I had this happen when I was growing strawberries, I found a heap of unripe berries hidden by my compost heap. I reckoned it was rats. I don't think birds would do it as they would only be interested in ripe fruits.

birds are animals :o)

magpies have done this to me before and they then share them with the rest of the family troupe. Could also be squirrels.

That reminds me of funny story that happened 30 years ago. We bought a strawberry tower, which was like a barrel about 3 feet tall. One day my husband said oh no something has been attacking the strawberries. Sure enough large chunks had been taken out of the fruits.
One look at my 2 year old's pink mouth and cheeks soon identified the culprit.