The Cornell Lab of Ornithology Handbook of Bird Biology (1-24) seems to be saying that the first birds hopped from tree branch to tree branch, and that some birds who now spend a lot of time on the ground now walk instead, but the changeover has been pretty patchy. For example the Greater Roadrunner runs, but has close relatives amongst the Cuckoos that hop. Most passerines hop, but Crows and Starlings walk. Adult Larks, Pipits and Wagtails walk and run, but juveniles hop.