Sister Species: Snow Goose and Ross’s Goose

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Sister Species: Snow Goose and Ross’s Goose
Adapted from a script by Bob Sundstrom

This is BirdNote.

          [Snow Goose and Ross’s Goose flight calls]

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…

          [Ross’s Goose calls]

…and the heftier Snow Goose.

          [Snow Goose calls]

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 2 million years ago, possibly because advancing glaciers separated the two populations.

After spending millennia in isolation, the Ross’s Goose evolved to be about ⅔ the size of a Snow Goose, with a noticeably shorter neck, smaller bill, and higher-pitched voice too.

          [Ross’s Goose calls]

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 lookalike species at our website, BirdNote.org. I’m Michael Stein.

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Senior Producer: Mark Bramhill
Producer: Sam Johnson
Managing Editor: Jazzi Johnson
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Bird sounds provided by The Macaulay Library of Natural Sounds at the Cornell Lab of Ornithology, Ithaca, New York. Ross's Goose ML113756861 recorded by Paul Marvin. Snow Goose ML71214541 recorded by Paul Marvin. Ross's Goose ML147565 recorded by Tom Johnson. 
BirdNote’s theme was composed and played by Nancy Rumbel and John Kessler.
© 2025 BirdNote October 2025
Narrator: Michael Stein

ID# evolution-05-2025-10-16    evolution-05

Reference:
https://www.google.com/amp/s/www.imperial.ac.uk/news/186799/sister-spec…;
https://u.osu.edu/biomuseum/2015/09/28/examples-of-sibling-species/comm…;
https://birdsoftheworld.org/bow/species/rosgoo/cur/systematics