeer 1969d), Oregon, 7 to 8 wk (Werschkul et al. 1977), Nova Scotia 7 to 8 wk (Quinney and Smith 1979), and California 81 d.
No data after fledging.
Associations With Parents Or Other Young
Fledglings have been observed to return to the nest to be fed by adults for a further 3 wk after initial fledging (Quinney and Smith 1979). Fledglings in British Columbia follow adults from the foraging grounds to the nest to be fed, or return to the nest alone (Butler 1997).
Ability To Get Around, Feed, And Care For Self
First flights for Great Blue Heron fledglings are clumsy, but their flying skills improve rapidly (RWB). Their foraging skills are poorer than adults (Quinney and Smith 1980, Butler 1991, Gutsell 1995). Strike and pace rates of newly fledged young are similar to adults, but capture rates are about half of adults during the 2 mo following nest departure (Quinney and Smith 1980, Butler 1991).
Great Blue Herons become independent from their parents a few weeks after their first flight. Young tend to wander in late summer, and in British Columbia can be seen foraging with adults on beaches until about Oct, after which juveniles mostly feed in grasslands and marshlands (Butler 1991, Butler 1997, Gutsell 1995).
See Migration for details on when they depart breeding grounds.