The tailbud is the posterior leading edge of the growing vertebrate embryo. Now, by measuring the three-dimensional cell flow field of the zebrafish tailbud, Scott Holley and co-workers reveal a posterior flow within the tailbud that reflects ordered collective cell migration (p. 573). They identify a transition in tissue fluidity at the tailbud tip where there is a decrease in the coherence of the cell flow but no alterations of cell velocities. Inhibition of Wnt or Fgf signalling reduces the coherence of the flow, but affects trunk and tail extension differently. By using computer simulations to interpret these complex phenotypes, the researchers show that a decrease in the coherence of the flow combined with a normal flow rate leads to a ‘traffic jam’ in the posterior tailbud and a severely contorted trunk, whereas decreases in both coherence and flow rate merely ‘kink’ the tip of the tail. Thus, the balance between flow rate and the coherence of collective migration within the tailbud steers zebrafish body elongation.