science, health & technology

Also in this section:
Are the lives of tropical birds really lived in the slow lane?
WHO and UNAIDS report on HIV and circumcision

Global forum on organ, cell and tissue transplants

Life histories of birds in the tropics

Is life in the sun really lived in the slow lane?

by Eric Jackson

Yes, we all know about the "life history" as the protracted exercise, usually of self-justification, that you will get from some people. When it comes in written form it's generally called an autobiography.

However, the term means something else to biologists. Ornithologist Jeff Brawn from the University of Illinois at Champaign-Urbana, who was back in Panama after a two-decade absence and spoke at the Smithsonian Tropical Research Institute's Tupper Auditorium on March 20 defined "life history" as he and his colleagues use it as "how an organism schedules its reproductive activities over a lifetime."

(That Brawn's lecture was about sex did not draw an unusually large crowd. That it was about birds did --- on this afternoon the Panama Audubon Society was out in force, as they usually are when a top-notch ornithologist, especially one who has worked in Panama, is the guest of honor at STRI's Tuesday afternoon science lecture series.)

One rule of thumb in birds' reproductive strategies, Brawn pointed out, is that "if you maximize one trait, it's at the expense of another." But how does that differ, if it does, according to latitude?

There has been a lot of research about bird life histories in the temperate zones, but not as much about tropical birds. Lately, however, a number of different scientists have been in their different ways doing comprehensive studies of the variations among birds and their reproductive behaviors by latitude and the sum of that research gives Brawn the confidence to draw some generalizations, and reject others.

One generalization that he rejects is about different overall survival rates in tropical and temperate zones. "The notion of systematic survival rate difference by latitude is unsupported," Brawn concluded. He added that birds who conduct long-distance migrations to or through the different latitudes seem to have higher survival rates than those who don't.

One of the striking differences that has been found, however, is in clutch size, that is, in the number of eggs hatched in a nesting season. It's smaller in the tropics, and even species that are imported from more northerly latitudes will quickly reduce the number of young they try to raise at one time when transplanted to an equatorial zone.

"There is some strong selective pressure here," Brawn noted, adding that there are several hypotheses of what they might be.

An obvious first guess would be the rate of predation. In the tropics, due to pressures by arboreal snakes, other birds, monkeys and various and sundry predators whom you might or might not suspect --- you find a bigger array of natural enemies here in Panama than in places a thousand or more miles to the north --- the predation rates are high. But if you look at the attacks on the young while in the nest, Brawn pointed out, within a single species the predation rates are not related to clutch size. Predation on nests, so it seems, is not the reason for the difference in clutch size.

However, Brawn said, when you look at the post-fledging survival rates of young birds in the tropics, then those from larger broods definitely do have less of a chance to live. And here there's a connection with other differences by latitude: birds down here care for their young longer than their northern relatives do. Using a study on the Western Slaty Antshrike as an example, he noted that parents spend substantially more time caring for their young after fledging than they do at the stage when they're feeding their chicks in the nest.

So, rather than nest predation, Brawn thinks that the controlling factor that keeps tropical birds' clutch sizes down is that larger broods are just too much for parents to care for in the post-fledging stage of raising their young.

"I don't think we should be looking at nestlings --- we shoould be looking at fledglings," Brawn opined.

So how does a scientist look at fledglings to get a good approximation of first year bird survival rates?

By catching young birds, fitting them with radios, and following them electronically. "The only way we can get at this is through telemetry," Brawn said, while admitting that this is difficult and expensive work.

Brawn's work in Panama, which has been carried on since the 70s by a succession of biologists, was concentrated along the Pipeline Road in Parque Soberania, and particularly in a 100-hectare study plot in the Limbo Basin, an area that includes new and old growth forest patches ranging in age from 75 to 400 years. Scientists have been capturing, banding and recapturing birds there, and from those studies trying to estimate bird survival rates. This type of long-term study is something for which STRI is justifiably famous. But the methodology in this particular long-running study makes it hard to distinguish between birds who have died and those who have just emigrated from the area and thus can only approximate a rate of "apparent death." This varies widely from species to species in the neighborhood.

So what if it's true that, on average, tropical birds live longer than their northern cousins? "That's not all that interesting," Brawn commented. "It's like saying that on average a family has 2.2 kids."

Are the lives of tropical birds really lived in the slow lane?
WHO and UNAIDS report on HIV and circumcision

Global forum on organ, cell and tissue transplants