Squirrels will dig up flowers/plants; I usually find the entire seedling a few feet away, shriveled up. They don't eat the plants, they want to see if anything is buried underneath.

I did not see entire zinnia seedling dug up - 2 seedlings had few mms of main stem above dirt. But the gerbera leaves have irregular marks(as if it had been poked with something sharp).

or can it be birds?

Anything I can do, so that it does not happen next time? I really want my seedlings to grow and see flowers stay.

Please help

Take a flashlight out after dark. It may be earwigs or slugs. If there are no bugs then I'd say there is a vole about. This is a wild rodent. It looks like a mouse without a tail. Years ago I had beautiful tomato plants. They were about 6 inches tall and I had them in a makeshift cold frame. I went out one morning and my tomato plants were eaten off to nubs. That night a couple of mouse traps went into the cold frame baited with peanut butter. The next morning I found 2 dead voles in the traps. I hadn't had a problem with them before this incident and never had one since then.

Thanks for the suggestions.

I left a mouse trap last night, but no result. Thankfully, other seedlings are intact. Also, checked at night - nothing of note. Will leave the trap in balcony for some time.

I lay a piece of chicken wire over my seedlings. And after they are 1" tall I remove that piece and then put a circular piece about 12" high and 12" in diameter around the sprouting. I don't remove it until the plant fills the space. This protects the plant from squirels, rabbits, and deer.