Virtual citizen science (VCS) projects have proven to be a highly effective method to analyse large quantities of data for scientific research purposes. Yet if these projects are to achieve their goals, they must attract and maintain the interest of sufficient numbers of active, dedicated volunteers. Although CSCW and HCI research has typically focussed on designing platforms to support long-term engagement, in recent years a new project format has been trialled -- using short-term crowdsourcing activities lasting as little as 48 hours. In this paper, we explore two short-term projects to understand how they influence participant engagement in the task and discussion elements of VCS. We calculate descriptive statistics to characterise project participants. Additionally, using calculation of correlation coefficients and hypothesis testing, we identify factors influencing volunteer task engagement and the effect this has on project outcomes. Our findings contribute to the understanding of volunteer engagement in VCS.