Chicks open wide, ultraviolet mouths. (Zoology).

The first analysis of ultraviolet (UV) reflections from the mouths of begging baby birds has revealed a remarkable display that birds can see but people can't.

The colors of chick mouths have attracted much scientific interest, says Sarah Hunt of the University of Bristol in England. An old theory held that the bright yellows and reds create conspicuous targets for parents delivering food in dim nests. Newer evidence shows that health influences mouth color, which suggests that various shades give parents a quick clue to a chick's condition and need for food.

To get a better idea of what the birds are seeing, Hunt and her colleagues measured UV reflection from chick gapes and nests for barn swallows, blackbirds, house sparrows, and five other European species. The gapes reflect a lot of ultraviolet, but the nests don't, Hunt and her colleagues report in an upcoming Proceedings of the Royal Society of London B. The big LTV difference between nest and chicks suggests that it's time to dust off the old conspicuous-target theory, says Hunt.--S.M.