They don't know. The assumption in other similar studies is that the chick is trying to pick the food up and eat it. But a chickens beak is equivalent to our hands and they make all sorts of judgments about the object they peck at without eating it. So, the first peck could well be establishing what the object is. Even with adult chickens if you provide them with something they are unfamiliar with, or they find something, they work out how they will categorize it by pecking at it. So every new object is a learning process.Categorized as "exploratory"...Do they mean they were exploratory or they possibly were just misses?
My sister had a related problem with her two Silkies when they arrived. They had apparently been kept in a super clean broody coop without a mum as many breeders do. When they got to free range in her garden they ate all sorts of stuff they really shouldn't have eaten. One ate so much of a herb plant that she developed an impacted crop that was bad enough to require surgery. It's taken some time for them to realise that not everything is food and some food you can eat lots of and others not so much.
So, it seems at least some of adult pecking behaviour is learned from their mother. There is no logical reason to doubt this as all creatures are the same in this respect, but not many other creatures are 'born' in an incubator without parents.
