As NJ said, Polly is indeed a nickname for Mary, at least in my family during the early 19th century. I have a many-great grandmother who was known as Polly Alexander in family oral history. Knowing her name was probably Mary enabled my mother to find her in old records. In the early 1960s, mama even tracked down the family bible which recorded her as Mary. Unfortunately, since then, people on Ancestry started listing her as Mary Polly Alexander and some even went so far as to assume that Polly was her maiden name! One of the many reasons I migrated to Wikitree.