Ross’s Geese and Snow Geese are both white-bodied waterfowl with black wingtips that can be difficult to tell apart. 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 their 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 ⅔ the size of a Snow Goose, with a noticeably shorter neck, smaller bill, and higher-pitched voice, too.

More info and transcript at BirdNote.org.

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