Mammoth chickens

Why heritage chickens are better:

By Susannah Locke on vox.com:

Here are three different breeds of chicken, raised on the exact same diet:

The left-hand chicken is a breed from 1957. The middle chicken is a breed from 1978. The right-hand one is a breed from 2005. They were all raised in the same manner for this paper and were photographed at the same age. Vox added the dates to this image. (Zuidhof, MJ, et al. 2014 Poultry Science 93 :1–13)

The one on the left is a breed from 1957. The middle one is a 1978 breed. And the one on the right is a commercial 2005 breed called the Ross 308 broiler. They're all the same age. And the modern breed is much, much, much larger.
In just 50 years or so, chickens have been bred to be much bigger. The image above comes from a study done by researchers at the University of Alberta, Canada, who raised three breeds of chickens from different eras in the exact same way and measured how much they ate and how they grew. This allowed them to see the genetic differences between the breeds without influences from other factors like food or antibiotic use. They recently published their results in Poultry Science.

What breeding has done to your chickens

1) Chickens today are much bigger than those in the 1950s: This one's pretty obvious. The 2005 chicken breed on the right ended up being about four times as heavy, on average, as the 1957 breed on the left — despite being fed the same foods.
2) Chickens today are more efficient at turning feed into meat: The reason for that is that modern-day chickens are more efficient at turning feed into breast meat. The researchers' metric for this was something they called the "breast conversion rate" of grams of feed into grams of breast meat. The 2005 breed was roughly three times as efficient as the 1950s one.
3) Modern chickens also have extra health problems: Previous research has noted increased bone, heart, and immune system problems in some contemporary chicken breeds. Health problems could come from several factors, including both unintentional genetic effects and behavioral differences such as diet and carrying around all that extra weight.
4) But the growth of chickens has helped make chicken a popular food: Over the past few decades, chicken has become a much cheaper food. And Americans have been eating more of it. (The price of poultry has risen at about half the rate of other consumer goods from 1960 to 2004.) In 2013, Americans consumed more than 83 pounds of chicken per person.

(USDA)

According to the Poultry Science paper, our ability to breed bigger, more efficient chickens had played a big part in that.
Hat tips to John R. Hutchinson and Francie Diep for calling my attention to this paper.