STATUS: Species of Special Status
DESCRIPTION: The white ibis is a medium-sized wading bird, about 56 centimeters in height; characterized by a long, narrow down-curved bill. Plumage is entirely white with the exception of the tips of the four longest primaries which are black (often appear iridescent green). Irises of adults are pale blue. The appearance of males and females is similar, but males tend to be larger than females and have a disproportionately longer bill. During courtship, the unfeathered portions of the face, bill, legs, and feet darken from pale pink to red. Juveniles have brown wings, neck, head, tail, and irises, and gray legs. They gradually assume adult plumage during the first 3 years.
HABITAT AND DISTRIBUTION: White ibises use both coastal and inland habitats for nesting and foraging, and during the nesting season are dependent upon brackish and freshwater habitats to obtain at least a portion of their diet. They typically nest in colonies and feed in shallow water. Generally occurs along coastal and coastal plain habitats from North Carolina through Texas, the West Indies, Central America, and northern South America. Range includes all peninsular Florida in marshes, wet prairies, mangroves swamps, and other wetlands.
LIFE HISTORY AND ECOLOGY: White ibis feed primarily on aquatic invertebrates and on fish, where such are densely concentrated. Crayfish is the predominant food in most freshwater habitats. Aquatic beetles, bugs, snails, and insect larvae are also consumed. In estuarine habitats fish, crayfish, and crabs are eaten. Diet is varied and generally opportunistic but feeding depends on suitable water conditions. Ibises feed singly or in tight flocks by probing in the water, in the ground, and among the roots of plants. Although white ibises frequently fly long distances in search of suitable foraging habitat, during the nesting season the majority of foraging flights are within 15 kilometers of the colony site.
Timing of nest initiation is variable, beginning any time after spring. White ibises use photoperiod as the initial cue to begin nesting. Rain appears to trigger nesting activity by reflooding nesting habitats and by increasing feeding habitats. The species breeds in intraspecific subcolonies, usually within a larger mixed species colony. Nests typically occur on islands in lakes, in marshes, or in mangroves. They are constructed of twigs and herbaceous materials and located in shrubs, cypress and marsh vegetation. White ibises lay 1 to 5 eggs. Both parents tend the eggs during the 20 or 21 day incubation period. Young are fed by the adults until they are 6 or 7 weeks old and attain independence. Approximately 1 young per nest successfully fledges. Nest abandonment due to changes in hydrologic patterns is the highest known cause of nest failure.
BASIS OF STATUS CLASSIFICATION: The white ibis was the most abundant wading bird in Florida. Although it remains so today, its numbers have declined considerably over the past 50 years. Changes in the ecology of the foraging habitat around nesting sites have had on the number, timing, and success of nesting birds is the most likely explanation for the continued white ibis population decline. Much of this foraging habitat is being threatened by destruction from human encroachment, which therefore poses a significant threat to maintaining the present population levels of the species. In response to the continued decreases in the breeding populations, the Florida Game and Fresh Water Fish Commission listed the white ibis as a "species of special concern" in 1994.
The most serious threats to this species are environmental contamination, loss of nesting sites, and loss of feeding habitat. Degradation of water quality can lead to significant changes in prey populations, availability of prey, and the vegetative characteristics of wetland habitats, and result in reduced reproductive success or abandonment of areas. Degradation of water quality can also increase the incidence of diseases. Infections by the nematode Eustrongylus ignotus is the most significant disease of wading birds in Florida. This disease appears to be linked to nutrient pollution and altered habitats. Encroachment of development and human activities into areas used by nesting white ibises frequently results in abandonment of nesting sites. White ibises may avoid otherwise suitable nesting areas when the buffer against human disturbances is reduced below a minimum tolerance level. The loss of wetlands to development or to development related hydroperiod alteration may also play a role in reducing the statewide foraging options for white ibises.
HABITAT GUIDELINES: Protection of this species consists of providing an effective zone that buffers the nesting sites. This "buffer zone" may extend into upland habitat. The extent of these buffers is based on existing State and/or Federal guidelines, or reflect the recommendations of the Florida Game and Fresh Water Fish Commission.
Any activity proposed within a buffer zone would be subject to review. Activities that could be allowed in the buffer zone would be those activities which would not have any long term detrimental impact on the nesting birds. The following would be considered when making this determination: (1) the type of activity or construction planned and its' long term impacts; (2) the nature of the natural community that makes up the buffer zone (does it provide a visual barrier for any proposed nesting activities?); and (3) the timing of the proposed construction (does it occur outside the nesting season?).
RECOMMENDATIONS: The preservation of nesting sites is critical to the species' survival, even if such sites are used only intermittently. Presently, most sites have some degree of legal protection under the federal Migratory Bird Treaty Act of 1981.
Of broader concern is the protection of feeding habitat. White ibises tend to be social, nomadic nesters, seldom using the same site for more than a couple of years in succession, depending on the location of suitable feeding areas. As a result, preservation efforts are more challenging. However, several aspects of white ibis biology are conducive to prudent management. For instance, we have good data on the biology of the species, which allows biologists and regulatory agencies to make accurate predictions as to the effects of certain activities. Also the species is linked to a group of habitats to meet both nesting and foraging habitat areas.
Preservation of habitat important to the long-term survival of Florida's white ibises can be accomplished by protection of colony sites and critical foraging areas. White ibis habitat protection measures should focus on active and recently active colony sites and areas of potential foraging habitat within 15 kilometers of active or recently active colonies.
The historically abundant white ibis, feeding and flying in tight flocks, has an esthetic appeal which has made it one of the most characteristic species of Florida. In addition, being still relatively common and feeding in dense flocks, this species has an important ecological impact on the wetlands it inhabits.
Frederick, P.C. 1996. White Ibis. Pp 446-474. In Rare and Endangered Biota of Florida.
Volume V. Birds. (J.A. Rodgers, Jr., H.W. Kale II, and H.T. Smith, Eds.) University Press
of Florida, Gainesville, FL.
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