Before revision:  We find that the cell deformation and cell rearrangement rate fields correlate strongly, which is compatible with a  viscoelastic liquid behaviour with a relaxation time \tau = 70 \pm 15 min. In our experiment, the monolayer behaves as a flowing material with a Weissenberg number close to one which shows that both elastic and viscous effects can have comparable contributions in the process of collective cell migration.
After revision:  We find that the cell deformation and cell rearrangement rate fields correlate strongly, which is compatible with a viscoelastic liquid behaviour with a relaxation time \tau = 70 \pm 15 ~min, is observed to be independent of obstacle size and division rate, and is increased by inhibiting myosin activity. In this experiment, the monolayer behaves as a flowing material with a Weissenberg number close to one which shows that both elastic and viscous effects can have comparable contributions in the process of collective cell migration.