It's a bit tricky since while genealogical data can't be copyrighted, sites do have rights to their indexes, etc. So WikiTree recomends that members not copy actual pages from sites unless they have the explicit written rights to them. For instance the person who shot a gravestone photo for findagrave has the right to allow a person to copy a photo to WikiTree, but you should have the permission in writting. For places like Ancestry.com and familysearch, they have source material you can copy and paste into Wikitree profiles but you shouldn't copy the actual source pages but link to them. You're not going to be able to do bulk transfer of sources in a form which will be very useful. Perhaps someday WikiTree will work deals to house data directly to our site, but for now it's cut and paste and link to data.
