Great tits are good at hiding their eggs; they cover them with animal hair, moss, and grasses—but no one knew why. Researchers suspected that the tits (Parus major) were trying to mask their nests from the prying eyes of their competitors, pied flycatchers (Ficedula hypoleuca). These odd birds spy on how many eggs tits lay; the more eggs, the better the habitat. To confirm this hypothesis, researchers uncovered tit nests in Finland and then placed a bird decoy nearby—either a pied flycatcher or an innocuous wax wing—and played the corresponding birdsong. A day later, they measured how thoroughly the tits re-covered their nests. The flycatcher’s song spurred the tits to pile on 41% more hair, and with 17% better coverage than with the wax wing’s tune, researchers report online today in BMC Evolutionary Biology. The flycatchers act like “information parasites.” By concealing their eggs, tits can fool them into thinking the habitat is inferior, so they will move on and leave the tits alone.

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