White ibises are white, long-legged wading birds with black wing tips and a long curved bill. On average, adult males weigh about 1,050 grams, which is much larger than adult females, who weigh about 750 grams. Likewise, female white ibises usually have shorter bills and wingspans. Their wingspans range from 56 to 68 cm. Newly hatched white ibises do not have feathers until they are 4 to 5 days old. When they do develop feathers, they are brownish gray at first and do not become completely white until adulthood. When they hatch, juvenile white ibises have a pinkish white bill that is mostly straight. After the first two weeks, the bill elongates and curves downward. Breeding white ibises may look different. Their pink bill fades and becomes black at the tip later in the breeding season and the legs and faces of breeding white ibises are red. A red gular pouch is also shown by female white ibises for about 10 days during the displaying phase of breeding. (Bildstein, 1987; De Santo, et al., 1990; Heath and Frederick, 2006; Heath, et al., 2003; Heath, et al., 2009; Kushlan-A, 1977; Petit and Bildstein, 1986)
White ibises (Eudocimus albus) are found near the coasts of southeastern North America. Year-round, they can be found from Delaware south to the shores of Central America. However, at the end of the summer white ibises can be found as far north as New York. During the spring breeding season, they are mostly found on the southern part 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. They may also breed on the northern coast of Venezuela. During the winter, white ibises migrate south of the Carolinas to coastal areas of the United States and internationally. (Frederick and Ogden, 1997; Frederick, et al., 1996; Frederick, 1987; Heath, et al., 2009)
White ibises feed in shallow water, so they are often found in coastal, aquatic habitats such as ponds and lakes or inland wetlands. White ibises are mostly found in coastal areas during the summer and winter months and move inland towards the beginning of breeding and nesting season. Because water depth is very important for their feeding and breeding behaviors, white ibises may move to other locations due to the rise and fall of water levels. For example, white ibises may move inland to fields and wetlands when floodwaters greater than 20 cm advance. (Bildstein, et al., 1990; Heath, et al., 2009)
White ibises are monogamous, meaning that males and females pair off and usually only mates within their pair. Their breeding season begins in early spring and includes 5 stages, display, copulation, egg-production, incubation, and chick rearing. During these stages, both males and females show major behavior changes. The first stage is display, which lasts about 10 days. This is when display flights happen and mates are chosen. During display flights, many colony members fly in a circular motion around the colony, diving up and down. Mate choice usually begins when females form their red gular sac, which is located on their throat. The females display this bright red patch and pairs form. Once they have paired, the male and female lock heads by wrapping their necks together. The pair then lunges their heads toward the ground. (Frederick, 1987; Heath and Frederick, 2006; Heath, et al., 2009; Kushlan-C, 1977)
Mating occurs during the second step. Males have many courtship behaviors such as rubbing their bill on the female’s head, preening, and head-nodding at the female. Males may become violent during this time and may sometimes injure the female. Because of this, females approach in a submissive manner and make a high-pitched squeal as a greeting. During this time, they begin building their nest. Female white ibises build the nest by grabbing twigs with their bill and forcing them into the nest framework, while males collect dead or living twigs for the females to use. Males may steal twigs and other wooden materials from other nests. Nests are normally found in the tops of trees, specifically in the crook of the trunk. However, nests have also been found on the ground. After the nest is built, the third stage of breeding, egg production, begins. Males are still extremely aggressive, which helps protect their young. The last stages are egg incubation, which lasts 3 weeks and chick-rearing, which lasts 6 weeks. (Frederick, 1987; Heath and Frederick, 2005; Heath and Frederick, 2006; Heath, et al., 2003; Heath, et al., 2009; Kushlan-C, 1977)
White ibises breed once a year during March and April. During the egg production stage, which happens 4 to 5 days after mating, female ibises lay one egg every other day, with an average clutch size ranging from 2 to 4 eggs. In one rare case, a clutch size as large as 7 eggs was found. Clutch sizes can vary depending on where they live. For example, inland populations tend to have larger clutches of 2 to 3 eggs, compared to coastal populations with an average of 1 to 2 eggs. The eggs can be brown or cream, with blueish green spots. On average, eggs weigh about 50.8 g and measure 58 by 39 mm. Once the eggs hatch, both parents help keep their young warm by laying on top of them, which last about 3 weeks. Both males and females become sexually mature after about 3 years. (Frederick, 1987; Heath and Frederick, 2006; Heath, et al., 2009)
While the eggs are being incubated, female white ibises leave to gather food for the young, while male white ibises stay to protect the nest. Eggs are incubated mostly by the male, for about 3 weeks. Because males are much bigger and more aggressive than females, they probably provide better protection for the hatchlings. While they are incubating the eggs, male testosterone levels drop, which may help them stay and watch over the young. Hatchlings are not able to move their weak legs right after hatching. Likewise, young white ibises cannot see for 1 to 3 days after hatching, because their eyes are not yet open and do not fully open until day 9. Due to this, the young are completely dependent upon their parents. Both parents help feed the young. Because young white ibises have underdeveloped neck muscles, hatchlings are unable to keep their neck upright, so they lie on their side for 1 to 2 days until their muscles develop. During this time, white ibises lie on top of their young to keep them warm. On their 2nd day, young ibises begin eating food. The young stay in the nest for about 40 days, while doing short flights and gaining the ability to find food. Adult ibises teach their young to fly by standing close and beginning to take off, the young mimic the adults. Once they are able to fly, juveniles are independent and leave their colony. (Frederick, 1987; Heath and Frederick, 2005; Heath and Frederick, 2006; Heath, et al., 2003; Heath, et al., 2009)
From banding data, the oldest white ibis found in the wild was 16 years and 4 months old. However, the average lifespan and survivorship of wild adult ibises has not been reported. The maximum lifespan seen in captivity was 27 years and 7 months, with an average in captivity of about 20 years. Nestling survivorship is much lower after the first 20 days post-hatching and varies with age and year, which may be explained by predation. (Brouwer, et al., 1993; Clapp, et al., 1982; Heath, et al., 2009)
White ibises are colonial birds. They are very social and are found in large colonies of 500 to 15,000 birds. These large colonies help protect them from predators. For example, if a predator threatens colony members, large groups of ibises come together to scare away the threat. The colony does many activities together including feeding, foraging, nesting, mating, and roosting. While searching for food, white ibises forage mostly by thrusting their bill into the wetland, while traveling at a very slow pace. Once prey are found and removed, white ibises swallow the prey, lunging their head forward. White ibises feed in large groups (>5,000 birds) mostly in the early morning or late evening, except during the nesting period. These birds may fly over 30 km in a single trip. While flying, they stay in a “V”-shaped group, which probably makes them more aerodynamic. Learning to fly in this formation is very important for young white ibises, those that do not, have a much higher mortality rate. (Frederick and Ogden, 1997; Heath, et al., 2009; Kushlan, 1979; Petit and Bildstein, 1986)
White ibises usually travel based on how much food is available and the conditions of their current habitat. They do fly to foraging areas, generally traveling less than 40 km at a time. Colonies of more than 1,000 breeding pairs cover over 800 square kilometers of wetlands. (De Santo, et al., 1997; Forrester and Spalding, 2003; Heath, et al., 2009)
White ibises find their prey by feeling with their bill. They are mostly quiet birds, but do communicate with "honking" noises to others in the colony. Newly-hatched ibises chirp and make "begging" calls. This helps parents determine which juvenile is theirs. During the breeding season, male and female ibises develop new colorations, which indicates the beginning of courtship behavior. Males also display as warning signals such as bill chattering and horizontal thrusting toward opponents. (De Santo, et al., 1990; Heath and Frederick, 2006; Heath, et al., 2003; Heath, et al., 2009)
White ibises search for prey in shallow waters, about 10 to 15 cm deep. Their diet includes insects, crustaceans, small fish, snails, frogs, and other aquatic organisms. White ibises hunt by feeling the ground with their long, curved bill. This long bill allows them to dig into the mud and find prey. Their long bill also helps search for food in slightly deeper water. However, food availability is limited by the water levels. For example, if water levels are too high, white ibises cannot forage to the bottom, but if water levels are too low, their prey may no longer be present. Adult white ibises feed in very large groups, while juveniles feed on the perimeter of the colony in the less desirable waters. Because males are larger, they need more food and forage longer than females. (Aguilera, et al., 1993; Bilstein, 1983; Heath and Frederick, 2006; Heath, et al., 2009; Kushlan, 1979)
White ibises have few predators, such as fish crows, which eat ibis eggs. To prevent this, white ibises rarely leave the nest unattended. Males show aggressive behaviors to fend off fish crows. Predation by these crows decreases the colony's nesting success and is a major cause of mortality for white ibises. Their eggs are also preyed on by black-crowned night herons, Virginia opossums, raccoons, and rat snakes. Adults have few predators, although they may be preyed on by alligators. (Dronen and Blend, 2008; Heath, et al., 2009; Shield and Pamell, 1986)
Colonies of white ibises affect local ecosystems by taking up nutrients while foraging and moving them to nesting areas. This allows nutrients, such as potassium, phosphorus, and calcium, to be dispersed throughout the habitat. Many parasitic organisms inhabit adult white ibises including 17 species of nematodes, 21 species of trematodes, 2 species of cestodes, 2 species of acanthocephalans, 3 species of mites, and 5 species of lice. (Forrester and Spalding, 2003; Heath, et al., 2009)
In Louisiana, crayfish producers have been significantly impacted by crayfish predation from white ibises. Crayfish farmers have been known to hunt ibises to protect the crayfish from predation. White ibises have no other known adverse effects on humans. (Heath, et al., 2009)
In the past, white ibises had been hunted and sold as a food source by Native Americans and individuals that lived near their colonies. In some areas, this still continues today. Some consider this bird to be a delicacy due to their taste, which is thought to be from their crayfish diet. (Audubon, 1835; Heath, et al., 2009)
According to the IUCN red list, white ibises are considered a species of least concern and they have no special status on the US Migratory Bird Act or the US Federal List. Currently, their population is large and mostly stable. However, according to the North American Waterbird Conservation Plan (NAWCP), there is some concern for their populations because of their migratory pattern and their limited range. Deterioration of their natural habitat is a big threat for their population. (Frederick, et al., 1996; Heath and Frederick, 2005; Heath, et al., 2009; Kushlan, et al., 2002)
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