<< Front page News March 19, 2004

The truth about birds and ants

Candidate for visiting assistant professor in ecology and evolution, Susan Willson Hillman, talked Monday about her research in Peru on army ants and the birds that follow them. Hillman is working to increase understanding about how similar and divergent species can coexist despite stiff resource competition.

Hillman studied five different bird species that find prey by following the shocking and awe-inspiring march of Eciton Burchelli, a prominent species of army ants.

“[Eciton] can spread out up to 30 meters to forage for prey,” Hillman said. “They tear up everything in their path, catching and dismembering all that they can pull apart.”

The birds snatch exposed insects running from the impending ant horde, always careful not to get too close to the ants themselves. There is fierce competition amongst the birds themselves.

“Birds are literally knocking each other off of branches to get a good position,” Hillman said.

Hillman’s dilemma was how five different bird species could maintain stable populations while all competing for the same food, especially when the larger black-spotted bare-eye, which Hillman referred to as the “top dog,” was better able to box out the “bottom of the barrel” white-throated antbird.

To answer this question, Hillman and her research team tagged birds with radio transmitters and trailed behind them and the ants almost every night for three years.

“We tracked the birds to find out which ant colonies they were following and why,” Hillman said. “It got be like a soap opera. I would wake up and ask where was S-YR last night?”

Hillman found that all five bird species followed not one but two different ant species. Labidus Praedator, a smaller species of army ant that lives primarily underground and only forages within two to three meter patches, was found to be much more important to the birds than Hillman had originally thought. One species, the hairy crested antbird, actually specializes in following Labidus rather than Eciton, and is territorial about its space.

Each bird species needs a home area large enough to encompass multiple Eciton colonies because the ants stop tearing up the underbrush when their queen’s 300,000 odd eggs are hatching. The birds augment their diet by dropping in on Labidus.

“Every morning you can watch these guys flying to the different colonies and peeping in at them to see if they’ll come out that day,” Hillman said, enthusiastically imitating her bird subjects. “The birds know which days are going to be slow and which days will be buffets. They do this for a living.”

Hillman found that annual fluctuations in bird populations were evened out between species and remained stable over time. Where one species declined one year, another would fill the gap. When competition decreased, the birds covered smaller areas.

“Coexistence is subtle and complicated,” Hillman said. “It’s not just about body size.”

When questioned on furhter species interdependencies, Hillman alluded to a species of butterfly called the “the skipper,” that specializes in eating the droppings of the birds that follow the ants skipper” that specializes in eating the droppings of the birds that follow the ants.


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