Merle is a word used to describe a bird that has a mottled or speckled color pattern. It is often used to refer to some types of blackbirds or thrushes that have a mix of dark and lighter colored feathers. Full definition
There are also some breed specific issues you will need to research if you want a certain pastoral type: for example those related to the Blue Merle gene in Border Collies, or hip problems in German Shepherds. (thehappypuppysite.com)
Only a couple years later, did I realize this was the result of double Merle breeding. (pethelpful.com)