Alimentary, My Dear Hoatzin

The average bird spends most of its day searching for the berries, insects, and other foods that provide the energy necessary for flight. Not the hoatzin, a pigeon-size denizen of South American floodplains, whose unique digestive tract allows it hours of leisure. Alejandro Grajal, a zoologist with the Wildlife Conservation Society, has found that hoatzins live on a diet of plants, more like a cow or sheep than a bird.

Only a few birds can make use of plant fiber at all. Ducks, geese, grouse, and ostriches can digest some plant matter in their hindguts, but they still excrete the bulk of what they eat. In 1989, Grajal discovered that hoatzins had foreguts--the equivalent of a second stomach--that presumably enabled them to extract more nutrients from plants, as cows and other ruminants do. But until now he didn't know just how efficient an herbivore the hoatzin was.

To find out, Grajal kept several hoatzins in cages with trays in the bottom to collect feces. He measured the weight difference between what the birds consumed and what they defecated to see how much of their food they were digesting. To see how fast they were digesting it, he fed them colored plastic markers and observed how long it took for the markers to come out the other end.

The hoatzin, Grajal found, digests about 70 percent of the fiber it ingests. "I was surprised with the results," says Grajal. "This is an animal the size of a pigeon or pheasant with the digestive efficiency of a sheep or small ruminant--which is ten times heavier and larger than a hoatzin." Getting the most out of food is a slow process, though: it takes almost two days for the hoatzin to pass its food, which again makes it more like a ruminant than a bird. Most birds pass their food within an hour (although turkeys may take as long as five).

Furthermore, hoatzins can eat plants containing toxic compounds, like alkaloids, that are supposed to ward off plant eaters. Grajal suspects that bacteria in the hoatzin's foregut protect the bird by breaking these compounds down.

All in all, the hoatzin doesn't face much competition in its quest for dinner. While other birds spend the whole day eating and looking for their next meal, hoatzins only have to gorge themselves twice a day for 30 minutes. "The search for food is a relatively small portion of their life requirement," says Grajal. "They only have to stretch the neck, literally, to eat." And what does the artful avis do with all its spare time? Says Grajal, "It doesn't do much. It sits for quite long periods of time, which is unusual for a bird."