Why can birds and honeybees see colours that humans cannot see?

Birds and honeybees can perceive colours that are invisible to humans primarily because they possess different types and numbers of visual pigments in their eyes, which allow them to detect wavelengths outside the human visible spectrum.31

Ultraviolet Sensitivity

While the human eye is blind to ultraviolet radiation, many insects, including honeybees, and many birds have evolved to perceive it. For bees, this sensitivity is part of a trichromatic colour vision system with peak sensitivities in the ultraviolet, blue, and green regions. This allows them to see patterns on flowers—such as nectar guides—that appear uniform or invisible to humans.21345

Biological Differences

The ability to see these colours stems from distinct physiological adaptations:
* Photopigments: Humans typically have three types of cone cells for colour vision. In contrast, many birds possess four or five types of cone cells, providing a broader range of spectral sensitivity.1
* Oil Droplets: In the eyes of birds and reptiles, specialized coloured oil droplets can modify the light absorbed by photopigments, further enhancing their colour discrimination.6
* Evolutionary History: The limited colour vision of many mammals is thought to be a legacy of an ancestral nocturnal or subterranean lifestyle, whereas birds and insects evolved visual systems coadapted to their specific environmental needs, such as foraging or social signaling.15