Cuckoos don't bother building their own nests - they just lay
eggs that perfectly mimic those of other birds and take over
their nests. But other birds are wising up, evolving some
seriously impressive tricks to spot the cuckoo eggs. Cuckoos are
what's known as brood parasites, meaning they hide their eggs in
the nests of other species. To avoid detection, the cuckoos have
evolved so that their eggs replicate those of their preferred
targets. If the host bird doesn't notice the strange egg in its
nest, the newly hatched cuckoo will actually take all the nest
for itself, taking the other eggs on its back and dropping them
out of the nest. To avoid this nasty fate for their offspring,
the other birds have evolved a few nifty ways to spot the fakes,
which we're only now beginning to fully understand. One of the
most intriguing finds is that birds have an extra color-
sensitive cell in their retinas, which makes them far more
sensitive to ultraviolet wavelengths and allows them to see a
far greater range of colors