Nearly eight billion chickens...

...are raised and killed for food in the United States each year. According to an opinion piece by Mark Bittman in the Times today, "A Chicken Without Guilt" (my emphasis): 

Chickens are grown so quickly that The Veterinary Record has said that most have bone disease, and many live in chronic pain. (The University of Arkansas reports that if humans grew as fast as chickens, we’d weigh 349 pounds by our second birthday.)