If not slugs then must be snails......look near soil for snails or trails? If you see yellow small balls (snail eggs) squish them between fingers to stop hatching. Spray with washing liquid.
Or it could be weevils.....they r finger-nail size brown bugs.....kill with insecticide.
T think you may find it is Blackbirds as they seem to take an interest in my flower baskets and ate a lot before they even got established so ending up having to use netting.
Headley, I would be most surprised if blackbirds were responsible. They were probably investigating the baskets with a view to removing spaghnum moss which they would use in their nests or looking to remove worms, grubs, etc. to feed their family or themselves. Slugs and snails are an enormous problem when growing impatiens and it really is surprising how much they can get through during the night.