of gradient sensing errors due to CCV is the existence of a biochemical or mechanical memory that can perform a time average over tens of minutes. There are several possibilities. First, memory could be external to the cluster – e.g. stored in extracellular matrix structure, or a long-lived trail [@lim2015neutrophil]. Secondly, supracellular structures like actin cables influence cell protrusion and leader cell formation [@reffay2014interplay], suggesting that collective directional memory could be kept by regulating actin cable formation and maintenance. Third, memory may be kept at the individual cell level by cells attempting to estimate their own bias level $\Delta^i$ and compensating for it. This contrasts with our straightforward average of the collective estimate ${\ensuremath{\hat{{ {\mathbf g}}}}\xspace}$, but could be an important alternative mechanism.

Our results are critical for understanding the ubiquitous phenomenon of collective gradient sensing. The importance of CCV provides a new design principle: CCV must either be tightly controlled or mitigated by time-averaging. We also established a surprising link between a central [*mechanical*]{} property of a cluster – its rheology – and its sensing ability. This connects mechanical transitions like unjamming [@bi2016motility] to sensing, opening up new areas of study. In addition, our results show cluster accuracy depends 