THEY'RE BAAAACK: Of more than 100 bird species breeding in Orange County, the one that draws the most attention each year is the cliff swallow, thanks to the enduring myth that it returns to Mission San Juan Capistrano each year on March 19. . . . The bird isn't that punctual and doesn't flock to the mission in the numbers of old, but it's still "our major tourist attraction," says Chamber of Commerce spokeswoman Mary Lou Bonsague. "People have to realize that 200 years ago, the mission was the only structure here." More buildings mean more potential homes for the birds.