A charismatic colony of purple martins along Ruston Way is moving. The dark purple and blue birds are North America’s largest swallow and often spotted among decrepit wooden poles at Dickman Mill Park, Titlow Beach and Chambers Bay. They are a native bird that has been migrating back to Tacoma and other areas of the West Coast for thousands of years, arriving each spring from Brazil and usually departing by late August. The birds – beloved for their pest control and throaty chirps known as the Dawn Song – live in wooden nest boxes or plastic gourds made by humans, …