Most biologists believe that these mechanisms always work in concert — that sex appeal is the sign of an objectively better mate, one with better genes or in better condition. But the wing songs of the club-winged manakin provide new insights that contradict this conventional wisdom. Instead of ensuring that organisms are on an inexorable path to self-improvement, mate choice can drive a species into what I call maladaptive decadence — a decline in survival and fecundity of the entire species. It may even lead to extinction.

To make those songs, the male club-wing needs unusual wing feathers. Those closest to his body are thickened and twisted, giving the species its name. Two are also twisted into knobs, like the handles of tiny shillelaghs, while the adjacent feather ends in a bent, sharp blade.

It took 145 years after the first description of these feathers to discover how they make their sounds. In 2005, high-speed video of singing males captured by the biologist Kimberly Bostwick in the Ecuadorean forest revealed that the male’s wing feathers oscillate over the bird’s back. With each oscillation, the blade-shaped feather rubs against the feather next to it, as if bowing a violin, causing the thickened feathers to resonate. This mechanism, called stridulation, is also used by crickets, katydids and cicadas. These birds could appropriately be called cricket-winged manakins.

But manakin beauty is not only skin deep. In subsequent research, Dr. Bostwick and colleagues demonstrated that the birds’ songs involve more than just unusual feathers and movements. They require evolutionary changes in the shape of their bones.

Avian wing bones are surprisingly uniform among species, because flight places such precise demands on the design of the wing. The 10,000 species of flying birds have tinkered only slightly with the design perfected over 135 million years ago, when Mesozoic birds evolved the modern flight stroke.