KEY POINTS:
Some birds are skilled at detecting imposters' eggs in their nests because they recognise that the shell colour is suspect, says an international research team that included New Zealand scientists.
The researchers - from the University of Auckland, the University of Birmingham and two Czech institutions, the Academy of Science and the Palacky University - suggest that some birds use colour to identify eggs of other species and eject them.
Dr Mark Hauber, of Auckland University's School of Biological Sciences, said the research showed song thrushes recognised small changes, particularly in ultraviolet wavelengths, and used these to distinguish between their own eggs and eggs of "parasite" species such as cuckoos.
"Birds have very different visual senses to humans - they can see ultraviolet wavelengths where we cannot."
He said the study introduced eggs painted in a range of colours from blue overtones to reddish tints into the nests of song thrushes. The birds rejected the eggs that showed differences in the reflecting of ultraviolet or short (blue) wavelength light.
Dr Hauber said birds such as cuckoos which parasitically introduced their eggs into the nest of other birds dramatically increased the amount of time and energy needed to provision the nest.
He said that even though the artificially coloured eggs had come from the female song thrushes, the birds still ejected them because of subtle differences from their natural colouring.
The research was published online this week in Biology Letters.
Dr Hauber told the Herald that previous research had looked into the way birds perceived colours in feathers, fruits and insects, but not eggs.
It was suspected that birds could recognise healthy eggs by theircolour.
An antioxidant, biliverdin, was responsible for blue-coloured eggs and it was thought the brighter the blue the more likely the egg would produce a healthy chick.
Dr Hauber said it was possible birds could eject less-healthy-looking eggs to increase survival rates.
