HOW does a bird know which of its brood to feed? It just checks how much UV light they reflect.

Birds are known to see UV light, but it wasn’t known why. To find out, Jesús Avilés at the Arid Zone Research Station in Almeria, Spain, examined European rollers (Coracias garrulus) and found that the foreheads of heavier chicks reflect less UV than weaklings.

The team then weighed 84 chicks born in nest boxes near the city of Guadix. They applied either a jelly containing a UV blocking agent or just jelly to the foreheads of the chicks. Four hours later they weighed them again to find out how much they’d been fed by their parents (Behavioural Ecology and Sociobiology, DOI: 10.1007/s00265-011-1164-8).

Parents with large broods preferentially fed UV-reflective nestlings over UV-blocked ones, suggesting that they used UV to decide which mouth to feed.