Um, Ancestry does not hold copyright to this book. They didn't publish it; they merely make page images available on their website. The Great Migration books are published by the New England Historic Genealogical Society, which is the copyright holder -- and if you republish the content, you are most definitely violating the NEHGS copyright.
Ancestry authorizes you to make and keep copies of the images you access on their website, but that authorization does not generally extend to republishing them -- and it most assuredly does not authorize republication of contents to which someone other than Ancestry holds the copyright. And even if there's no copyright involved, Ancestry's terms of service tell members not to republish images obtained from Ancestry. Violation of their terms of service could lead to termination of your Ancestry membership.
And lest there be any question, posting an image on WikiTree is publication of that image.