Birds live longer in the tropics

Adult tropical birds have higher annual survival rates than their temperate counterparts, researchers found, which seems to help balance the smaller clutch sizes of these forest passerine birds.

“Birds in lower latitudes near the equator lay fewer eggs per clutch than birds in higher latitudes,” said Gonçalo Ferraz, an assistant professor at the Universidade Federal do Rio Grande do Sul in Brazil and senior author of the recent study published in The Auk: Ornithological Advances. “It’s well documented in the New World and in the Old World, in the northern and southern hemispheres, and it’s a very strong pattern. It’s not even controversial. What has been controversial is, how does it stay like this and how are populations maintained?”

Alejandra Munoz, lead author of a recent study on tropical birds, teaches a student how to remove a passerine bird from a net. The team found adult birds in the tropics live longer than adult birds in temperate areas. ©Gonçalo Ferraz

Ferraz realized something in their life history must compensate for these small clutch sizes. He and his colleagues measured the difference between juvenile and adult survival rates in 40 species in Manaus, in the Brazilian Amazon, two degrees south of the equator. Past studies hadn’t looked at survival in different age groups since birds in the tropics are hard to age, Ferraz said, but with recent ornithological advancements, researchers can determine age with the birds’ molt cycles.

Comparing survival in 175 species from Peru to Alaska, they found adults in the tropics had higher survival than temperate species, which appeared to allow the populations to persist despite the small clutch sizes.

Ferraz said this raises questions for further studies such as the possibility of other aspects of the birds’ life history resulting in their ability to survive despite small clutches. Since tropical birds aren’t limited by cold winters, he said, they may compensate by laying more clutches throughout the year.

“Even though this may be how the current pattern is maintained, we don’t know how it evolved,” Ferraz said. He suggests looking into resting metabolic rates of tropical versus temperate birds.

“Any knowledge of population dynamics could have potential use for conservation,” Ferraz said.