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II. HABITAT AND
DISTRIBUTION
Regional occurrence:
A. herodias occurs throughout most of North America, including Alaska,
Quebec and Nova Scotia. The range extends south through Florida, Mexico and the
Caribbean to South America, including the Galapagos Islands.
IRL Distribution:
A. herodias is distributed
throughout the Indian River Lagoon.
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
The largest heron in North America, great
blue herons attain a height of 38 inches, with a wingspan of 70 inches.
Abundance:
The great blue heron is generally
the most well known and widespread heron species in North America. It is common
throughout the Indian River Lagoon.
Reproduction:
Though sympatric with other species of
wading birds, the great blue heron forms monospecific breeding colonies. This
behavior differs from species such as the snowy egret and white ibis that nest
in large mixed flocks. Colony size in great blue herons can be large, ranging
between 5 – 500 nests per colony, with an average of approximately 160 nests
per colony.
Nesting in the Florida Everglades and Lake
Okeechobee begins between late November through mid-March and April, with nests
sometimes located as high as 20 m above the ground (Butler 1993; Smith 1997).
Clutch size ranges from 3 – 6 eggs, with 1 brood being raised each year.
Should eggs be abandoned or destroyed, a replacement clutch can be laid (Ehrlich
et al. 1988).
Reproductive success in great blue herons is
limited by the ability of parent birds to gather food. Energy availability (as
measured in food resources) appears, in part, to determine when eggs will be
laid. Butler (1993) observed that egg laying in great blue herons occurs after a
threshold level for successful egg laying is surpassed for several consecutive
days. Chicks are generally present after parental food availability peaks. This
leads to a potential problem in nourishing nestlings. Parental birds have been
shown to consume up to 4 times as much food when they are feeding young chicks
than when egg laying or incubating eggs (Butler 1993); however, chicks do not
reach their metabolic peak until approximately 26 – 41 days after birth. Thus,
if food availability has already peaked before chicks appear, it is then
potentially more difficult for parents to nourish nestlings. There is some data
to support this view. Collazo (1981) found that starvation was not only the
primary cause of nestling mortality, but that it most often coincided with the
peak energy demands of 3 – 6 week old chicks.
A. herodias
is negatively affected by human disturbance, particularly during the early
stages of the breeding cycle. Repeated human intrusion into nesting areas often
results in nest failure, with abandonment of eggs or chicks (Carlson and McLean
1996). Great blue herons thus tend to choose nest sites in undisturbed wetland
areas around lakes or rivers, well removed from roads (Gibbs and Kinkel 1997)
and other signs of human activity. In one Ohio study, foot traffic was observed
to be the most detrimental disruption to breeding great blue herons, sometimes
leading to nest abandonment. Nesting birds were shown to tolerate routine
mechanical disturbance, even if loud, as long as no humans were nearby (Carlson
and McLean 1996). Nest success increased when nest sites were chosen near
barriers such as fencing, ditches or moats that excluded human intrusion. Nests
located in areas with this type of buffer zone, even if small in overall area,
had higher nest success and fledging rates than those located near disturbed
areas.
Embryology:
Eggs are incubated for approximately 28
days and hatch asynchronously over a period of several days (Custer and Peterson
1991). The first chick to hatch becomes more experienced in food handling and
aggressive interactions with siblings, and so often grows at a faster rate than
its nestmates. Male and female chicks show little difference in hatching weight,
but males tend to grow somewhat faster than females. By the time fledging
occurs, males often weigh up to 13% more than females. Young fledge after 56 –
60 days (Ehrlich et al. 1988).
IV. PHYSICAL TOLERANCES
Temperature:
A. herodias is eurythermal. This species has been known to overwinter as
far north as British Columbia and New England. There are also records of great
blue herons overwintering in bays along the Alaskan coast. However, many of
these Alaskan birds do not survive harsh winters.
V. COMMUNITY ECOLOGY
Trophic Mode:
As large wading birds, great blue herons
are able to feed in deeper waters, and thus are able to exploit a niche not open
to most other heron species. Primary feeding behaviors in this species include
standing crouched, standing upright, and wading slowly. Great blue herons have
high striking efficiency (the percentage of strikes that result in the capture
and ingestion of prey) when standing crouched on banks. However, this behavior
is used less often than either standing upright or wading slowly. This suggests
that the goal for great blue herons may not be achieving maximum feeding
efficiency; but rather, maximizing food intake through the infrequent capture of
high quality prey (Willard 1977). Data seem to reinforce this point, as it has
been shown that great blue herons that utilize the rather inefficient slow wade
as a feeding strategy tend to feed upon more large fish such as eel and perch
(Willard 1977).
In a Canadian study of great blue herons,
Butler (1993) found that A. herodias fed primarily upon sticklebacks,
shiners, sea perch, pipefish, sculpins, etc., and that they require
approximately 30% more energy above basal metabolic costs to fuel their
activities.
Activity Time:
Butler (1993) stated that the nocturnal
feeding behavior observed in great blue herons was important to resource
partitioning and reduced competition among wading birds. In a Cedar Key, Florida
study, Black and Collopy (1982) found great blue herons to be equally active in
either daylight or evening hours, though most evening feeding occurs during
early evening low tides than later in the evening.
Associated Species:
A. herodias is generally considered
a solitary feeder, though it has been observed to associate with other wading
birds of various sizes while feeding. While breeding, A. herodias avoids
mixed flocks in favor of nesting in monospecific colonies.
VI. SPECIAL STATUS
Special Status:
Great blue herons are among the most well
known and most common of the wading bird species. It is not federally listed as
either threatened or endangered, nor is it listed in Florida as a Species of
Special Concern (SSC).
Benefit in IRL:
The environmental sensitivity of wading
birds, coupled with the relative ease of assessing their numbers, makes them
attractive as biological indicators of ecosystem health and habitat quality
(Custer and Osborn 1977; Powell and Powell 1986; Powell et al. 1989).
Economic Importance:
None.
Report by: K. Hill,
Smithsonian Marine Station
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irl_webmaster@si.edu
Page last updated: July 25, 2001
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