Ezra Klein:

The carbon implication is that vegetarianism is best, but if people insist on eating meat, chicken is far better than beef. This puts the carbon argument at odd with the animal rights movement. For them, chicken is far worse than beef. It takes a human being years to eat a cow but only a single dinner to consume a chicken. The death toll of a poultry diet is far higher than a beef diet. And chicken are treated far worse than cows.