On Alaska's rugged Shumagin Islands, no fruit tree dares take root, yet the smell of fresh lemons and tangerines hangs heavy in the air.

A nearby colony of lovesick crested auklets are the culprits: Males produce a citrus odor to attract females. And as new research in Behavioral Ecology reveals, it's the strength of their citrusy scent along with the size of their crests that really matters.

Researchers have long known that male auklets with larger crests—a tuft of head feathers that curl forward—have more sexual appeal. But unlike an elaborate display or vocalization, these ornaments cost little energy to produce, making them an unreliable signal of fitness to females looking for a robust male.

Scientists assumed there must be something