I know it sounds like a difficult subject but it just means that although things that we might go through in life do not change our actual DNA, they can modify the way enzymes, proteins, etc act on the chromosomes and what they found was that these 'modifications' are inherited by the offspring. They mention the case of European women who were starved during WW II who, getting pregnant, gave birth to very small babies -something that, of course, one expects when the mother is starved. But the kicker is that, even when those babies grew up eating normally, they also had very small babies!
Now, think about the bad conditions that commercial breeders keep their birds under and do the math on the baby birds people buy from them...
https://www.zmescience.com/medicine/epi ... -242789930