The name "Birdcatcher" comes from a Thoroughbred stallion named Birdcatcher foaled in Ireland in 1833 by Sir Hercules. He has white hairs along on the base of his tail and throughout his coat. His unique markings were then dubbed Birdcatcher spots. For more information on Birdcatcher, visit the website Thoroughbred Heritage: http://www.tbheritage.com/Portraits/Birdcatcher.html
Birdcatcher spots are small, round, white spots that have been reported to be as small as pearls and as big as coins. They usually appear over the entire body in a random pattern, and there are some examples of Birdcatcher spots clumping together. Also, there are reports of the spots developing with age and appearing and disappearing. There are no links to it being genetic.