They'll not just sitting on the window sill pecking their reflection jingl, but flying full tilt into the glass.
Only one shape is hawk shaped, two are blackbirdy/sparrowish and one definitely a swift. I think it just tells the birds the glass is there. RSPB also sell butterfly shapes which apparently work just as well.
It’s Monday and we are still here!!
Your username explains how you die , how do you die?
How to deal with doom-y thoughts

I have been known to pin a bit of net curtain up until the one for it passes. That does stop it. Robins can do it all through the Summer.
