How come birds are so much better at spotting the cuckoo in the nest? It has a lot to do with the fact that birds see ultraviolet light. These are well outside our visual range. And it is known that many birds' eggs reflect UV wavelengths.
So, a team of researchers wanted to see how well birds can identify their own eggs using UV light. They surmised that if they altered the reflected UV spectrum of an egg, the bird would perhaps not be aware that it was a different egg.
Starling Feeding a Cuckoo:
The study was conducted in the Czech Republic in a nearby forest. The researchers tested a bird known as the Blackcap. They carefully researched to find the nests with well established clutches.
But they didn't just put cuckoo eggs in the nests, they went a step further. They found abandoned blackcap eggs and tried to introduce those into the nests. What they did was to put a UV blocker on one set of eggs, while coating the remaining eggs with Vaseline. The eggs coated in Vaseline did not have any effect on the UV reflectivity. The studied the birds' nests for 5 days.
There were a total of 16 eggs coated in Vaseline and 11 were accepted by the Blackcaps. Of the ones that were UV blocked, the majority were rejected. This shows how important the UV reflectivity is for these birds.
Another interesting study would be to see whether cuckoos try to trick their prey by choosing eggs whose UV reflectivity closely resembles their own reflectivity.