Biology
Color vision and UV reflectance in mate choice of budgerigars
Quick fact
Budgerigars have four types of color photoreceptors, including one that detects ultraviolet light, which allows them to see patterns on each other's plumage that are invisible to human eyes. The feathers on a male's crown reflect UV light, and females prefer males with brighter UV reflectance.
Why this is interesting
You watch a pair of budgerigars courting, and to you they look identical. Yet the female is clearly choosing one male over another. How can she tell them apart when you can't?