If you don't have squirrels then voles and mice are the main culprits, though some birds, like jays, will take them too. If you wait till they are fully ripe then you won't get a look in, not least as they often drop before then anyway. I'm not sure when they would normally ripen in your neck of the woods, but here (Wales and formally the south of England) I'd be looking to pick them in September on average (might be earlier this year with this good weather) - keep an eye on them and when you see the first ones just starting to turn brown don't leave it too much longer. If you keep them in the husk they will ripen fine (they used to traditionally store them in a crock pot, something 'sound' that could still breathe), but they are edible while still slightly green and have a lovely milky texture. I've noticed that when ripened in storage those that slip out of the dried husk are usually okay, but those where the husk stays firmly attached to the shell are not good. So far it's looking to be another good hazelnut year here...