The tropical majority: have studies of temperate-zone birds warped our view?

Science News, Dec 1, 2001 by Susan Milius

Eugene Morton and Bridget Stutchbury don't chase Alice down a rabbit hole when it's time for fieldwork. But the pair of ornithologists does end up in territory where familiar patterns reverse and truisms turn inside out.

Their wonderland lies in the tropics of the Western Hemisphere, where birds often fail to follow the rules, at least as temperate-zone scientists have explained them.

Morton, now at the Smithsonian Institution in Washington, D.C., recalls that his first hint of the ecological surprises waiting in the tropics came from his grad-student fieldwork in Panama. He began wondering whether clay-colored robins there had gone crazy. "They breed at the wrong time of the year," he says.

In the temperate zone, the breeding season for most bird species is determined by baby food, baby food, and baby food. Chicks need protein in daunting amounts, and the spring frenzy of nesting typically sends the parents foraging right when food supplies peak.

Morton found that clay-colored robins, tropical cousins of the red-breasted migrant, breed with tight synchrony but in the dry season, when insects dwindle to their scarcest. Chicks often starve in their nests, and those that: do survive typically start their independent life small and frail. When Morton brought chicks extra food himself, he boosted the percentage of chicks that successfully flew away from the nest.

As he puzzled over the birds, Morton began to wonder if some seasonal surge in attacks by coatis or other predators might have driven the birds to nest at an otherwise terrible time. Some observations support that explanation. Morton found that during the wet season, predators destroy about 85 percent of bird nests. During the dry season, however, nest losses drop to 58 percent.

Since then, Morton's come up with ideas for additional forces pushing claycolored robins to breed at a strange time, and research on other species has revealed yet more possibilities for unexpected breeding-system dynamics. Such rule breaking in the tropics extends to other aspects of bird life, say Morton and Stutchbury, who are based at York University in Toronto. In their new treatise, Behavioral Ecology of Tropical Birds (Academic Press, London, 2001), they describe species that don't fit the previously described pattern for territory defense, hormone surges, or outside-the-nest trysts.

Despite the surprises, the researchers refuse to say that bird life gets peculiar in the tropics. Such talk comes from a perspective they don't endorse. "It is ironic that tropical birds are viewed as strange, and perhaps even bizarre, when they vastly outnumber temperate-zone species," write Morton and Stutchbury. "Our premise is not `why tropical birds are so different' but rather `why temperate-zone birds are so atypical.'"

Temperate zone species seem like the plain vanilla of ornithology because they're what people have studied most, Morton contends. By the mid-1990s, published studies in behavioral ecology had featured that midlatitude classic species, the red-winged blackbird, more often than all the tropical birds combined.

Morton predicts that anyone who bothers to count will find the same overwhelming abundance of studies on other temperate standbys, such as barn swallows and great tits.

"There's no question that is the case," says Robert Ridgely of the Academy of Natural Sciences in Philadelphia. "We do have this bias."

However, Ridgely sees no deep psychology in the skew. Although he's just finished some 20 years of work on a guide to the birds of Ecuador, most professional ornithologists stay closer to home. And home has mostly been in temperate countries.

Compared with the tropics, these milder climes seem bird poor. Panama covers land the size of South Carolina but has some 900 bird species--about the number in the whole of North America. The difference in variation shows up with particular drama in certain kinds of birds. For example, eastern Canada and the United States offer homes to one genus of hummingbirds and one of tanagers. In tropical Brazil, ornithologists have found 30 genera of each.

To explain differences between the low-latitude crowd and its temperate relatives, Morton and Stutchbury turn to the basics or tropical habitats. Temperatures and day length tend to vary only a little during a year. Seasons change, but mostly from wet to dry, not from warm to freezing.

Without a temperate springtime, when's a bird to nest? Some species reproduce year-round, but most show what Morton calls a surprising degree of seasonality. The season isn't the same for all species, leaving ecologists with quite a complicated phenomenon to explain.

Peaks in food supply do dovetail nicely with nesting season for some species. Hawaii's little nectar-sipping iiwi do most of their breeding when flowers bloom abundantly. The timing of southern Florida's white-crowned pigeons' breeding similarly reflects the abundance of fruit of the Florida poison tree.

However, there's food and then there's food, cautions Morton. Two species of manakins in Costa Rica eat mostly fruit, but they breed during a sorry time of year for collecting it. Morton proposes that such species feed insects to their young, since the nesting season does coincide with a buggy season.