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II. HABITAT AND
DISTRIBUTION
Regional Occurrence:
In the continental United States, Eudocimus
albus, the white ibis, occurs from Virginia south along the Atlantic coast
to the Gulf of Mexico. It is found on both coasts of Mexico, and ranges as far
south as Columbia and Brazil.
IRL Distribution:
The white ibis is distributed throughout the Indian River Lagoon.
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
The white ibis is a medium sized wading
bird that attains a height of 22 inches, with a wingspan of 38 inches. It may
live as long as 16 years in the wild, and 20 years in captivity (Kushlan and
Bildstein 1992).
Abundance:
Frederick et al. (1996) reported that the
Florida Everglades population of white ibis is clearly in decline, with a more
widespread overall decline in progress throughout the United States. Available
records indicate there were approximately 125,000 breeding pairs in 1933;
170,000 breeding pairs in 1976; 51,000 pairs in 1991; and 43,000 pairs in 1992.
Kushlan and Bildstein (1992) have suggested that these figures may not represent
actual declines in population. Rather, they may indicate that range expansion of
this species throughout the southeast, coupled with the nomadic lifestyle of the
white ibis cause fluctuation in population numbers in any one area. These
authors stress that population figures as a whole continue to be relatively
high. One exception occurs in the Florida Everglades, where water management
practices have altered the natural hydrologic regime of the system, causing a
marked population decline (Kushlan and Bildstein 1992)(see Reproduction section
below).
Locomotion:
White ibises walk at a rate of
approximately 25 – 40 steps per minute. They fly with rapid wingbeats at the
rate of 3.3 flaps per second. Flying is alternated with gliding for 60 – 100
m, sometimes as fast as 45 km/hour (Kushlan and Bildstein 1992).
Reproduction:
White ibises are highly social breeders
and nest in large mixed colonies numbering from hundreds to ten thousands of
nests. They begin nesting in their third summers, and will nest yearly under
optimal conditions. In Florida, nesting begins as early as mid-March and April
(Smith and Collopy 1995; Smith 1997). Further to the south, in Costa Rica,
nesting takes place between May and July (Leber 1980). Nest success is highly
correlated to low and receding water levels in freshwater areas, as nestling are
highly susceptible to salt stress. Between 2 – 5 eggs are laid per clutch, and
1 brood is raised per year. Renesting often occurs following early-season nest
failure.
White Ibises shift breeding locations in
response to rainfall patterns, with breeding success shown to increase in years
with high rainfall (Bildstein 1990). In south Florida, it has been observed that
35 times as many breeding birds are present in wet years than in dry years (Kushlan
1976). In dry years, clutch size is decreased, nests are more likely to be
abandoned, and chicks are more likely to die from starvation (Bildstein et al.
1990). In Florida, the active management of water levels in wetland areas such
as Lake Okeechobee and the Florida everglades 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 late winter to
early spring nesting season. Low water levels help to concentrate prey in small,
shallow areas, thereby making it simpler for parent birds to feed nestlings. It
also contributes to the foraging success of fledglings (Smith and Collopy 1995).
Nest takeovers and nest parasitism are
sometimes problems in ibis breeding colonies. A pair that is nesting, but
without eggs will occasionally attack a nesting female with eggs and force her
from her nest. If eggs are present, the intruding female will peck at eggs to
destroy them, and then eject them from the nest. Nesting material is then
rearranged and the intruding pair will begin copulation. The ejected pair will
often return to the nest and chase off the intruders; however, in all cases
where this behavior was observed, nest failure resulted (Frederick 1986).
Conspecific egg dumping, or nest parasitism also occurs during nesting, where
non-nesting females will deposit eggs into a nesting female’s nest and leave
the nesting female to raise chicks that are not her own (Kushlan and Bildstein
1992).
White ibises from wild populations in
Venezuela and Florida are known to hybridize with the scarlet ibis, Eudocimus
ruber. Hybridization with the scarlet ibis also readily occurs in captivity
(Ramo and Busto 1987).
Embryology:
Eggs are incubated 21 – 23 days.
Hatching is asynchronous and occurs over several days. The oldest chick quickly
gains experience at food handling and aggressive behaviors toward its siblings,
and thus grows at a faster rate than its nestmates. Nestling mortality is
greatest in the first 20 days of life (Kushlan and Bildstein 1992).
At hatching, the skin, legs and feet of chicks
are flesh colored. Blue-gray down covers the body by the third day, with black
down covering the head and neck. Feathers begin to emerge around day 5. The eyes
open after 1 – 3 days, with chicks becoming fully alert by day 9. Chicks are
mobile at 8 days of age, and by day 15 often begin leaving the nest to join
crèches (social groups) of similarly aged birds. Chicks in crèches continue to
be fed by adults. Fledging occurs at 28 – 35 days.
IV. PHYSICAL TOLERANCES
Salinity:
White ibis chicks are susceptible to depressed growth
rates and increased mortality (Kushlan and Bildstein 1992) due to salt stress when
they are fed on fiddler
crabs (Uca spp.). These crabs inhabit estuaries and coastal areas
and are osmoconformers, meaning that they cannot regulate salt content within their
bodies. Rather, they simply conform to environmental conditions such that salt
concentration outside the body equals that of inside the body. For this reason,
parental birds do not offer Uca spp. to nestlings when foraging
conditions are good. Rather, chicks are fed primarily on freshwater fish, crayfish and
insects.
V. COMMUNITY ECOLOGY
Trophic Mode:
White ibis feed primarily on species of
crustaceans (Kushlan and Bildstein 1992) from both saltmarsh and freshwater
wetland habitats. They are tactile feeders that probe around soft bottom areas
with their long bills in search of prawns, crayfish and fiddler crabs. Tactile
foraging is somewhat more inefficient in capturing fish, so it is used rarely
when foraging for fish (Kushlan 1979). Adult birds that are feeding chicks visit
freshwater foraging sites more frequently than saltmarsh sites. However, once
young have fledged, parental visits to saltmarsh feeding habitats double.
Competitors:
White ibises compete for food with other
wading birds.
Habitats:
The white ibis has been described as a
nomadic species, rather than a migratory species (Frederick et al. 1996). As
such, it quickly colonizes wetlands having good food resources, and readily
abandons areas where resources have become scarce. It utilizes both freshwater
and estuarine wetlands such as mangrove and cypress swamps, bottomland hardwood,
and marshes. In Florida studies, its preferred breeding habitat is in freshwater
areas where winter and early spring water levels are low or receding (Smith and
Collopy 1995).
Associated Species:
White ibises are highly gregarious and
readily associate with other species of medium sized herons and egrets. They are
known to nest in mixed flocks with cattle egrets (Kushlan and Bildstein 1992).
VI. SPECIAL STATUS
Special Status:
The white ibis is listed as a Species of
Special Concern (SSC) in Florida. It is not federally listed as threatened or
endangered.
Benefit in IRL:
In contrast with migratory birds that
commonly utilize the same habitats from year to year, the white ibis is nomadic,
readily abandoning habitats that can no longer support it. Its presence in
wetlands is thus a good indicator of overall environmental health and habitat
quality (Frederick et al. 1996).
Cost in IRL:
As a nomadic species, the white ibis is
thus unlikely to be insular and easily protected because any conservation effort
must address habitat quality over extensive areas. Should more evidence of a
declining U.S. population continue to accumulate, the continued survival of this
species will depend both regional planning and a continued commitment to
maintain high quality habitat areas (Frederick et al. 1996).
Economic Importance:
Though not of economic value, in
some regions throughout its range, the white ibis continues to be hunted for
food. Their appealing taste is perhaps due to the ibis’ dietary preference for
crabs and other crustaceans (Kushlan and Bildstein 1992).
Report by: K. Hill,
Smithsonian Marine Station
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Page last updated: July 25, 2001
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