A. Barnett and P. Shaw; Journal of Zoology, Vol. 294, Issue 4, pages 224–233, December 2014
With some studies, you go into the field site with the idea and a series of questions ready formed, while others leap out and clamour for your attention while you are already there and doing something else. The study on uacaris and their influence on the foraging success of bird species is an example of the second.
With a group of Brazilian biologists, I was researching the feeding ecology of the Golden-backed uacari, a monkey which lives in igapó, the seasonally-flooded forests along the sides of blackwater rivers in central Amazonian Brazil. At the time almost nothing was known about the animal’s lifestyle, so what it ate seemed a pretty good place to start.
Paddling our small wooden canoes through the igapó we noticed that small antbirds would follow uacaris whenever they were in their territory. They would move along in the general direction of the uacari band, stopping when the monkeys left the bird’s territory. This didn’t make a lot of sense initially, because the antbirds feed on tiny insects that live in the moss and crevices on treetrunks. Not the kind of insects that you’d imagine get disturbed by a band of monkeys.
It was a different story, though, with the nunbirds and the jacamars, who are sit-and-wait predators feeding on grasshoppers and moths – exactly the kind of insects you can imagine leaping out of the way when monkeys are crashing around. Checking out how often these birds made their feeding sallies when monkeys were and were not around, we found that, indeed, yes, they fed more when monkeys were present. But what about the antbirds?
Well, it turns out that its not only grasshoppers that get out of the way of monkeys, small hawks do too. We’re not sure if its because they just find the monkeys annoying and leave, or if its because eagles feed on the monkeys and tend to follow them. And eagles also eat hawks. Either way, the upshot is that when and where there are monkeys there are fewer hawks. Which is good news for the antbirds, because its exactly these small hawks that are their major predators.
So, there is quite a lot going on, with two different groups of birds benefiting from the monkeys’ rumbuctious activities in quite different ways (while the hawks are probably off cursing in a corner somewhere).
Not only does it show the complexity of ecological interactions, the study is only the second time that anyone has actually shown that the presence of monkeys can increase feeding rates in some birds. People seeing monkeys and birds together have always assumed that the primates are acting as some sort of beater, but only one other study (on how gorillas visiting swamps increase lilly-trotter feeding rates) had actually shown it.
The work was done in Jau National Park in central Amazonian Brazil, and we continue our work there, trying to unravel the fascinating ecology of the seasonally-flooded igapó forests in which the monkeys and the birds in this study make their home.
University of Roehampton / Instituto Nacional de Pesquisas da Amazônia