Birch reduction involves an SET from the metal to the pi-system to give the radical anion. This species abstracts a proton from the solvent forming a radical. A second SET occurs giving the anion. This abstracts another proton from the solvent. Subtle changes in product can be obtained by controlling solvent pKa. For more information look in the Chemogenesis
webbook sections on redox
chemistry and substitution,
transfer, abstraction, displacement (STAD).