|
Species Description:
Egretta tricolor
is a medium sized, long necked wading bird of the Order Ciconiiformes (herons
and storks). In this species, sexes are similar in appearance. Overall body
color is gray along the neck, back and wings; and white along the foreneck and
much of the ventral area. The legs are greenish or yellow in color, and the
black-tipped bill can range from shades of blue to yellow.
II. HABITAT AND
DISTRIBUTION
Regional Occurrence:
E. tricolor
occurs along the Atlantic coast from Massachusetts south through Florida, the
Gulf of Mexico and much of South America.
IRL Distribution:
Occurs throughout the Indian River Lagoon.
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
Tricolored herons attain a maximum size of
approximately 22 inches, with a wingspan of 38 inches.
Abundance:
Tricolored herons are common
inhabitants of the Indian River Lagoon.
Reproduction:
Tricolored herons begin nesting in
early to mid-March in Florida (Smith and Collopy 1995). There is a positive
correlation between receding water levels in Lake Okeechobee and nesting sites
chosen by tricolored herons. It is believed that areas with receding water
levels are preferred because they have the effect of concentrating prey in a
smaller area, thus making it simpler to feed nestlings (Smith and Collopy 1995).
Between 3 - 7 eggs are laid per clutch. One brood is raised each year; however,
should eggs be abandoned or destroyed, a replacement clutch can be laid.
Predation and disturbance rates determine nest
success in all wading bird species. Cold weather events are particularly
detrimental to nest success in many species. Additionally, in Florida, the
active management of water levels in wetland areas such as Lake Okeechobee may
also contribute to nest success (Smith and Collopy 1995). It has been suggested
that in order to enhance nest success and later fledgling success, water levels
in managed wetlands should be maintained at low or receding levels throughout
the December to March nesting season. Low water levels help to concentrate prey
in a small area, thereby making it simpler for parent birds to feed nestlings.
It also contributes to the foraging success of fledglings (Smith and Collopy
1995).
Embryology:
Rodgers (1980), in a study conducted in
Hillsborough County, Florida, observed a mean clutch size in tricolored herons
of 2.79 eggs per nest, with a 96% hatching rate. Approximately 70.5% of
nestlings survived from birth to 2 weeks of age.
Eggs are incubated for approximately 21 – 25
days. As in all heron and egret chicks, hatching is asynchronous, and occurs
over several days. The first chick to hatch quickly gains experience at food
handling and aggressive interactions with siblings, so is often able to
outcompete its siblings. Thus, growth rates are unequal among nestmates. In one
California study, it was observed that as chicks age, predation gives way to
starvation as the primary cause of death (Alfaro and Russi 1989).
Young fledge after approximately 35 days.
IV. PHYSICAL TOLERANCES
V. COMMUNITY ECOLOGY
Trophic Mode:
Strong et al. (1997) observed tricolored
herons to be solitary feeders, with only 3% of birds associating with mixed
flocks of other wading birds. Tricolored heron in Florida waters primarily
consume snook, followed by guppies and gobies (Miranda and Collozo 1997). E.
tricolor in the Florida Everglades and Lake Okeechobee areas show low diet
diversity compared to other heron species, with a preference for fish of
intermediate size (Smith 1997). Kent (1986) observed tricolored herons
capturing, but always releasing, small crabs.
Food offered to nestlings includes
approximately 27 different species, though the most important items in the diet
were sailfin mollies, followed by mosquitofish, killifish, and golden topminnows
(Smith 1997).
Smith (1997) observed that while other heron
and egret species such as the great blue heron and the little blue heron will
alter their diets in response to changing hydrological and habitat conditions,
tricolored herons and snowy egrets do not. Rather, they change their foraging
habitats and feeding strategies in order to continue to encounter preferred prey
items.
In foraging efficiency, the tricolored heron
falls last behind the little blue heron and the snowy egret in terms of striking
efficiency (i.e., the percentage of strikes that result in capture and ingestion
of prey). However, in terms of food intake per minute (g/min), the tricolored
heron outcompetes both the little blue heron and the snowy egret. Of the 3
species, Kent's (1987) study estimated that the tricolored heron, due to its
preference for somewhat larger sized prey, obtained its daily energy requirement
fastest. In terms of caloric intake per minute, the little blue heron was
required to forage 0.6 times as long as the tricolored heron, while the snowy
egret required twice as much time as the tricolored heron to take in the same
amount of energy. Willard (1977) observed the primary foraging strategies of
tricolored herons to be: 1) standing still on banks waiting for prey to swim by,
2) wading slowly, and 3) active disturb-and-chase behavior. Tricolored herons
achieve the greatest striking efficiency with less active behaviors such as
wading slowly (Kent 1986); however, this highly efficient behavior is used
equally as often as inefficient disturb-and-chase strategies (Willard 1977; Kent
1987). As evidenced by these foraging behaviors, tricolored herons may not have
the goal of achieving maximum feeding efficiency, but rather try to maximize
their energy intake through the infrequent capture and ingestion of high quality
prey (Kent 1987).
Competitors:
E. tricolor
competes with other species of wading birds for food resources and nesting
space. However, habitat partitioning
and resource partitioning among species reduces direct
competition such that species can coexist.
Habitats:
The tricolored heron shows
significant habitat overlap with other species of wading birds, particularly the
great blue heron, the little blue heron, and the snowy egret (Kent 12986;
Willard 1977). However, it appears to prefer small pools over lakes and bays
(Chavez-Ramirez and Slack 1995). Strong et al. (1997) observed that tricolored
herons were consistent in their choice of habitat among both wet and dry years,
tending to avoid inland marshes and sloughs in favor of mangrove habitats and
coastal marshes. When inhabiting freshwater areas such as Lake Okeechobee,
Florida, tricolored herons were particularly attracted to Hydrilla and
lotus (Nelumbo lutea) plants, using the leaves of the lotus for support
while fishing among Hydrilla mats (Smith and Collopy 1995).
Associated Species:
Though tending to be somewhat solitary in
its feeding behavior, the tricolored heron can often be found in association
with other medium sized wading birds.
VI. SPECIAL STATUS
Special Status:
E. tricolor
is listed as a Species of Special Concern (SSC) in Florida, but is not federally
listed as threatened or endangered.
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
Submit additional information, photos or comments to:
irl_webmaster@si.edu
Page last updated: July 25, 2001
|
