A new study finds that mother birds are blessed with UV vision that allows them to accurately assess the nutritional status of their chicks.
According to researchers at the University of Bern in Switzerland, mother birds' ability to detect ultraviolet light reflecting off the feathers of their young means that they can favour the strongest chicks in times of environmental hardship and constantly monitor their chicks' healthy glow.
Growing feathers that reflect UV light is metabolically expensive. Birds that are healthy can grow these feathers easily, but malnourished or ill individuals cannot.
In species that can see in the ultraviolet part of the spectrum, this is used to select food and help choose potential mates from the crowd.
Researchers Marion Tanner and Heinz Richner found that in some species even nestlings had UV reflective feathers.
"We really wondered why juveniles would build these energetically expensive feathers when they were incapable of breeding," New Scientist magazine quoted Tanner, as saying.
They suspected that UV reflection from baby bird feathers could be sending a health signal to the parents to guide their feeding decisions.
In order to examine the phenomenon, the researchers observed 155 wild nestling great tits in 25 different nest boxes near Bern. They weighed the chicks and covered half of them in an ointment that blocked UV reflectance, and the rest in a similar ointment that had no blocking properties.
Fledglings were then kept separately in cages on a tree near their nest-box for one day. Cameras filmed the parents' visits and feeding. In the evening, the fledglings were collected and weighed.
While males were unaffected by the presence or absence of UV reflection, females were found to feed reflective offspring preferentially, indicating that they were favoring high-quality offspring over weaker offspring.
This might be to ensure that at least one chick survives should resources become scarce.
"It's an amazing discovery that UV vision is being used in parental/chick relationships," said avian biologist Megan Ross at Lincoln Park Zoo in Chicago, US.
However, according to Tanner, parental response to UV may change. When food supplies are limited or parasites are common, parents may use UV reflectance differently.
The study is published in Behavioral Ecology.