They tried to but they were killed by red kites, pine martins. gos hawks, buzzards,otters ,polecats,sea eagles and one nest was destroyed when a tree fell on it by a beaver
I'm not sure are you joking or just misinformed
http://britishbirds.co.uk/news-and-comment/common-cranes-breeding-in-Scotland
Seem to be doing well on that evidence.
I'm sure it was a joke mate at the fact of all the predators they have released