Research focusing on how collaborative writing takes place across multiple applications and devices and over longer projects is sparse. We respond to this gap by presenting the results of a qualitative study of longer-term academic writing projects, showing how co-writers employ multiple tools when working on a common text. We identify three patterns of multi-application collaboration as well as four common types of motivations for transitions between applications. We also extend existing taxonomies of collaborative writing by proposing a categorization of the functions served by the text as object and backbone of the collaboration. Together, these contributions offer a framing for understanding transitions within and across artifact ecologies in work around a common object. Our findings highlight ways in which features like concurrent editing may in fact challenge the collaborative writing process, and we point to opportunities for alternative application models.