Its the shine or their reflection we used to have a crow that attacked the bathroom window....... at dawn.......

They see their reflection in the mirror and think it is another magpie trying to take over their territory and so attack.

And the magpies are doing nothing to justify shooting them.

The crows I mentioned were really only a problem in the breeding season

Keep it running - that way, they won't think there's another magpie they need to defend their nests from. Shouldn't be an issue once breeding is over.

I got fed up with cleaning blackbird spit off my double-glazed windows which give a reflection, so I stood an open book against the window showing a picture of a kestrel flying. The blackbird came again to 'fight off his competitor' then stared in amazement at the book - and we've had no trouble since!

Some gc's sell black images of predators to stick or hang near a window

In find those plastic windmills stuck in the ground deter bigger birds.

Thanks for all your helpful remarks. Thought you would like a follow up. We can't leave the feature on at night as the noise would disturb in a built up area. We were already trying a blanket cover as the birds were beginning to scratch the mirror - but they were lifting it up to try to find their enemies! What seems to deter are a couple of my grandchildren's plastic windmills as suggested by Drc726. The magpies are very suspicious of them, but seem to have stopped attacking the mirror. So again thank you.

Hope the windmills continue to work!

Thats good to hear Lizjag, I have a plastic windmill by my pond and have not seen a Heron since I put it there, I do change it every spring because they fade and get tatty and I think its the colour as well as the movement that seems to do the trick.