This study presents the results of a preliminary experiment comparing physiological metrics and task performance between different workload conditions of operating multiple UAVs. With the development of UAV automation technology, one operator is able to supervisory-control multiple-UAVs. However, as the number of UAVs assigned increases, the amount of information involved is also expected to increase, intensifying the workload for the operator and potentially reducing UAV control performance. Thus, this study compares potential metrics to assess the different levels of workload conditions based on human-in-the-loop experiments. The results indicate statistically significant differences between high-vs. low-workload conditions in eye-tracking data and heart rate data. Future research will build an on-line workload metrics based on this study's comparative results.