A fascinating study has recently come out of Spain, using a bird species called the European roller (Coracias garrulus). This bird, like many others, has “asynchronous hatching,” meaning that the eggs do not hatch all at once. Since baby birds grow at a tremendous rate of speed, a bird that hatches out one day before its sibling will already be quite a bit larger, and a bird that hatches out two or three days later than the first will be comparatively tiny.
Which means the parents need to adapt their feedings to the varying sizes of their chicks. This can get complicated in a nest full of squirming chicks all opening their mouths expectantly, but it turns out that European rollers have a sure-fire way of determining which chicks need food the most, regardless of size.
They look to see who has a glowing forehead.
Birds can see UV light, and European rollers happen to have highly UV-reflective skin, especially in the forehead area. Since hatchlings don’t have any feathers, there’s nothing to block that UV reflection. Researchers Jesús M. Avilés, Deseada Parejo and Juan Rodríguez decided to find out whether the reflectivity was tied to the size and health of nestlings.
The first thing they learned, write the scientists, was that “heavier roller nestlings had less brilliant and less UV saturated skins than weaker nestlings.” In other words, a more reflective skin probably meant the chick was in need of food.
So they set about changing the rules a bit, and used UV-blocking jelly to alter the reflectivity of some nestlings. After four hours, the blocked nestlings already weighed less than reflective nestlings in the same age and weight bracket — they weren’t getting fed as much. The parents were indeed using UV reflectivity to determine which of their chicks needed food the most.
If you’ve ever peered into a bird nest, and seen four wide open mouths gaping at you, you can appreciate how handy it would be to have a bright, glowing signal of need. European rollers have it figured out.
(Photo from Wikipedia.)