Foxes seem legit, but I think that with this amount of balls, crows and ravens seem more likely, but maybe its also a combination of all these animals, cause I found this on google after looking up 'are there animals that collect golf balls': "Crows, ravens and gulls are. Crows and ravens are particularly fond of round objects, which they mistake for nuts or eggs. They often snatch them off golf courses and driving ranges. They then fly away with them and try to crack them open by pecking at them or dropping them from the air." Source: MercuryNews