Adult plumage is virtually all scarlet. The feathers may show various tints and shades, but only the tips of their wings deviate from their namesake color. A small but reliable marking, these wingtips are a rich inky black (or occasionally dark blue) and are found only on the longest primaries – otherwise the birds' coloration is "a vivid orange-red, almost luminous in quality. Scarlet ibises have red bills and feet however the bill is sometimes blackish, especially toward the end. They have a long, narrow, decurved bill. Their legs and neck are long and extended in flight.
A juvenile scarlet ibis is a mix of grey, brown, and white. As it grows, a heavy diet of red crustaceans produces the scarlet coloration. The color change begins with the juvenile's second moult, around the time it begins to fly: the change starts on the back and spreads gradually across the body while increasing in intensity over a period of about two years. The scarlet ibis is the onlyshorebird with red coloration in the world.
Adults are 55–63 centimetres (22–25 in) long, and the males, slightly larger than females, typically weigh about 1.4 kilograms (3.1 lb). Their bills are also on average around 22% longer than those of females. The life span of the scarlet ibis is approximately sixteen years in the wild and twenty years in captivity. An adult scarlet ibis has a wingspan of around 54 centimetres (21 in). Though it spends most of its time on foot or wading through water, the bird is a very strong flyer: they are highly migratory and easily capable of long-distance flight. They move as flocks in a classic V formation.
Mating pairs build nests in a simple style, typically "loose platforms of sticks" of a quality sometimes described as "artless". They roost in leaf canopies, mostly preferring the convenient shelter of young waterside mangrove trees. Scarlet ibises like wet, muddy areas such as swamps, but for safety they build their nests in trees well above the water. If they can, they nest on islands, where their eggs and chicks are less likely to be in danger from predators.
To attract a female, the male will perform a variety of mating rituals such as "preening, shaking, bill popping, head rubbing, and high flights. As with most birds, mating does not involve any coupling or insertion: instead, a transfer of seminal fluids occurs during external contact between the cloacal openings. After a gestation period of five to six days, the female lays a clutch of three to five smooth, matte eggs which typically incubate for 19–23 days. After a successful courtship, pairs remain faithful and cohabitant, sharing parental responsibilities for the young.
Feeding:Their distinctive long, thin bills are used to probe for food in soft mud or under plants. Popularly imagined to be eating only shrimp, a recent study in Llanos has found that much of their diet consists of insects, of which the majority were scarabs and ground beetles . One species in particular, a scarab beetle Dyscinetus dubius, formed a large part of the diet. In contrast, the diet of the co-occurring American white ibis there differed, the latter consuming more bugs, fish and crustaceans.They do, however, eat much shrimp and other similar fare like small crabs, mollusks and other crustaceans. The large quantity of shrimp and other red shellfish produces a surfeit of astaxanthin, a carotenoid which is the key component of the birds' red pigmentation. When kept in zoos, the birds' diet often contains beetroot and carrot supplement to maintain color vibrancy in their plumage.The Llanos are notable in that these wetland plains support seven species of ibis in the one region. Here, scarlet ibis are the most aggressive, and attack other species to steal their food. They have also been observed trailing white-faced whistling ducks (Dendrocygna viduata) and domestic livestock, and catching insects disturbed by them.