Add up the world’s cats, dogs, pigs and cows and there would still be more chickens. Toss in every rat on earth and the bird still dominates. The domestic fowl is the world’s most ubiquitous bird and most common barnyard animal. More than 20 billion chickens live on our planet at any given moment – three for every human. The nearest avian competitor is the red-billed quelea, a little African finch numbering a mere two billion or so.