It may be rats. We started getting similar sized holes, then larger and larger ones. We trapped them for a while but didn't seem to be making any impression on the population. One of our neighbours complained to the council about an unkempt garden nearby (we were far too polite!) and the council insisted that they tidy up and only put food out for the birds in the morning. The council also put down bait. Within two weeks we stopped catching any in our trap and have seen no signs of fresh activity.

As for cats, they are a double edged sword. Great allies in the war against rodents but a pain when it comes to birds and their toilet habits. Our birds seem to be smart enough to get out of the way of any visiting cats and we don't seem to have much trouble with toilet activities. We use alot of cow manure and mulches, but I don't know if this helps. A cut out cat with a sparkly eye is supposed to be effective and not too difficult to make.

Good luck