The New Neotropical Companion

head. Its plumage is a combination of soft browns, with

rich buff on breast and wings. Hoatzins are weak fliers,

a feature that contributes to their primitive appearance.

They live in noisy groups that occupy dense riverine

vegetation. Their nonmusical, guttural vocalizations

add to the auditory experience at Neotropical oxbow

lakes.

Though originally considered taxonomically related

to chickens, Hoatzins have proven difficult to classify.

For a while they were linked with the cuckoos, but the

most recent genetic analyses suggest that Hoatzins

are related more closely to cranes and shorebirds. The

species is placed in its own family, Opisthocomidae,

and order, Opisthocomiformes. It has an unusual

diet, unusual breeding system, and unusual juvenile

b ehavior.

Hoatzins are among the few avian folivores, feeding

mostly on leaves (over 80% of the diet), often from

plants that are typically loaded with secondary

compounds, such as plants of the arum family, including

philodendrons. (Recall the discussion of folivory in

chapter 11.) Leaves are bitten off, swallowed, and

ground into a large bolus in the bird’s oversize crop (the

anterior of the digestive tract). With the aid of a diverse

and abundant microflora housed within the expanded

crop and esophagus, the bolus slowly ferments and is

digested. The birds benefit not only from some of the

digestive products of their microflora, but the bacteria,

which are as concentrated in Hoatzins as they are in

bovines, also help detoxify secondary compounds.

The odd amalgamation of partially decomposed leaves

gives the bird an unpleasant odor (rather like cow

manure), a benefi cial characteristic, since it renders

the flesh distasteful to human hunters. Though a few

other bird species are known to eat leaves, Hoatzins

represent the only known case of a bird species that

exhibits foregut fermentation, a unique adaptation

resulting from coevolution with microorganisms that

enables the birds to survive on a diet of normally

indigestible leaves from numerous plant species.

Hoatzins are communal breeders, and between

two and seven birds cooperate in a single nesting.

Nonbreeding birds called helpers typically assist the

pair responsible for the eggs. Nests with helpers are

considerably more successful at fledging young than

nests lacking helpers. The helpers aid in incubation

and feeding young, enabling the juvenile birds to grow

more quickly and thus reduce their vulnerability to

predators. The streamside nest is a cluster of thin sticks

so loosely constructed that the eggs are usually visible

from beneath.

Juvenile Hoatzins bear a superficial resemblance to

Archaeopteryx, one of the first birds, whose fossilized

remains established that birds evolved approximately

150 million years ago during the Mesozoic era, when

dinosaurs flourished. Young Hoatzins possess claws on

their first and second digits that enable them to climb

about in riverside vegetation. Should they be faced with

danger, they escape by dropping from the vegetation

into the water; they swim and dive efficiently. When

danger passes they use their wing claws to help in

climbing back onto the vegetation. Wing claws were

also present on Archaeopteryx, though no one suggests

that the resemblance between the modern Hoatzin

and the first bird is other than coincidental. Young

Hoatzins lose their wing claws as they become adults.

Plate 12- 54. The Agami Heron in a typical posture as it quietly

stalks fish to capture. Photo by Kevin Zimmer.

Plate 12- 55. The ever- so- odd Hoatzin. Photo by Gina Nichol.

chapter 12 cruising the rivers to the sea 225