American white ibis

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 (Virginia, the Carolinas and 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. 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. 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. Adults have black wingtips that are usually only visible in flight. 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. 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 . 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. These lines fly in an undulating pattern as they alternately flap and glide. Soaring in a circular pattern is also seen.

The main call of the American white ibis is a honking sound, transcribed as urnk, urnk, or hunk, hunk. 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

thumbnail|right|An American white ibis at [[Riverside Park (Jacksonville)|Riverside Park, Jacksonville, Florida]]

right|thumb|Adults in shallow water at [[Merritt Island National Wildlife Refuge near the Atlantic coast of Florida]]

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. thumb|Adult American white ibis on pavement outside of Orlando, Florida 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. 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.

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. Two species, one living and one extinct, have been recovered from the Talara Tar Seeps in northern coastal Peru. Eudocimus peruvianus was described from a tarsometatarsus that differed slightly from E. albus, whose remains were also found there. Remains of neither species are common in the beds. The tar seeps have been dated at 13,900 years old. The American white ibis is still found in Peru.

Behavior

thumb|American white Ibis birds in [[Dade City, Florida|Dade City, Florida ]]

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. American 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 deep and flick water over themselves with each wing in succession. Hundreds of birds may bathe together around the time of courtship.

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. These extra-pair copulations are usually done after the within-pair copulations, and make up about 45% of all total matings, although only about 15% of all extra-pair copulations are successful.

thumb|left|Juvenile in [[Everglades National Park. Some of its brown feathers have molted and have been replaced with white feathers.]]

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 success rate of parents raising one or more young to 20 days of age ranges widely from 5 to 70% of nests, and varies greatly between nearby colonies. Studies have also shown that years with higher nesting numbers had significantly faster spring drying rates of water bodies than years with low nesting numbers. This is because faster drying rates means that there are fewer fish and increased available area where crayfish can be hunted.

The main cause of nest failure among the species is due to nest abandonment, the leading cause of which is inundation from extremely high tides. Parents abandoned 61% of all nesting starts either during or immediately after extremely high tides. The eggs float out of the flooded nests, or get washed out into the sea by wave action. Incubating parents usually abandon the nest when the water or tidal levels reaches above the bottom of the nest cup. Nevertheless, there have been instances where the parents have been observed to transport their eggs to another nest in an attempt to salvage some eggs. However, despite the fact that some nesting sites face high chance of tidal damage every breeding season, American white ibises still continue to nest in these areas because of other favorable conditions such as abundant nearby food sources and low egg predation rates.

thumb|right|Juvenile in Florida

The eggs hatch after about three weeks and the young are attended by both parents. Males are present around the nest for most of the day, and females most of the night. The parents exchange nest duties in the morning and in the evening. Most of the feeding of the chicks occurs during the period where they swap nesting duties. Little feeding is done in the period of the day that is between the two duty swaps and no feeding is done between midnight and 6 a.m. Chick mortality is highest in the first twenty days post hatching, with anywhere from 37 to 83% of hatchlings surviving to three weeks of age in the Everglades. For many bird species that have sexually dimorphic nestlings, mortality rates are higher for larger-sized male nestlings as a result of the parents' inability to meet its greater nutritional needs. However, in the case of the American white ibis, the male nestlings actually have a lower mortality rate as compared to the females despite being on average 15% greater in mass as compared to its female counterparts. Other predators of eggs and young include the boat-tailed grackle (Quiscalus major), black-crowned night heron (Nycticorax nycticorax), gulls, and possibly vultures, as well as the common opossum (Didelphis marsupialis), raccoon (Procyon lotor), and rat snakes (Elaphe species). Outside the nesting season, the diet is highly variable, as abundance and types of prey depend on both the region and the habitat. In the Llanos, located in 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. 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. 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 American white ibis is found in mixed-species foraging flocks with the glossy ibis (Plegadis falcinellus) in flooded fields, and the two species select different food items with little overlap; the former foraging for crabs and aquatic insects and the latter feeding mainly on grain. An isolated event of intraspecific predation in juvenile American white ibis has been observed, where a juvenile attacked and consumed a chick from another nest.

Foraging

thumbtime=1|thumb|Video of adults foraging on Bonita Beach in [[Bonita Springs, Florida]]

thumb|Adult white ibis foraging for food in a front garden in [[Port Orange, Florida]]

During the summer, the American white ibis roams along the coast of tidal flats and mangrove swamps as the inland marshes are usually flooded. However, as the water level recedes in the fall, populations at the coast shift their foraging area inland, to inland marshes and swamplands. and is one of a number of wetland-dependent bird species which forages in man-made ponds on golf courses in the southwest of the state.

The American white ibis is a tactile, nonvisual forager, which limits its ability to choose from a wide variety of prey. For the most part, the American white ibis forages for food by tactile probing. It wades slowly through shallow water and sticks its long, downcurved bill into the substrate of the water body with its bill held at around agape at the tip, and sweeps its long bill back and forth across the bottom to pick out suitable food items. Along with the scarlet ibis, the species coexists with another five species of ibis in the Llanos in Venezuela. American white ibis males are aggressive to and take prey items from smaller ibises, but the smaller females are more often the victims of this behavior.

Juveniles have lower foraging efficiency compared to adults and in most feeding flocks, the juveniles are usually outnumbered by the adults. They usually tend to stay close to one another and forage for food together at the peripheral region of the group. During the breeding season, adult male ibises have been recorded raiding other parent ibises who are feeding their young in the colony. The raiders force their bill down the throat of the victim—either the parent about to disgorge their food or recently fed young—and extract the ball of food. This behavior allows the otherwise starving adult males to obtain food without having to spend long periods of time away from the nest, and prevent its female mate copulating with another male ibis, which would reduce its own reproductive success. Females and juveniles almost never try to drive off the larger and more aggressive pirating males, but instead try to avoid or move away from them.

Parasites and mortality

Causes of death of adult ibis are not well known. Alligators could feasibly prey on nesting ibises but there has been little research in the area.

A total of 51 species of parasitic worm have been recovered from the American white ibis, predominantly from the gastrointestinal system and particularly the small intestine. These include Cestoda (tapeworms), Acanthocephala (thorny headed worms), Nematoda (roundworms), Digenea and Spirurida. Several roundworm and spirurid species have been found in the lining of the gizzard. Nematodes are more prevalent in American white ibis from freshwater habitats, and cestodes more frequent in those from saltwater areas. One nematode found in adult birds, Skrjabinoclavia thapari, is borne in the fiddler crab as an intermediate host, while the thorny headed worm species Southwellina dimorpha is carried in crayfish and infests both adult and juvenile ibis. and another species, Haemoproteus plataleae, has been recovered from the blood of adults and nestlings, and can hence be transmitted before the young leave the nest. The larvae of two species of mite of the family Hypoderidae, Phalacrodectes whartoni and Neoattialges eudocimae, have been recovered from under the skin. Two species of the louse suborder Mallophaga, Plegadiphilus eudocimus and Ardeicola robusta, also parasitise the bird.

Status

The American white ibis is classed as being of least concern on the IUCN Red List. 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. Hormone levels in males are affected, leading to a decrease in the rates of key courtship behavior, and fewer approaches by females during the mating season. In addition, methylmercury also increased male-male pairing behaviors by 55%. Both the chemically induced "homosexual" behavior and the diminished ability to attract females by males have reduced reproduction rates in affected populations. and it also makes them more likely to abandon nests owing to the disruptive effect of the pollutant on the bird's hormone systems, which in turn affects parental care behavior.

In culture

Native American folklore held that the bird was the last to seek shelter before a hurricane, and the first to emerge afterwards. The bird was thus a symbol for danger and optimism. For this reason, the University of Miami adopted the American white ibis as its official athletics mascot in 1926, and the yearbook was known as The Ibis from that year.

Similar to the University of Miami, South Georgia Tormenta FC uses the ibis as its mascot and nickname, incorporating the bird into the team's crest.

References

External links

American white ibises on Arkive. Shows videos made by the BBC natural history unit.