Bird eyes carry a fourth type of cone cell sensitive into the ultraviolet, a range human vision does not cover. Measured with a spectrometer, the blue tit crown has a strong reflectance peak in the near-ultraviolet, and male crowns reflect more strongly than female ones. To a blue tit, the sexes are not the near-identical pair they appear to be in a field guide.
Experiments have tested this by applying an ultraviolet-blocking sunscreen to the crown, which leaves the bird unchanged to human eyes. Females presented with treated and untreated males shifted their preferences, indicating the ultraviolet component itself carries weight in mate choice.
The usual over-reading is that blue tits therefore see a fundamentally different world. What ultraviolet vision actually adds is a further dimension to colours that are also visible to us, not a separate hidden picture, and the crown's structural blue is produced by feather nanostructure rather than pigment.