I think birds have put holes in fleece covering

Does sound like bird peck holes, although how they'd know seed was there is a bigger question. If possible, keep watch on this to see what culprit does this. I can't think of any bugs that would drill holes and squirrels would simply tear or pull fleece away.

Is the fleece new, or has it been used for several years?  Eventually, UV light can rot fabrics, causing holes (though probably not holes which are beak-shaped!)

on a similar theme, I have some flimsy weed suppressing plastic down on most of the beds and I had noticed distinctive muddy fox paw prints on the surface.  Yesterday I found 2 of the beds had been shredded, and one with a crop in had been dug over.  Bl**dy fox, or so I thought. :evil:

However, during one of the my (many ) breaks, I saw a magpie fly and land on the bed.  It didn't do anything but it did make me wonder ...