How about putting a couple of strands of barbed wire length wise on the boxes?.

I think you mean Mourning Doves... when you used the netting did you raise it above the level of the window boxes? Barbed wire is a little barbaric Rogi. You could also try my shrubby plants that the doves will not flatten.

The things that happen to people ! It must be warm by the brickwork of the house, and safe from predators.

if you look in cities they put wire pointing upwards everywear they don't want them to lande . seams nasty but they just wont land .

You can buy cheaply 'Wide Plastic Pigeon Spikes' which sit down the centre of the windowsill or in your case plantbox and deter birds from landing without hurting them.or you could try sticking twigs in the boxes.

drc has got the idea there not silly they wont impale there selves .

Moon growe, yes the netting was way above but the birds just flatten it down and procedure to nest. I do see as some had suggested some bird spikes you can use that will not harm the bird but will stop them from nesting in my window boxes.

Thank you to everyone for the info.

your welcome