The White ibis (Eudocimus albus) is a species of bird in the ibis family, Threskiornithidae. It is found from Virginia via the Gulf Coast of the United States south through most of the coastal New World tropics. This particular ibis is a medium-sized bird with an overall white plumage, bright red-orange down-curved bill and long legs, and black wing tips that are usually only visible in flight. Males are larger and have longer bills than females. The breeding range runs along the Gulf and Atlantic Coast, and the coasts of Mexico and Central America. Outside the breeding period, the range extends further inland in North America and also includes the Caribbean. It is also found along the northwestern South American coastline in Colombia and Venezuela. Populations in central Venezuela overlap and interbreed with the scarlet ibis. The two have been classified by some authorities as a single species.
The white plumage and pink facial skin of adult American white ibises are distinctive. Adults have black wingtips that are usually only visible in flight. In non-breeding condition the long downcurved bill and long legs are bright red-orange. During the first ten days of the breeding season, the skin darkens to a deep pink on the bill and an almost purple-tinted red on the legs. It then fades to a paler pink, and the tip of the bill becomes blackish. It is difficult to determine the sex of an adult American white ibis from its external appearance, since the sexes have similar plumage. However, there is sexual dimorphism in size and proportion as males are significantly larger and heavier than females and have longer and stouter bills. A study of the American white ibis in southern Florida yielded weight ranges of 872.9 to 1,261 g (1.924 to 2.780 lb) for males and 592.7 to 861.3 g (1.307 to 1.899 lb) for females, with average weights of 1,036.4 g (2.285 lb) for males and 764.5 g (1.685 lb) for females. The length of adult female and male birds ranges from 53 to 70 cm (21 to 28 in) with a 90 to 105 cm (35 to 41 in) wingspan. Among standard measurements, American white ibis measure 20.5–31 cm (8.1–12.2 in) along each wing, have a tail measurement of 9.3–12.2 cm (3.7–4.8 in), a tarsus of 6.75–11.3 cm (2.66–4.45 in) and a culmen of 11–16.9 cm (4.3–6.7 in).
The newly hatched American white ibis is covered with violet down feathers, deepening to dark brown or black on the head and wings. The chest is often bare and there can be a white tuft on the head. The irises are brown. The exposed skin is pinkish initially, apart from the tip of the bill which is dark gray, but turns gray within a few days of hatching. The bill is short and straight at birth and has an egg tooth which falls off between days five and nine, and develops three black rings from around day six, before turning gray by around six weeks of age. The gray to sandy gray brown juvenile plumage appears between weeks two and six, and face and bill become pink a few weeks later, while the legs remain gray. The irises have turned slate-gray by this stage. Once fledged, the juvenile American white ibis has largely brown plumage and only the rump, underwing and underparts are white. The legs become light orange. As it matures, white feathers begin appearing on the back and it undergoes a gradual molt to obtain the white adult plumage. This is mostly complete by the end of the second year, although some brown feathers persist on the head and neck until the end of the third year. Juvenile birds take around two years to reach adult size and weight.
Like other species of ibis, the American white ibis flies with neck and legs outstretched, often in long loose lines or V formations—a 1986 field study in North Carolina noted over 80% of adult ibis doing so, while juveniles rapidly took up the practice over the course of the summer. The resulting improvement in aerodynamics may lower energy expenditure. These lines fly in an undulating pattern as they alternately flap and glide. Soaring in a circular pattern is also seen. Heights of 500 to 1,000 m (1,600 to 3,300 ft) may be reached as birds glide over flights of 20 km (12 mi) or more. More commonly, birds fly between 60 and 100 m (200 and 330 ft) above the ground, gliding or flapping at a rate of around 3.3 wingbeats a second.
Immature American white and scarlet ibises are very difficult to tell apart, although scarlet ibises tend to have darker legs and bare skin around the face. An immature American white ibis could be mistaken for an immature glossy ibis, but the latter is wholly dark brown and lacks the white belly and rump. The adult is distinguishable from the wood stork, which is much larger and its wings have more black on them.
Distribution and habitat
The White ibis is most common in Florida, where over 30,000 have been counted in a single breeding colony. It also occurs throughout the Caribbean, on both coasts of Mexico (from Baja California southwards) and Central America, and as far south as Colombia and Venezuela. The non-breeding range extends further inland, reaching north to Virginia, and west to eastern Texas.Adult American white ibis on pavement outside of Orlando, FL.
The species is known to wander, and has been sighted, sometimes in small flocks, in states far out of its usual range.
In North America, breeding takes place along the Atlantic coast, from the Carolinas south to Florida and thence west along the Gulf Coast. Laguna Cuyutlán is an isolated and regionally important wetland in the state of Colima on México’s west coast where a breeding colony has been recorded. American white ibises are not faithful to the sites where they breed, and large breeding colonies composed of ten thousand birds or more can congregate and disband in one or two breeding seasons. Breeding populations across its range have fluctuated greatly with wholesale movement between states.
The White ibis is found in a variety of habitats, although shallow coastal marshes, wetlands and mangrove swamps are preferred. It is also commonly found in muddy pools, on mudflats and even wet lawns. Populations that are away from the coast and shoreline, particularly in southern Florida, often reside in other forms of wetlands such as marshes, ponds and flooded fields. In summer, these move to more coastal and estuarine habitats as inland waterways become flooded with summer rains and the ibis find the water levels too deep to forage effectively.
A field study late in the Florida nesting season revealed that on an average day, adult American white ibis spent 10.25 hours looking for food, 0.75 hours flying, 13 hours resting, roosting, and attending to their nests. Much of the time roosting is spent preening, biting and working their feathers with their long bills, as well as rubbing the oil glands on the sides of their heads on back plumage. White ibis generally only preen themselves, not engaging in allopreening unless part of courtship behavior. Bathing often takes place before preening; ibis squat in water 5–20 cm (2.0–7.9 in) deep and flick water over themselves with each wing in succession. Hundreds of birds may bathe together around the time of courtship.
The white ibis is territorial, defending the nesting and display sites against intruders. Agonistic or threat displays include lunging forward with the bill in a horizontal posture, and standing upright and snapping the bill opposite another bird engaging in the same display. Birds also lunge and bite, often holding onto an opponent’s head or wings.
Breeding and lifespan
The white ibis pairs up in spring and breeds in huge colonies, often with other waterbird species. Nesting begins as soon as suitable foraging and nesting habitat is available. The female selects the site, usually in the branches of a tree or shrub, which is often over water, and builds the nest, and males assist by bringing nest material. Anywhere from one to five eggs are typically laid, with two or three being the most common. The eggs are matte pale blue-green in color with brown splotches, measure 5.8 cm × 3.9 cm. Throughout the mating and incubation period, the male undergoes a period of starvation to stay close to the nest and aggressively defend his nest and mate from both predators and other ibises in preference to foraging for food.
Although the American white ibis is predominantly monogamous and both sexes provide parental care to their young, the male often flies off to engage in extra-pair copulation with other nesting females after mating with its primary female partner.
The breeding success of the American white ibis is sensitive to the hydrological conditions of the ecosystem such as rainfall and water levels. Low and decreasing water levels predict good prey accessibility. Water level reversals, where levels rise in the breeding season, disperse prey and impact on foraging success. Nest numbers and average clutch sizes are smaller in periods of reduced prey availability.
The white ibis prefers to eat crayfish and other crustaceans, but also takes aquatic insects and small fish. Outside the nesting season, the diet is highly variable, as abundance and types of prey depend on the both region and habitat. In Los Llanos, located on the border of Colombia and Venezuela, the most frequent prey are insects, such as fly larvae and beetles. Generally in North America the main prey are crustaceans, mostly crayfish. In the Everglades and cypress swamps, the diet is primarily made up of crayfish, while those that feed in willow ponds eat predominantly fish. American white ibises that feed in mangrove swamps focus on crabs. The tactile nature of the ibis’s probing for food in mud means that it catches prey that are too slow to evade the ibis once located by its bill. In the Everglades, this means that crayfish make up a large part of the diet, but a more diverse array of invertebrates are taken in coastal areas. Although crayfish are sought by foraging ibises, prey switching to fish does occur if fish are found in great abundance. It is unclear whether the fish are more easily caught if overcrowded, or whether sheer numbers of fish mean that ibises are catching them instead of crayfish—normally, fish are more agile than crayfish and hence elude the ibis’s bill more easily. Fish are a more energy-rich source of food for the American white ibis. In the breeding season, American white ibises in a colony at Pumpkinseed Island travelled further to forage in freshwater wetlands and catch crayfish, than nearby saltwater areas where fiddler crabs predominated, indicating their benefit was worth the extra energy expended in fetching them for their young. This travel results in the wholesale transport of nutrients across the landscape by the colony; in a successful breeding year the colony at Pumpkinseed Island was estimated to have contributed a third as much phosphorus to the neighboring estuary as other environmental processes.
The white ibis is classed as being of least concern on the IUCN Red List. The population consists of 150,000 mature adults, and is stable, although some populations have unknown trends. A partial survey of under 50% of the North American population published in 2007 found an almost six-fold increase in the last four decades. The estimated breeding range is huge, at 1,200,000 km2 (460,000 sq mi). Fluctuating breeding populations and high mobility of colonies make estimating the population difficult. Attempted censuses of breeding colonies across Texas, Louisiana, Florida, and the Carolinas yielded a count of 166,000 breeding birds in 2001, and 209,000 in 2004. The conservation status has been listed in two states—it is a Species of Special Concern in Florida, and a species of Moderate Conservation Concern in Alabama. The preservation of colony sites and freshwater foraging areas is important to maintaining populations; however, the highly mobile nature of breeding colonies makes this challenging.
The main call of the American white ibis is a honking sound, transcribed as urnk, urnk, or hunk, hunk. The call is used in flight, courtship or when disturbed. Birds also utter a muted huu-huu-huu call while foraging, and make a squealing call in courtship. Young in the nest give a high-pitched zziu as a begging call.