A few years ago, biologists Adrian Barnett and Peter Shaw were in Brazil doing field research. They’d gone with the intention of studying the feeding behavior of a monkey called the Golden-backed uakari, but another research idea came flying out of the trees begging for their attention.
Golden-backed uakaris migrate between dry forests and flooded forests in the central Amazon tracking their most important food source, unripe seeds from hard-husked fruits. They travel through the upper canopies of the trees in groups that can contain anywhere from 12 to 100 monkeys. As a group moves around, it can make quite a ruckus, shaking branches and disturbing or scaring other animals.
While Barnett and Shaw tracked the uakaris from a canoe on the flooded forest floor, they realized that they weren’t the only ones tailing the monkeys. Wherever the uakaris went, they saw, an assortment of birds followed them. When the uakaris stopped to forage or eat, the birds stopped, too, and then started trailing them again when they moved along. The game of follow the leader didn’t make much sense, so the researchers decided to investigate.
Barnett and Shaw thought that the uakaris might be acting as flushers for some of the birds, startling insects as they moved through the trees and making them easier for the birds to catch. Two of the birds the scientists had seen, the bronzy jacamar and the black-fronted nunbird (above), feed on flying and leaping insects like moths and grasshoppers. These bugs tend to clear out of the way when a band of uakaris comes storming through and would be easy pickings when chased out of the canopy. When the researchers compared the number of hunting attempts the birds made when the uakaris were around to when they were absent, they found that both birds indeed hunted and ate much more when the monkeys were nearby.
Two other birds that followed the uakaris, the black-crested antshrike and the black-winged antbird, eat tiny insects that live in moss or crevices on tree trunks and hide when they’re disturbed by uakaris instead of fleeing. For these birds, monkeys crashing through the branches doesn’t make it easier to grab a meal, and they didn’t hunt or eat more when the uakaris were around. So what’s the benefit of following them?
Barnett and Shaw soon saw that it’s not just insects that get disturbed and flushed out by uakaris, but also small raptors like the slaty-backed forest falcon and grey-line hawk. These birds—which eat antshrikes and antbirds—often cleared out when the uakaris came along and were seen less often when the monkeys were present than when they were absent. The researchers aren’t sure if the raptors are simply annoyed by the uakaris or if they flee because they and uakaris are both prey for harpy eagles, which are also known to follow the monkeys around. Either way, more monkeys meant fewer raptors and some degree of protection for the antshrikes and antbirds.
“So, there is quite a lot going on,” Barnett writes at the Journal of Zoology blog, “with two different groups of birds benefiting from the monkeys’ rambuctious activities in quite different ways (while the hawks are probably off cursing in a corner somewhere).”