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For the bay's Savannah sparrow, a subspecies that lives in salty tidal marshes, increased immigration of its inland cousins over the past century has definitely been bad news.

The Bay Area has seen a 90% decline in tidal marshland since the 1800s, which has reduced Savannah sparrow populations to the level that interbreeding with immigrants is affecting their unique saltwater adaptation.

For comparison, he collected blood or tissue samples from living birds he had encountered over the last decade studying Savannah sparrow populations around the Bay Area for his Ph.D. thesis from the University of Montana.

The researchers found that the genetic diversity of the Southern California subspecies, the Belding's Savannah sparrow, had declined slightly, but that it had been low even in the 1880s.

In the case of the Savannah sparrow, such outbreeding reduced adaptation to the environment.