27-3-2017 RANCHO NATURALISTA - WHITE IBIS (Eudocimus albus)

The American white ibis (Eudocimus albus) is a species of long-legged wading bird that often flies in long loose lines or V formations. During the breeding season, American white ibises gather in huge colonies near water where males don’t hesitate to pirate food from unmated females and juveniles. Populations of this species in central Venezuela overlap and interbreed with the Scarlet ibis and the two have been even classified by some authorities as a single species.

American white ibises breed along the Gulf and Atlantic Coast and the coasts of Mexico and Central America. Outside the breeding period, the range of these birds extends further inland in North America and also includes the Caribbean. They are also found along the northwestern South American coastline in Colombia and Venezuela. American white ibises live in a variety of habitats, although shallow coastal marshes, wetlands, and mangrove swamps are preferred. They also frequent muddy pools, mudflats, and even wet lawns. Populations that are away from the coast and shoreline, particularly in southern Florida, often reside in marshes, ponds, and flooded fields. In summer, these move to more coastal and estuarine habitats as inland waterways become flooded with summer rains and ibises find the water levels too deep to forage effectively.

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 wader. 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 that 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 the 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.

American white ibises are gregarious birds, however, they are territorial and defend the nesting and display sites against intruders. When threatened or defending a territory American white ibises perform displays that include lunging forward with the bill in a horizontal posture, standing upright, and snapping the bill opposite another bird engaging in the same display. Birds also lunge and bite, often holding onto an opponent's head or wings. American white ibises are active during the day and for the most part, they forage for food by tactile probing. They wade slowly through shallow water and stick their long, downcurved bill into the substrate of the water body and sweep it back and forth across the bottom to pick out suitable food items. Groping with a wide open bill is a technique used by ibis in deeper water when alone, as is head swinging, in which the ibis swings its wide open bill widely in open water. On land, American white ibises locate prey by sight and pecks and do not have to insert their bill into the substrate. When not feeding they spend their time resting, roosting, attending to their nests, and flying. American white ibises fly with necks and legs outstretched, often in loose lines. These lines fly in an undulating pattern as they alternately flap and glide. Soaring in a circular pattern is also seen. Heights of 500 to 1,000 m (1,600 to 3,300 ft) may be reached as birds glide over flights of 20 km (12 mi) or more. More commonly, they fly between 60 and 100 m (200 and 330 ft) above the ground, gliding or flapping. When roosting these birds spend much time 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. Before preening ibises often bathe; they squat in water 5-20 cm (2-7.9 in) deep and flick water over themselves with each wing in succession. Hundreds of birds may also bathe together around the time of courtship. The main call of American white ibises is a honking sound, transcribed as ‘urnk, urnk’, or ‘hunk, hunk’. This 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.