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Spotted Antbirds shifting to specialist strategy?

Fern Davies

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...according to recent research in the journal Ecology, the spotted antbirds on Barro Colorado Island just may be taking it professional. Whereas the species has traditionally opted for a mixed approach — filching from ant swarms but also finding food on its own — the island-bound antbirds appear to increasingly depend on army ants to scare up their every meal.


Janeene M. Touchton a researcher associated with the Smithsonian Tropical Research Institute and Princeton, and the principal author of the report, is now trying to identify the personality traits that may facilitate a spotted antbird’s leap from amateur to polished parasite. Is it boldness, aggressiveness, a love of novelty? Or maybe a lack of aggressiveness, a nonchalance about territory and a refusal to pick a fight? She is collaborating on the project with Martin Wikelski of the Max Planck Institute for Ornithology.


Michaela Hau of the Max Planck Institute and the University of Konstanz, who has studied breeding patterns among antbirds, said Dr. Touchton was “getting fantastic data out of the system — and she’s gathered the data through heroic fieldwork, through crazy, really hard work.”

The antbird story also demonstrates the vividly baklavaian nature of parasitism in a tropical rain forest. Researchers have identified three species of butterfly that specialize in following antbirds. The reason? The butterflies feed on bird droppings, and though guano is a notoriously unpredictable resource in a rain forest, these butterflies know where their suppliers are likely to be found.

As the biggest, most dominant member of the antbird guild and an “obligate” parasite — one that cannot find food on its own — the ocellated antbird monopolizes the leading edge and snaps up the meatiest prey. The bicolored antbird, also an obligate parasite, occupies the side position and takes in the second-order invertebrates. Spotted antbirds content themselves with the dregs at the rear. After all, they’re only discretionary parasites and can compensate later with independent foraging.

In the new work, the researchers compared the standard three-part scrimmaging of antbirds seen on the Panamanian mainland with the situation on Barro Colorado Island, where the ocellated antbird recently went extinct. They expected that the bicolored antbirds on the island, as the beta birds of the pecking order and obligate parasites besides, would be the biggest beneficiaries of the loss of competition.

Instead, it seems that the spotted antbirds are the ones making the most of the newly opened niche. For one thing, while the island’s population of bicoloreds has stagnated and may even be shrinking, the number of spotted antbirds is on the rise. Moreover, some spotted antbirds on the island are clearly losing interest in maintaining and defending specific territories, as spotted antbirds on the mainland vigorously do.

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