I've seen that term here and there and I think I know what it is... it's when hatcheries put a few extra chicks in with your order, right? And you may not know what breed or sex they are?
How often do hatcheries do this? Is it something they all do or just some of them? Are you more likely to have it done if you've ordered a small amount of chicks?
How often do hatcheries do this? Is it something they all do or just some of them? Are you more likely to have it done if you've ordered a small amount of chicks?