Peculiarities of an Airborne Leafeater
Most of us, sometime during childhood, learned a little verse to use in one of life's unpleasant moments. There are variants, but the basic form is: "A little birdie flying by/ Dropped a present in my eye/ But I'm a good kid, I don't cry/ I'm just glad that cows can't fly."
Readers, I regret to inform you that they've found a flying cow.
Let me clarify, before you grab for the nearest hard hat. It's not that scientists have stumbled on a herd of winged Holsteins lurking in some hidden meadow. What they have discovered is that one bird has a remarkably cow-like digestive system. To science, that was nearly as surprising as finding a bovine version of Pegasus.
The hoatzin (hoe-AT-zin) lives in the tropical zone of the Western Hemisphere, ranging from the Guianas to Brazil. It's a member of the cuckoo tribe, but looks rather like a small skinny chicken in fancy dress. (For the issue of Science magazine reporting the research, the cover photo showed a hoatzin in closeup: the bird had a thick, slate-colored beak, a bright red eye surrounded by bare blue skin, and tufts of golden-orange feathers forming a tall scraggly crest and bushy sideburns.)
Hoatzins caught the attention of early naturalists not because of their gaudy getups--many tropical birds have spectacular plumage--but because of a remarkable feature on the young of the species. They have functional claws on the first and second digits of their wings. Before they can fly, they can use these claws to climb through the branches to avoid predators. For the continuation of the hoatzin species, this is a very good thing, because they can't fly at all until they are 60 to 70 days old. Even the adult birds are poor fliers.
A second point of interest for hoatzin observers is what the birds eat: leaves. That's an extremely rare diet for flying creatures. Leaves are a comparatively low-energy food source, and flying demands a lot of energy. They are also bulky food, so a bird is better off eating more compact energy sources like seeds, fruit, or something that eats leaves (like a caterpillar, or--as Alaska's hawks would recommend--a hare).
Recently, a team of U.S., Venezuelan, and British scientists, led by Alejandro Grajal of the University of Florida, took a long careful look at hoatzin feeding habits, anatomy, and digestion. They found that the birds selected mainly new growth from relatively few kinds of plants. The preferred food items were higher in both water content and nutritive value than the run-of-the jungle leaves.
The birds have a unique digestive system anatomy. The hoatzin crop and upper esophagus are comparatively huge, so enlarged in proportion to the bird's size that there is not much room on the sternum (the wishbone) to attach flight muscles. That is why hoatzins fly poorly.
The interior workings of that strange digestive system are what make hoatzins the virtual equivalent of flying cows. As Grajal's team stated with justifiable pride in their report, they had found "a well-developed ruminant-like digestive system," the first ever found outside the mammals.
Technically, it's known as foregut fermentation. Just as in cows (and caribou and moose, for that matter), bacteria do the hard work. The byproducts of bacterial processing nourish all ruminants. The enlarged hoatzin crop and upper esophagus, like the bovine rumen, serve as a kind of food storage chamber in which resident microorganisms break down the hard-to-digest leafy matter. Cows have to reduce the size of the food bits with their teeth, chewing their cuds, but hoatzins manage that chore internally. Their crops have horny ridges on the inner surface, so that--in effect--chewing and fermentation occur in the same place.
With our seasonal supply of leaves, hoatzins wouldn't do well in the north. Perhaps we should be glad. As the researchers noted, earlier investigators really should have figured out that there was something odd about hoatzin digestive systems. The birds smell very like fresh cow manure.
