Problem:
I want to know this in particular about animals that no longer eat what they used to eat. The best example in my opinion are chickens. Chickens are often held to eat all kinds of food we would otherwise throw away. Most of this, they would never find in nature. Chickens almost eat everything, but there are things they don't. Also, they often have a clear preference for some food.
Question: How can they differentiate this food?
Question: Do they use smell? Looks?
Question: How can they know smells of food they never had before?
Please answer the questions in detail.