White Ibis

White Ibis
Photo by Themassiah, licensed by CC BY-SA 3.0.

The white ibis, Eudocimus albus, stands about two feet tall and has a wingspan of three feet. Its plumage is entirely white, except for black wingtips that are usually only visible in flight. It has a long, down-curved pinkish-orange bill, and pinkish-orange facial skin and legs. The sexes have similar plumage; however, there is sexual dimorphism in size and proportion as males are significantly larger and heavier than females (weighing, on average, over half a pound more) 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 wader. The decurved beaks of white ibises, along with their black-tipped flight feathers, easily distinguish them from white egrets. Similarly, the other two species of ibis found along the Texas coast, the white-faced ibis and glossy ibis, are smaller and dark-colored – not likely to be confused with the actually white ibis. The species name is the Latin adjective albus, meaning "white.” Alternative common names that have been used include Spanish curlew and white curlew.

White ibises are native to the coastal regions of southeastern North America. Year-round, their range extends from Delaware to the shores of Central America. During the end of the summer months, white ibises can be found as far north as New York. During the spring breeding season, their population seems to be localized on the southern portion of the Atlantic coast (south of Virginia) including the entire U.S. and Mexico Gulf coast, Cuba, Dominican Republic, the Brazilian and Colombian coast, and Hispaniola. Remote breeding locations can also be found on the northern coast of Venezuela. During the winter months, white ibises migrate south of the Carolinas and localize in the coastal areas of the United States and internationally. Casual visitors are found in the eastern Edwards Plateau and are accidental in the Trans-Pecos region.

Because they require shallow water for feeding, many are found in coastal, aquatic environments such as swamps, marshes, wetlands, and wet agricultural fields. Because water depth is of the upmost importance for their feeding and reproductive behaviors, white ibises may shift locations due to the rise and fall of water levels, such as moving inland to fields and wetlands when floodwaters advance.

White ibises are foraging birds that search for their prey in shallow waters. Their diet includes crustaceans, small fish, snails, frogs, insects, etc. White ibises hunt through non-visual, tactile probing with their long, curved bill. Bill tips are very sensitive, and contact with prey elicits a reflex closing action. To find prey, ibises thrust their long bill into the wet substrate, while wading at a very slow pace. Once prey are snapped up, white ibises swallow by rapidly lunging their head forward. Food availability is extremely limited by the water levels. If levels are too high, they can’t forage the wetland substrate their prey inhabits. In contrast, when water levels are too low, many of the aquatic organisms they consume are no longer present.

While adults can feed on fiddler crabs and such in brackish waters, the salt toxicity prevents their nestlings from developing normally on brackish-water prey because their salt-excreting glands are not yet fully developed, so the adults fly inland to freshwater habitats and catch crawfish for their nestlings. They have also been known to supplement their chicks' diet with items such as cockroaches and rotting food from human garbage in poorer years, when fish and crawfish are scarce. White ibises often feed in large groups, sometimes more than 5000 birds. When other species are around, they tend to seek small prey that can be swallowed whole, since other predators such as herons and egrets often take the opportunity to rob the ibis of its catch. Other wading birds even follow behind the white ibis and catch prey that has been disturbed by the ibis’s probing.

White ibises practice a colonial lifestyle. These birds are highly sociable and are often found in large colonies of 500 to 15,000 birds. The colony participates in many activities together including feeding, foraging, nesting, mating, and roosting – but not preening. Much of the time roosting is spent preening, working their feathers with their long bills, as well as rubbing the oil glands on the sides of their heads on back plumage. They generally only preen themselves, not engaging in allopreening (social grooming) unless part of courtship behavior. Hundreds of birds may also bathe together around the time of courtship. They are relatively quiet birds, but are known to communicate with a honking sound – transcribed as urnk, urnk or hunk, hunk – to others in the colony. 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.

Like other species of ibis, the white ibis flies with neck and legs outstretched, often in long loose lines or V formations. Learning to fly in this formation is essential to the survival of adolescent white ibises. Juveniles that are unable to develop this behavioral pattern exhibit a significantly higher mortality rate. A 1986 field study in North Carolina noted over 80% of adult ibis flying in this manner, while juveniles rapidly took up the practice over the course of the summer. The V formation may improve aerodynamics, thus lowering energy expenditure, an important skill to learn when these birds regularly fly up to about 20 miles just for foraging. Heights of 1,600 to 3,300 feet can be reached as birds glide. More commonly, birds fly between 200 and 330 feet above the ground, gliding or flapping at a rate of around 3.3 wingbeats a second.

In Texas, according to colonial waterbird censuses, the annual breeding population fluctuates widely between about 1,600 and 32,000 pairs. In general, the wetter the winter/spring season, the larger the number of breeding birds. March thunderstorms commonly trigger courtship flights. White ibises are a monogamous species that go through five stages during the breeding season, which begins in early spring. The five stages include display, copulation, egg-production, incubation, and chick rearing. These stages are tied to major behavioral changes exhibited by both sexes. The first stage is display. This is when mate choice is decided and display flights take place. The legs and faces of breeding ibises turn red, and females display a red gular sac (featherless throat pouch). Involving many individuals of the colony, display flights consist of birds flying in a circular motion around the perimeter of the colony while diving up and down. Mate choice seems to be catalyzed by the gular sac on the female. Once pairing has occurred, the male and female lock heads by wrapping their necks together, then thrusting their heads toward the ground.

Males display many courtship behaviors during the second stage, including rubbing their bill on the female’s head, preening, and head-nodding at the female. Due to the aggressive nature of displaying males, females generally approach in a submissive manner. Males take the process of pairing extremely serious and may attack other males or abandon the nest if another male has bred with his mate. Although, while the 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 his primary female partner. These extra-pair copulations make up about 45 percent of the total attempted matings, though only about 15 percent of extra-pair copulations are successful. Females are receptive towards extra-pair copulations, but male mate-guarding greatly reduces the rate of successful female involvement in attempted extra-pair copulations by other males.

Nest construction begins during this time and is dependent on both parents. Males collect twigs for the females to use, and females construct the nest by forcing twigs into the framework. Males may steal twigs and other materials from other nests. Completed nests are platforms of sticks, reeds, and marsh grasses and are located in trees, bushes, reeds, or even on the ground. In Texas, most nest in shrubs and low trees on barrier islands, salt marshes, and dredge-material (spoil) islands. The birds nest colonially near or above a marsh or wetland, often with herons and spoonbills. In fact, the presence of earlier nesting species may prompt ibises to choose the same site. After the nest construction period, the third stage of breeding, egg production, begins. Males are still extremely aggressive towards other males of the colony, and even to their own mate, as a way of protecting their young.

During the egg production stage, female ibises lay one egg every other day for about a week, with an average clutch size of 2 to 4 eggs. Inland breeding populations tend to have larger clutches of 3 to 4 eggs, while coastal populations average 1 to 2 eggs. The eggs are brown or cream, with blueish green spots. Hatchlings are initially unable to move much or see. Their eyes do not fully open until day 9. They also have underdeveloped neck muscles, and so are unable to sustain the weight of their neck and have to lie on their side for 1 to 2 days until their muscles properly develop. Their bills are relatively straight at this point, and they lack feathers until they are 4 or 5 days old. (The bill begins to elongate and curve downward after about 2 weeks.) Due to these limitations, the young are completely dependent upon their parents. Newly-hatched ibises have a distinctive chirping or "begging" call. This verbal communication enables the parents to distinguish their young from the rest of the colony. Female white ibises leave to gather food for the young, while the males stay to protect the nest. Because males are significantly bigger and more aggressive than females, this likely provides better protection for the hatchlings. Hatchlings are preyed on by fish crows, black-crowned night herons, opossums, raccoons, and rat snakes.

The last stage, chick rearing, takes about 6 weeks. Adult ibises teach their young to fly by standing close to them and beginning to take off. This allows the young to mimic the adults’ technique. Fledglings have largely brown plumage and only the rump, underwing, and underparts are white. The legs become light orange. Once they are able to fly, juveniles are independent. As they mature, white feathers begin appearing on the back and they undergo 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. Sexual maturity for both males and females is reached at age 3. From banding data, the oldest white ibis found in the wild was 16 years and 4 months old. The maximum lifespan observed in captivity was 27 years and 7 months, with an average in captivity of about 20 years.

In the past, white ibises have been hunted and sold as a food source. In some areas, this practice still continues today. Some consider this bird to be a delicacy due to their appealing taste, which is thought to be derived from their crawfish diet. John James Audubon reported that they have orange flesh and a strong fishy taste. Crawfish farmers in Louisiana sometimes shoot them for foraging in crawfish ponds. Overall, the impact of hunting is not thought to be major. White ibises have no other known adverse effects on humans. According to the IUCN red list, white ibises are listed as least concern and have no special status on the US Migratory Bird Act and the US Federal List. Currently, their population is large and relatively stable. Although white ibis coastal breeding colonies are protected, their inland feeding areas are subject to rapid human development. They cannot sustain their populations without both habitats, so deterioration of inland habitat is a threat to their population. Human pollution has also affected the behavior of the white ibises in some areas due to an increase in the concentrations of methylmercury, which is released into the environment from untreated waste. Exposure to methylmercury alters the hormone levels of the white ibis, affecting their mating and nesting behavior and leading to lower reproduction rates.

Where I learned about white ibises, and you can too!

The Texas Breeding Bird Atlas

Animal Diversity Web

IUCN Red List