https://academic.oup.com/bioscience/article/50/10/854/233996

Male blue tits(Parus caeruleus) have a UV-reflectant crown patch that plays a role in courtship displays and antagonistic encounters with other males. The two photos at left show a normal blue tit photographed in visible light (top) and UV light (bottom). When researchers applied a sunscreen to male birds, hindering UV reflectance (right, with visible light [top] and UV light [bottom]), the birds’ mates produced fewer male offspring, supporting the notion that UV reflectance plays a role in male attractiveness.