Songbirds are able to communicate with potential mates using plumage colors while remaining inconspicuous to avian predators, Swedish researchers suggest in PNAS this week. They do so by using colors that the larger birds are less able to discriminate from the background.

Ultraviolet plumage coloration, which reflects light in the range of 355–380 nanometers, has long been known to serve as a secret communication channel in songbirds, exploiting a shortfall in the mammalian visual system. But it has not been clear how avian predators, which can see ultraviolet, are excluded.

Ornithologists Olle Håstad, Jonas Victorsson, and Anders Ödeen, all based at Uppsala University, present evidence that small passerines such as the robin Erithacus rubecula, brambling Fringilla montifringilla, and golden oriole Oriolus oriolus exploit differences in the maximum sensitivities of their own visual systems and those of their potential bird predators.

Using retinal models—an equation that describes how...

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