I have an elegant sufficiency of voles, the fields are full of them...if I was running a vole farm I would be a rich man.Well , check with the RSPB but my gut instinct is you won't encourage them unless you can increase their prey species which is a massive task in itself. Supply and Demand really.
Not sure about raptors mate but when I bred chickens I was advised to us 2"x2" squared poles because round ones aren't so good for their feet so I guess the same might apply to raptorsMaybe I'm being dense or just too precise, is there a recommended height/ diameter to encourage Raptors to perch ?