unless their primary food source (the American Chestnut) is restored. A Chestnut blight began at the turn of the 20th century and quickly spread across North America. The blight eventually fells the Chestnut tree reducing them to just stumps with suckers. New Chestnut trees eventually succumb to the blight preventing them from bearing nuts. Attempts were made to stop the blight from spreading but eventually that was given up.
Decades later some folks have revived the attempt to back cross and cross breed Chinese Chestnuts, which are blight resistant, to help reestablish the Chestnut trees in North America. That effort continues today. Most states have American Chestnut groups who help identify trees in the wild that have survived the blight and help at developmental plots to eventually reestablish the Chestnuts. Here's one for Penn State https://ecosystems.psu.edu/research/chestnut. My wife an I found a Chestnut tree growing in Minnewaska state park. She shot a picture and sent it to the NY group who maintain a website of know tree locations.
As for the Passenger Pigeon, at one time it was estimated that if you were to grab 10 random birds in North America, 4 would be Passenger Pigeons. They were that common. So common, no one thought they would disappear. Their disappearance helped bring awareness to people that we are stewards of our realm.
There is a site dedicated to this bird: http://passengerpigeon.org/index.html
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