The Gouldian finch, found in northern Australia, looks like a bird painted by Gauguin. Its palette includes a purple chest, yellow belly, green wings and cyan highlights. But it’s the head that really matters. They come in red or black (there’s a very rare yellow variant too, but we can ignore that here), and they strongly prefer to mate with partners of their own colours. This isn’t abstract fussiness – genetic incompatibilities between the black-heads and red-heads mean that their offspring are often infertile and feeble. Indeed, these two variants could be well on the way to becoming separate species.

Red and black finches are so easy to tell apart that scientists could be forgiven for neglecting how they do so. But Templeton suspected that the act of choosing a mate was more complicated that anyone had thought.

Another type of finch – the zebra finch – provided a clue. The males prefer to watch their intended females with their right eye. The right eye feeds information to the left half of the bird’s brain, and there the asymmetries continue. Genes that are switched on when neurons fire tend to be more active in the left half of a courting zebra finch’s brain than the right half. Maybe the left brain, and thus the right eye, dominates the selection of mates.

Templeton tested this idea with Gouldian finches. She covered the left or right eyes of black males (who show the strongest preferences for their own colours) and presented them with a choice of potential mates. If they could see through both eyes, or just the right one, they spent more time staring at black females than other finches. If they could only see through their left eye, this preference disappeared. Likewise, males were more likely to serenade black females, but only if they could see with their right eye.