I have a lot of birds in the garden and although several 10's of metres away
the next door neighbour has a cat - who likes birds! - Game on -
I use 2 electronic devices bought from an RSPB recommended resource -
Concept Research and if you keep moving them around they seem not to like
it.
http://www.conceptresearch.co.uk/cats_1.htm
http://www.conceptresearch.co.uk/cats_2.htm
Not cheap but ......
I hope that helps
O. N.