how do I stop birds eating solanum berries

Why would you want to stop the birds eating the berries? If you're worried because they're poisonous, I seem to remember reading recently that they're poisonous to us, but not to birds.
I can't think of anything you can put on them to stop them being eaten.

I don't think that in general birds eat things that are poisonous to them. They seem to have built-in knowledge of what is safe.

It's a bit like the reverse rule of thumb for deciding whether a particular fungus is edible - never go by whether it has been nibbled by a squirrel or other woodland animal - it may be just fine for them and very unpleasant for us!

Leave the birds alone to eat the berries - they will do them no harm, just don't eat them yourself :-)

Yes, are you worried about the welfare of the birds or the loss of ornamental value of your plant?

The birds as said will be fine eating the berries. That is what berries are designed for (including berries that would be toxic to us).

Nature can be very subtle at times. Berries fine for birds to eat as their seeds pass through quickly undamaged but made toxic for mammals with their pesky seed crushing teeth. How selective and how clever. It's the plants that really control the animals.

Fractal - you've hit it on the head!

No harm will come to the birds or your plant, the berries have to come off sometime, let the birds have them.