When it comes to mallard bills, brighter is better: A bright yellow bill is duck-speak for “I’m healthy,” attracting more female ducks than dingy green ones. After discovering that avian semen has antibacterial properties, scientists then found that the semen of brighter-billed males killed more bacteria than the semen of darker-billed ones. It implies that by seeking out bright-billed males, female ducks are protecting themselves against bacteria-related sexually transmitted diseases.
In her experiment, University of Oslo researcher Melissah Rowe collected semen from ducks (a feat unto itself—the videos in this link are amazing, but watch at your own risk) of various bill colors, and then tested how well the semen killed bacteria such as E. coli. She found that ducks whose bills had more carotenoids—an organic pigment that brightens bills—also had semen that more effectively killed E. coli. However, they discovered that the semen’s effectiveness against the bacteria S. aureus wasn’t associated with bill color, possibly implying that this bacteria doesn’t pose much harm to ducks.
If you want to impress a female great bustard, going clean-shaven is probably the wrong approach. According to biologist Juan Carlos Alonso and colleagues at the Spanish National Museum of Natural Sciences, the size of a male bustard’s “whiskers and beard” is correlated with its reproductive success.
The great bustard is a beloved but endangered bird found in Spain and other locations scattered across Eurasia. Males of the species are possibly the heaviest flying birds in the world (rivaled only by the male kori bustard), and each sports whisker-like plumage on either side of its beak, along with neck feathers that resemble a beard. They also engage in showy mating displays, strutting about “like a vicar in a tutu,” according to naturalist Chris Packham in this BBC video.