This is birdnought.

In winter, flocks of white geese gather by the thousands in fields and marshes of the U.S. and Mexico.

All the birds in this flock flying through North Texas have white bodies, black-tipped wings, and pink legs.

But on closer inspection, they appear to come in two different sizes.

That's because this flock is a mix of both the petite Ross's goose and the heftier snow goose.

Okay. and the heftier snow goose. The pair are a great example of what science calls sister species,

which occur when a portion of one species takes an evolutionary step away from the rest,

resulting in a species split. In this case, genetic studies

suggest that the Ross's goose branched away from its snow goose relatives more than two million years

ago, possibly because advancing glaciers separated the two populations. After spending millennia in isolation,

the Ross's goose evolved to be about two-thirds the size of a snow goose,

with a noticeably shorter neck, smaller bill,

and higher-pitched voice, too.

So next time you spot a flock of white geese, see if you can tell the sisters apart.

We've got tips and tricks for distinguishing between look-alike species at our website, birdnote.org.

I'm Michael Stein.