Jackdaws pecking on a chimney cap

Hi everyone, 

Does anyone know why jackdaws will peck on chimney caps and how they might be deterred?  I have been wondering why bits of mortar had been falling off our roof, and thought it was frost....until I could feel the hammering and caught them in the act!  Today I watched them on my neighbour's cap - one pecked off an egg sized lump of mortar and was pecking off and seemed to be swallowing bits of it  before it flew off and dropped it.  Perhaps they do it to aid breakdown of food in their crop? 

The damage is not inconsiderable - we are losing mortar in the interstices of the bricks now, and I'm a bit concerned about water penetration and freeze thaw... any suggestions for deterrent welcome - would pigeon spikes work?  Many thanks!