Yes Bobbie they do call them butcher birds here too. Its because they skewer their food on a branch and rip it up.
And yes they do look like Northern Mockingbirds,Mak see pic for comparison.
Not the greatest
Saw this Butcher Bird behaviour in Majorca - quite amazing. I think it was a Red-backed Shrike, quite an attractive fellow. And MAK - once you have seen a shrike, there is no second-guessing with anything else. Trick is to see one!