Species Description: Eudocimus albus, the white ibis, is a member of the Order Ciconiiformes (herons and storks). It is a long-necked wading bird in which the sexes are similar in appearance. Major identifying characteristics include its long decurved (downward curving) bill, longer in males than in females; entirely white body coloration, pink bill, and legs, black tipped outer primary feathers, and distinctive bare face which ranges in shades from red to pink. The eyes of adults are blue.
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.
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').
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 tens of 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 nestlings are highly susceptible to salt stress. Between 2 - 5 eggs are laid per clutch, and 1 brood is raised per year. Re-nesting 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 for 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.
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.
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).
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 become evident, the continued survival of this species will depend upon 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).
Bildstein KL, Post W, Johnston J, Frederick P. 1990. Freshwater wetlands, rainfall, and the breeding ecology of white ibises in coastal South Carolina. Wilson Bull 102: 84-98.
Frederick PC, Bildstein KL, Fleury B, Ogden J. 1996. Conservation of large, nomadic populations of white ibises (Eudocimus albus) in the United States. Conserv Biol 10: 203-216.
Frederick PC, Shields MA. 1986. Suspected intraspecific egg dumping in the white ibis (Eudocimus albus). Wilson Bull 98: 476-478.
Kushlan JA. 1976. Wading bird predation in a seasonally fluctuating pond. Auk 93: 464-476.
Kushlan JA. 1979. Feeding ecology and prey selection in the white ibis. Condor 81: 376-389.
Kushlan JA, Bildstein KL. 1992. White Ibis: Eudocimus albus. Amer Ornithol Union.
Leber KK. 1980. Habitat utilization in a tropical heronry. Brenesia 17: 97-136.
Ramo C, Busto B. 1987. Hybridization between the scarlet ibis (Eudocimus ruber) and the white ibis (Eudocimus albus) in Venezuela. Colonial Waterbirds 10: 111-114.
Smith JP. 1997. Nesting season food habits of 4 species of herons and egrets at Lake Okeechobee, Florida. Colonial Waterbirds 20: 198-220.
Smith JP, Collopy MW. 1995. Colony turnover, nest success and productivity, and causes of nest failure among wading birds (Ciconiiformes) at Lake Okeechobee, Florida (1989 - 1992). Archiv for Hydrobiologie, Adv Limnol 45: 287 - 316.