The original is, 'Hay en esta cuidad unas beatas que ganan su vida ensenando [sic] a hablar a los loros,' — i.e. by teaching parrots to speak. Mr Pinkerton has probably seen hablar in the title-page of some spelling book, and supposed that it meant grammar.
I don't think anyone has suggested recently that parrots are able to acquire grammar in any sense. Were the True Cross beguines the only ones to earn a living training birds to talk clean?