The Chicken Update, also known as the September 9, 2016 Update, was an update which added Chickens (which were actually designed from a 2013 blog post by Gelling) and eggs, as well as some other changes.
September 9, 2016 - Added new animal and item. - Added item decay. Only the following items decay: rat brain, rat tail, gray wolf heart, white wolf heart, and the new item. If any of these items are currently in your vault, the decay process will not start until the first time you pick the item up, so old structures and old items will never decay if left alone. However, once picked up, or if new items of these types are added to your vault, they will eventually decay, even if they are part of a structure or are left alone afterwards. - Decay times (in real life time) are 8 days for rat brains, 12 days for wolf hearts, and 4 weeks for rat tails. - All decayable items in the game (and future decayable items) will give a status of the decay state of the item when picked up. If an item does not give a decay status, the item will never decay. - When polymorphing, the skill focus for the new creature will now be whatever your player was focusing on before the polymorph. If the new creature does not have that skill, your focus will be whatever skill is most similar. - Slightly upped silver dagger stats, and upped required skill to 30 (up from 25). Upped spiked shield skill to 10 (up from 5). Lowered bone shield skill to 20 (from 25). Terror shield remains at 35, but other bone shields are now evenly spaced between 20 and 35.