There are no genetically modified chickens, however for several decades, chickens have been traditionally bred for higher productivity including the larger size and health of the bird.
Ashley Peterson, senior vice president of scientific and regulatory affairs for the National Chicken Council answered a very similar question and states,
“We start with good breeding. Over the years, we’ve selected chickens with the best traits (size, health, leg strength, etc.) for breeding, to help get the best start possible. Modern advances in farming including state-of-the-art housing, nutrition, climate controls and biosecurity, coupled with good animal husbandry and cooperation between farmers and veterinarians, all help us raise larger, healthier birds.”
For more Q&A on the topic of animals and animal feed, please review this GMOs in Animal Agriculture booklet that comprises many of the common questions from the public on this topic. This is a great resource!