This is Informtion the Requerement to Upgrade it. So you can plan it early..
ROOKERY
To upgrade the 'Rookery', we must first upgrade the 'Range'.
So, for example, upgrading a 'Keep' requires a 'Wall', and it requeres a 'Rookery', so while the clock is still ticking for other buildings, we can plan to upgrade the 'Range' before it's needed.