Why Do Tropical Birds Have Fewer Chicks?

There’s an egg question that’s been puzzling scientists for decades, and it has nothing to do with chickens. Tropical birds consistently lay fewer eggs and raise fewer chicks than birds in more temperate climates. The pattern has been shown again and again; a comprehensive 2008 analysis of 5,290 bird species found that, on average, birds in the tropics lay two eggs while those at northern latitudes lay four and a half eggs per clutch. This holds true from warblers to hawks—size, lifestyle, and even habitat preference don’t matter nearly as much as latitude.

A common explanation, dating to the 1940s, figures that chick survival relies on two factors: parents' ability to find food for their nestlings, and pressure from nest predators. Parents lay as many eggs as they can properly feed, and raise them as quickly as possible. In temperate areas, breeding happens in the summer when there is a glut of food, so songbird parents can spend lots of time raising chicks—thus, they have more of them. Down at the equator, however, there is both less food and hungrier predators, so tropical songbird parents have fewer chicks and those chicks leave the nest sooner.

"It wasn’t really a scientifically tested idea, but it made sense on its own ground," says evolutionary ecologist Stephen Stearns of Yale University. "It's one of those things that made people feel kind of comfortable."

But it doesn't make Thomas Martin feel comfortable. The University of Montana evolutionary ecologist studies songbirds nesting in temperate, high-elevation Arizona forests, as well as those nesting in the tropical forests of Venezuela and Malaysia. When he looked through his years of data on 72 songbird species, he noticed that the rate at which chicks put on weight were consistently lower than those of temperate songbirds. This is not what you would expect if tropical chicks supposedly grow more quickly to escape nest predation.

Furthermore, Martin’s data shows that tropical bird parents bring food to the nest just as frequently as those in the Arizona mountains, which suggests “food is not hard for them to find," says Martin. "That indicated that food was not driving what was going on, and we needed to think about other things."

One of those things, he suspected, was how well the chicks survived after they left the nest—something previously unconsidered. One thing that helps chicks survive outside the nest, Martin figured, is wing length. And he’d conveniently collected data on that for almost 30 years.

He discovered that tropical nestlings grow longer wings sooner than temperate nestlings. This means they’re smaller when they fledge, but they’re more competent fliers. "The better they can fly, the better they are able to avoid predators," he explains. His report on wing length at fledging time was published yesterday in Science.

But what does this have to do with the number of chicks tropical birds raise? While the new study “helps us understand a lot of things about the reproductive strategies of birds,” Martin's finding alone doesn’t explain this phenomenon, says Jeffrey Brawn, an animal ecologist at the University of Illinois who was not involved in the research. But it does add to a growing body of research about the tropical post-fledgling period.

Brawn has found that Western Slaty-antshrike parents continue to feed their young for months after they leave the nest, and suspects that other tropical songbirds do the same. Martin’s research suggests that the birds sacrifice size to grow longer wings, so that explains why the parents would need to keep feeding the chicks after fledging. And: "If they're going to feed young that long and intensely, they may only be able to do it with a reduced number of young," Brawn says.

Of course, Martin’s research only applies to the 52 tropical songbird species that he happened to have data on—it may not apply to tropical birds at large. "The notion that you can divide the world up by latitude and make sweeping generalizations is suspect," says Stearns, who was not involved in the research.

So, it’s still early days for this theory, but the research could help solve an age-old mystery. And that would be pretty clutch.