In the US, there are more than 20 chickens for every human. All of them poop and a single giant chicken farm can buildup more than 3,300 tons of manure each year. A lot of this ends up in rivers and streams, disrupting natural ecosystems and killing wildlife. The nitrogen and other chemicals in the manure also often soak down into the groundwater, threatening municipal water supplies. There are many important reasons not to eat chickens. But these are big ones.