One of the signs of spring over the past month or so has been the adult black-headed gulls (Larus ridibundus) developing their dark-brown head colouration. For most of the year their heads are white with only a dark smudge behind each eye.
Walking home from the station one afternoon, I was struck by how sudden and definite the change to dark brown colour was, and this got me thinking along the following lines: The colour change comes on during the breeding season so it obviously evolved in response to sexual selection or mate choice. One of the theories of sexual selection is that in it animals are (unwittingly) selecting for mates that have fewer parasites. Suppose that the lice and ticks of gulls are light coloured (dark coloured ones being easier for the gulls to to see and pick out from their mainly white feathers). Now, in selecting a mate with a dark head in which there are no white spots a gull is (unwittingly) ensuring that this mate has no lice or ticks. That was all fairly obvious to me, but the problem was why is it only the head that changes to dark brown? Then the answer hit me: think of how a bird can groom itself and remove lice and ticks: it uses its beak, and which part of its body can it never reach with its beak? Its head! If a bird has a louse or tick infestation then the head is precisely where you should look to find out!
I felt very pleased with myself for having worked this out. Sexual selection for mates without lice and ticks seems to explain both when the black-headed gull's head turns dark and why it is only the head that turns dark. Presumably the same reasoning can explain the irridescent green heads of male mallard ducks.
Later that evening I told Zoe of my 'discovery' but she wasn't impressed. Maybe she is just too young.