They can't do this. They don't have the passenger pigeon DNA, not one copy, apparently, 100% intact, and you'd need dozens at LEAST to give you anything LIKE enough genetic variability for successive generations of the resurrected species to reproduce and create healthy, viable, not horribly deformed due to extreme inbreeding specimens.
Except that they do. It just takes a pinhead size portion of tissue to get the DNA. There are about 2000 species currently in museums, all with intact DNA.
"Confound these ancestors.... They've stolen our best ideas!" - Ben Jonson