The bluetit (Parus caeruleus) has been classified as sexually monochromatic, which means that male and female are the same colour. This classification is based, though, on human colour perception, not on bluetit colour perception.
Like all birds, bluetits can see ultra-violet light and the front of their heads glow brightly under UV light. Researchers have found that it is the males who choose the females with the brightest heads; this is unusual because it is usually the females who do the choosing.
Females with bright ultra-violet heads are more successful mothers; they rear more chicks to the point of fledging.
All pictures by DKG
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