When it's time to settle in for the night, red postman butterflies (Heliconius erato) often roost in groups of four or five. To figure out why, researchers hung several thousand fake versions of the insects around the forest in Panama and Costa Rica. To measure bird attacks, they counted beak marks on the dummies' modeling-clay bodies and wax-coated paper wings. Individuals perched alone or in pairs were more than six times as likely to be attacked as were models perched in groups of five. The effect went beyond a simple sharing of risk among group members: Each roost of five, considered as a unit, was less likely than a singleton to experience an attack, the team reports online today in the Proceedings of the Royal Society B. The researchers argue that the butterflies' bright markings, which advertise their toxicity to predators, are more effective when amplified in a group.