Here in the dour understory of the Panamanian rain forest,the best way to find the elusive and evolutionarily revealing spotted antbird is to stare at your boots because sooner or later you will finally step into a swarm of army ants boiling out across the forest floor.
At that point you should step right back out of the swarm and start looking for the characteristic flitting and hopping of the thrush-size antbird,listening for its vibrato peee-ti peee-it call. Because wherever there are army ants out on a hunting raid,peckish antbirds are almost sure to follow.
The birds are not foolish enough to try to eat them: Army ants are fiercely mandibled and militantly cohesive. Instead,they hope to skim off a percentage of the ants labour,by snatching up any grasshoppers,beetles,spiders or small lizards that may jump to the side in a frantic attempt to elude the oncoming avalanche of predatory ants.
Its a gleeful reversal of the conventional notion of parasites as little,ticky things that plague large,poorly dressed hosts. Here the big vertebrates are the parasites,freeloading off insects a fraction of their size.
And the parasitic strategy is so irresistible that according to recent research in the journal Ecology,the spotted antbirds on Barro Colorado Island just may be taking it professional. 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 antbirds leap from amateur to polished parasite.
The antbird story also demonstrates the vividly baklavaian nature of parasitism in a tropical rain forest. Researchers have identified three species of butterfly that specialise in following antbirds. The butterflies feed on bird droppings,and these butterflies know where their suppliers are likely to be found.
I always end my talks by showing a slide of this complex,quadruple-tiered relationship, said Joseph Tobias of the University of Oxford,who studies antbird song and has worked with Touchton. You have the ants themselves,followed by a gaggle of antbirds,and behind the antbirds are the butterflies,and behind them are a couple of bird watchers.
Antbirds belong to an old and almost purely tropical avian family of some 200 species,only a fraction of which have anything to do with ants. The new research looked at three swarm-stalking species that live in the same region of Panama: the spotted antbird,the slightly larger bicoloured antbird and the even larger ocellated antbird.
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 bicoloured antbirds on the island 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 islands population of bicoloureds has stagnated,the number of spotted antbirds is on the rise. In tricky field experiments just now under way,Touchton is using checkered flags to determine whether antbirds that score low in neophobiafear of the neware the ones most likely to roam. As she explained,if ants are always on the move,their true moochers must follow. Home,for them,must be where the swarm is.