BY LANCE MURRAY Tarrant County bird watchers will joins others nationwide Friday through Monday in the Great Backyard Bird Count, a joint project of the Cornell Lab of Ornithology and the Audubon Society.Enthusiasts are being asked to spend at least 15 minutes on any of those days counting and noting the species of birds that they see in their neighborhoods.Tarrant County communities will be participating again this year.Fort Worth Audubon Society members will be present at several Tarrant County parks to assist persons with their counts, said Gail Morris, president of the Fort Worth group.Included among those locations will be River Legacy Parks in Arlington, which will hold its Great Backyard Bird Count/Family Fun Saturday: We Love Birds!