A dinosaur-age bird's extravagant tail feathers may have helped it win over mates, but the fluffy rump was also literally a drag during flight, a study of a well-preserved fossil finds.

The bird's tail is truly "bizarre," the researchers said; it had two lengthy plume feathers that were more than 150% of its body length.

At the tail's base, a stiff fan of short feathers likely helped the bird fly, the researchers said.

"We've never seen this combination of different kinds of tail feathers before in a fossil bird," study co-researcher Jingmai O'Connor, a paleontologist at the Field Museum in Chicago, said in a statement.

The unique combination of a short tail fan and two long feathers, known as a pintail, is seen in some modern birds, such as sunbirds and quetzals.

However, scientists have never found a fossil bird or nonavian dinosaur with that combination, O'Connor said.

Other enantiornithes had either plumes or tail fans, but not both, said study first author Min Wang, a researcher at the Chinese Academy of Sciences.

"The tail fan is aerodynamically functional, whereas the elongated central paired plumes are used for display, which together reflect the interplay between natural selection and sexual selection," Wang said in the statement.

"Scientists call a trait like a big fancy tail an 'honest signal,' because it is detrimental, so if an animal with it is able to survive with that handicap, that's a sign that it's really fit," O'Connor said.

"A female bird would look at a male with goofily burdensome tail feathers and think, 'Dang, if he's able to survive even with such a ridiculous tail, he must have really good genes.'".

"Birds that live in harsher environments that need to be able to fly really well, like seabirds in their open environment, tend to have short tails," O'Connor said.