Pigeons!

[Plocket]

I have been feeding the birds using hanging feeders for a couple of years now.    The feeders hang over concrete because I got fed up of picking out seedlings and I sweep up regularly.

My problem is that a pair of pigeons have been regular visitors for a couple of months now, and they are scaring away the blackbirds and robins.  The pigeons can't find much food - they just stroll around the garden - and occasionally my cat chases them, but can anyone suggest how I can deter them?