You don't want to know what sort of oils this bird is smearing all over itself.
Sally King/NPS
You don’t want to know what sort of oils this bird is smearing all over itself.

If animals are doing something with glands and oils and rubbing, it’s usually a fair bet that it has something to do with sex. Scientists used to think that birds and their “preen glands” were an exception to this rule — that birds would “extract oil from the glands and rub it on their feathers and legs” just to make their feathers stronger. Ha! Silly scientists. Turns out, this also has to do with sex.

A new study found that birds rub their smelly secreted oil all over themselves in order to show potential mates that they’re healthy and can make beautiful babies. (Or in Rihanna-speak, “Sex in the air, I don’t care, I love the smell of it.”) Science Daily reports:

“This study shows a strong connection between the way birds smell near the beginning of the breeding season — when birds are choosing mates — and their reproductive success for the entire season,” [researcher Danielle Whittaker] said. “Simply put, males that smell more ‘male-like’ and females that smell more ‘female-like’ have higher genetic reproductive success.”

Female birds also use smell to help select male birds to raise their nestlings. But, because birds are sexist jerks, the female birds were more likely to choose males with a stronger “female-like” smell to raise their kids, the study found.