In order to do this he had a squadron of Peregrine Falcons which hunted down "suspect" pigeons. (I don't know if the falcons asked for ID first.)
Lord Tredegar came up with a plan to bewilder Abwher pigeons by dropping British pigeons from aircraft over Germany. ( Yes, yes, when I read it the obvious problem sprang immediately to mind, but wait.)
The first load of pigeons were unfortunately sucked into the plane's slipstream and "defeathered". For the second attempt the pigeons were dropped inside paper bags. Really. I didn't make this up, only Terry Pratchett could come up with this as fiction.
These pigeons apparently survived their rough treatment and invaded the German pigeon lofts. Of course after a few days R&R they returned to their home lofts in the UK, being homing pigeons and all. (Personally, having been dropped from a plane in a paper bag I think I'd have set up home where-ever I landed.) My source doesn't reveal how many of the returning pigeons were taken out by the friendly fire of the falcons.
Lord Tredegar complained about the failure of his plan to Lady Baden-Powell, who promptly had MI5 lock him up in the Tower of London for revealing top secret government business.
Don't believe me? This link has more information, including the fact that two enemy pigeons were captured and interned for the duration of the war.
