Ionic migration is a key ingredient for applications such as oxide electrolytes and resistive switching memories. We investigate the evolution of ionic conduction pathways based on optical contrast in an epitaxial Bi0.7Ca0.3FeO3-delta thin film where oxygen vacancies are spontaneously produced. We visualize electroforming processes in the hundred-micrometer-scale material channels between coplanar electrodes with a constant electric bias at an elevated temperature, systematically varying the channel orientation with respect to the crystal axis. At the initial stage of electroforming, conducting filaments are created and propagate nearly along the crystal axes ⟨100⟩. The local density of conducting filament regions increases with the elapsed time of bias application and also exhibits a linear dependence on the spatial position at a given time. We also find that the filament-type ionic conduction is abruptly transformed to the bulk conduction when the filament density reaches similar to 30%. These results offer useful insight into collective ionic migration in crystalline solids.