Gray shoulder and back, pale/white belly; tail pumping along the distant shoreline's edge
ID 80%, based on overall dull brown warbler with partial eye ring, eyestripe, and a clear supercilium. Throat is slightly striped, breast appears pale, and bright yellow rump/undertail.
Ruby? White-faced? Neither?
4th instar. I don't like it's chances of making it to Mexico.