First experiments on birds of paradise tie metabolism, ecology

December 10, 2003

GAINESVILLE, Fla. — Birds of paradise, those resplendent, elaborately-shaped birds of the South Pacific, hardly seem like a potential source for ironclad biological fact.

In the first experimental work performed on the birds, a UF researcher has discovered a remarkably strong relationship between the birds’ metabolic rates – how much energy they use – and their environment.

In a paper scheduled to appear Thursday in the journal Nature, UF zoologist Brian McNab reports he can account for 99 percent of the variation in metabolic rate among different species of birds of paradise based on three factors: body weight, food habits and the altitude at which the birds live. While such strong correlations are not unusual in the physical sciences, they are rare in ecology, where the complexity of natural interactions and the difficulty of separating their relationships make weaker or caveat-studded conclusions the norm.

“I’ve had some friends joke to me, ‘Hey, Brian, biology is not physics,” McNab said. “With this work, we’re getting into a system of analysis, at least for certain traits, that is awfully determinative.”

New Guinea is home to most birds of paradise. Some species are relatively plain, while others are wildly colored and shaped. Members of the most striking species have what appear to be furry tails, uncannily long tail feathers, or even pairs of large flashy feathers jutting from their foreheads.

The birds get their name from the unique circumstances surrounding their discovery by Europeans in the 1500s. When the first preserved specimens reached Spain, their feet had been removed. The oddity of these seemingly footless birds, combined with their unique colors and shapes, prompted the Spanish to conclude they spent their whole lives aloft – in the metaphorical paradise of the sky – without ever alighting on Earth.

The birds have been the object of much study in the wild because their behavior is often

as unique and interesting as their appearance. For example, some species breed in “leks,” in which groups of males line up and simultaneously display their feathers to females as they “window shop” for the best suitor, McNab said.

However, he said, no one had ever done experiments on the birds, because they are not only rare but also difficult to capture and transport. In fact, only a handful of zoos worldwide have birds of paradise, many of which are protected as endangered species by international law, he said.

McNab said he was visiting Papua New Guinea while researching other birds when he was surprised to discover that the Papua New Guinea University of Technology in Lae had some 14 species of birds of paradise in captivity in a rainforest conservation exhibit.

With permission from the university, he began his experiments about five years ago, making three extended visits for the research. The experiments involved placing the birds in sealed chambers, pumping in air and measuring how much oxygen they consumed – a determination that can yield what is known as their basal metabolic rate, or the amount of energy used to keep them alive.

As with all animals, by far the biggest factor in determining metabolism was body size, McNab said. The bigger the bird, the more energy it used, with body size alone accounting for 91 percent of the variation in metabolism between species, he said.

However, his research also found that the birds’ food habits influenced its metabolism. Species that restricted their diets to fruits had a lower metabolism than those that ate fruit and insects. He found further that birds living at altitudes below about two-thirds of a mile, or about 3,500 feet, had lower metabolisms than those residing at higher elevations.

Together, his calculations showed the three factors accounted for 99 percent of the variation in metabolic rates among different species, a “really unusual” result, he said.

McNab said it isn’t clear why it takes less energy to keep fruit-eating, lower altitude-living birds of paradise alive. One possible explanation, he said, may be that fruits are seasonal so there may be some periods when they are unavailable, and birds that use less energy are better equipped to survive such periods of scarcity. Lower altitudes also tend to be warmer, so lower-altitude birds would not need as much energy to remain warm and active, he said.

James Brown, a distinguished professor of biology at the University of New Mexico in Albuquerque, said that McNab’s choice of birds of paradise for his study was a good one, because all birds of paradise are closely related, non-migratory, and fruit or insect-eating. That those factors could be held constant provided an “exceptional” opportunity to study the effect of other variables – body size, diet and elevation – that effect metabolic rates, he said.

He said he is particularly impressed by McNab’s conclusions about the influence of body size.

“The very strong influence of body size is striking,” he said. “It has been known since the 1930s that metabolic rate varies predictably with size, but Brian’s work shows how strong this influence can be. This also allows him to make some very strong inferences about the adaptive significance of the remaining variation that is due to diet and elevation.”