Some birds are relatively easy to study.

Ross Crates studies the ones that aren't.

He's part of the Difficult Bird Research Group at the Australian National University.

"All our study species are quite challenging to study for various reasons, mostly because they're really rare and highly mobile."One of those "difficult birds" is the critically endangered regent honeyeater.

They're medium-sized songbirds—with bright yellow tails and black-and-white chests.

And though they once roamed Australia in flocks of hundreds, fewer than 300 remain in the wild today.

Crates and his team tracked the birds over a five-year period.

If they encountered a male,they'd record his song.

And they noted whether the males were paired up with females.

They found that a quarter of the birds sang variations of the traditional honeyeater song.