We describe the results of a field study in which the social robot RUBI-5, was left alone for a 28 day period to interact autonomously with 16 toddlers at an Early Childhood Education Center. The study is part of the RUBI project, which started in 2004 with the goal of exploring the potential of social robotics for research and enrichment of early childhood education environments. As part of the 28 day field study RUBI-5 collected data about the facial expressions, activities, and spatio-temporal proximity of the toddlers. We found that RUBI-5 could use the facial expression data to accurately predict the children’s preference for different activities: on average robot agreed with human judges as much (Pearson Correlation =0.67) as human judges agreed with each other (Pearson Correlation = 0.68). In addition RUBI discovered some useful aspects of the social structure of the toddler’s group. The study is an important milestone in social robotics, both for the length of time the robot could interact autonomously with children, and for the richness of the data that it provided. The results indicate that social robots have the potential to act as low cost, autonomous “digital ethnographers” in a manner that may revolutionize the science and technology of early childhood education.