The American white ibis (Eudocimus albus) is a species of bird in the ibis family, Threskiornithidae. It is found from the southern half of the US East Coast (from southern New Jersey to Georgia), along the Gulf Coast states (Florida, Alabama, Mississippi, Louisiana and Texas) and south through most of the Caribbean coastal regions of Central America. This particular ibis species is a medium-sized wading bird, possessing an overall white plumage with black wing-tips (usually only visible in flight), and having the typical downward-curving bill of the ibises, though of a bright red-orange color, the same hue as its long legs. 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.
Their diet consists primarily of small aquatic prey, such as insects and small fishes. Crayfish are its preferred food in most regions, but it can adjust its diet according to the habitat and prey abundance. Its main foraging behavior is probing with its beak at the bottom of shallow water to feel for and capture its prey. It does not see the prey.
During the breeding season, the American white ibis gathers in huge colonies near water. Pairs are predominantly monogamous and both parents care for the young, although males tend to engage in extra-pair copulation with other females to increase their reproductive success. Males have also been found to pirate food from unmated females and juveniles during the breeding season.
Human pollution has affected the behavior of the American white ibis via an increase in the concentrations of methylmercury, which is released into the environment from untreated waste. Exposure to methylmercury alters the hormone levels of American white ibis, affecting their mating and nesting behavior and leading to lower reproduction rates.
Taxonomy The American white ibis was one of the many bird species originally described by Carl Linnaeus in the 1758 10th edition of his Systema Naturae, where it was given the binomial name of Scolopax albus. The species name is the Latin adjective "white". Alternative common names that have been used include Spanish curlew and white curlew. English naturalist Mark Catesby mistook immature birds for a separate species, which he called the brown curlew. Local creole names in Louisiana include bec croche and petit flaman.
Johann Georg Wagler gave the species its current binomial name in 1832 when he erected the new genus Eudocimus, whose only other species is the scarlet ibis (E. ruber). There has long been debate on whether the two should be considered subspecies or closely related species, and the American Ornithologists' Union considers the two to be a superspecies as they are parapatric. The lack of observed hybrids was a large factor in the view that the species were separate.
However, in a field study published in 1987, researchers Cristina Ramo and Benjamin Busto found evidence of interbreeding in a population where the ranges of the scarlet and white ibises overlap along the coast and in the Llanos region of Colombia and Venezuela. They observed individuals of the two species mating and pairing, as well as hybrid ibises with pale orange plumage, or white plumage with occasional orange feathers; their proposal that these birds be classified as a single species, has been followed by least one field guide. Hybrid ibises have also been recorded in Florida, where the scarlet ibis has been introduced into wild populations of American white ibis. Birds of intermediate to red plumage have persisted for generations.
Ornithologists James Hancock and Jim Kushlan also consider the two to be a single species, with the differences in plumage, size, skin coloration and degree of bill darkening during breeding season forming the diagnostic characters. They have proposed the populations recontacted in northwestern South America after a period of separation, and that the color difference is likely due to the presence of an enzyme that allows uptake of pigment in the diet. They have questioned whether white-plumaged birds of South America are in fact part of the ruber rather than the albus taxon, and acknowledge that more investigation is needed to determine this.
Description
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. This species is moderately large for an ibis but is relatively short legged, compact and bulky for a large wading bird. A study of the American white ibis in southern Florida yielded weight ranges of for males and for females, with average weights of for males and for females. The length of adult female and male birds ranges from with a wingspan. Among standard measurements, American white ibis measure along each wing, have a tail measurement of , a tarsus of and a culmen of .
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 may be reached as birds glide over flights of or more. More commonly, birds fly between above the ground, gliding or flapping at a rate of around 3.3 wingbeats a second.
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.
Similar species 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 American 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. 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 southern New Jersey south to Florida and thence west along the Gulf Coast. (It nested in New Jersey for the first time in 2020, having appeared there rarely before then.) 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.
Until the 1940s, the species bred only in large numbers in Florida, mostly within the Everglades. Drought conditions elsewhere in the United States led to more than 400,000 American white ibis breeding there in the 1930s. In the 1950s and 1960s, large colonies appeared in Alabama, Louisiana, and then North and South Carolina and the Gulf Coast of Florida, and finally Texas in the 1970s. Between the 1970s and early 1990s, breeding colonies declined and disappeared in South Carolina and Florida, and greatly increased in North Carolina and Louisiana. Colonies last between one and seventeen years, their longevity related to size and quality of nearby wetlands. The longest-lasting are associated with wetlands over in size. Degradation of wetland or breeding sites are reasons for abandonment. The population of American white ibises in a colony at Pumpkinseed Island in Georgetown County, South Carolina, dropped from 10,000 to zero between 1989 and 1990 as Hurricane Hugo had inundated nearby freshwater foraging areas with salt water.
The American 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.
Fossil record