Disastrous mulberries

This happened to me last year, and I put it down to freakish weather conditions, but the mulberries on my tree are not making it to fruition. 

When I approach the tree, there is a smell of fermented fruit, and plenty of rotten mulberries lie on the ground. 

None of the berries have gone black, and when I reach up to pick the more deeply-coloured ones, they either fall apart in my fingers like raspberries, or they are dry and rotten. Clouds of flies (fruit flies?) surround the tree. 

The harder, more complete berries I have successfully picked are either sour, or they taste rotten. 

I suspect that it might be bird damage - is that possible? That the birds peck them and then they die on the tree? I really can't harvest a single one, though - there are no bursting black mulberries at all - can birds be that destructive? I have lived with mulberries all my life and never known this. I can post a picture if that helps. 

Please help - this tree is an old friend and it's no exaggeration to say that picking mulberries is usually the high point of my year. If there is something intrinsically wrong with the tree I'd be devastated. 

I just need to know what, if anything, I can do to stop this from happening. 

Thanks for your help.