Where birds reign, ants rain.

Ants that haul themselves up tree trunks in search of insects and other food sometimes take the easy way down: They jump. What's more, like any cliff-hanging action figure, they'll jump to get away from the bad guy--or bird.

In the pine forests of northern Sweden, ecologist Paul D. Haemig of Umea University set out to see just what effect a bird in a tree has on the northern wood ant (Formica aquilonia). He measured ant traffic moving up and down trees outfitted with liquid-filled traps to catch any falling "ant rain." Half of the trees were baited with suet to attract birds.

Without any birds around, 0 to 12 percent of the ant traffic took the jump, Haemig reports in the July Animal Behaviour. When a woodpecker or other bird appeared, 3 to 28 percent of the traffic came down as rain. The longer the birds hung around, the heavier the ant rain became.

Some of the raining ants may simply have been blown or jostled off the tree by the foraging bird, Haemig offers, but he and other researchers have observed ants "deliberately" leaping off trees or laboratory tables. The ants may be acting like aquatic insects that launch themselves into the current of a stream as a means of escape, he says. Still, some ants would rather fight than flee. In earlier work, Haemig found that worker ants can attack foraging birds.