Cover with chicken wire which you can remove when the bulbs start sprouting, but leave a gap so the critters can't get their little mitts through.

Chicken wire is a well known deterrent to squirrels. Just make sure that you press it down and tuck it hard over and around the edge of the tub. Sprinkling curry powder over the soil surface also seems to deter them but doesn't harm the plants.
There's one other serious pest that loves tulips even if the squirrels don't get at them and that's slugs.

We have had bird feeders demolished by squirrels....almost no matter what we have done.......they have knawed through wire and are very, very persistant. Perhaps the smell of curry powder masks the smell of tulips buried 5 inches down. Should I hide behind the large tub with a high pressure water pistol?

I also read that chilli powder does the same.

Chilli flakes last longer and deter rabbits too.....
You can coat peanuts etc in chilli powder it doesn't affect birds it seems?

I spotted a squirrel in my back yard two days ago, picking up the food that the birds are ignoring. Can't think how far it must have travelled as, apart from the overgrown buddleias a few houses away, the nearest trees are the sycamores in the street at the front of the house, then it's a mad dash across the road, round the corner, over the wall and up the embankment to the railway line....
The only thing that stopped me calling pest control was the furry tail but if I saw a rat that size I would be on the phone so quick.

I put the chicken wire about 1" below the soil that way the wire isn't visible and the squirrels if they do dig don't get the bulbs.