In Lafayette Park in St. Louis, Missouri, a dozen or so Eurasian Tree Sparrows from Germany were released. But unlike other introduced species such as the similar-looking House Sparrow, they did not spread very far and remained in a limited area in the Midwest. It’s not fully clear why, but it’s possible that they just behave differently. And the aggressive House Sparrow may have helped prevent the tree sparrows from spreading further by competing for the same nesting sites.

More info and transcript at BirdNote.org. 

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