The following report provides good evidence that some passenger pigeons may have been able to survive (after there exinction in the 1800's)at least into the 20th century. However, this is very doubtful, as there wouldn't be very many left, and passenger pigeons only breed when there is a large number of them all togethor in one area, so if there were just a few left, they wouldn't breed, and eventually would just die out.

"Passenger Pigeon"

Anonymous; Science News Letter, 17:136, 1930.

The passenger pigeon, long thought to be totally extinct, has been reported alive in the wilds of Northern Michigan.

Prof. Philip Hadley, bacteriologist at the Univeristy of Michigan, has notified the journal, Scienece, that while hunting in an uninhabited wilderness in the northern peninsula of michigan, his companion, familiar with the country, pointed out a bird which he declared to be a passenger pigeon, such as he had seen in vast numbers in his younger days. As Prof. Haley looked, the bird took flight, and all he could see was its pigeon-like form and its pointed tail.