This research examines the "Tree Investigators" project to support science learning with mobile devices during family public programmes in an arboretum. Using a case study methodology, researchers analysed video records of 10 families (25 people) using mobile technologies with naturalists at an arboretum to understand how mobile devices supported science talk related to tree biodiversity. The conceptual framework brings together research on technological supports for science learning and research on strategies that encourage families to engage in conversations that support observation and explanation practices. Findings suggested that families engaged in high levels of perceptual talk (describing and identifying) while using mobile computers. Commonly, families articulated scientific observations when supported by prompts, visuals, and scaffolds delivered by the mobile computers. Families struggled to make explanations about the biological importance of what they saw in relation to ecological principles; however, families made connections to their everyday life within explanations they developed at the arboretum. Our research showed the importance of mobile supports that provided on-demand, localised sense-making resources for explanation building while limiting observational complexity.