Within the photoreceptor cells of avian retinas, cryptochrome-4a (Cry4a) has been proposedas a magnetic compass sensor in migratory songbirds. Recent photochemistry studies have demonstrated that Cry4a from the night-migratory European robin (Erithacus rubecula) exhibits greater magnetic sensitivity compared to the nonmigratory bird species. In ErCry4a, blue-light excitation of the flavin triggers stepwise electron transfer along a tryptophan tetrad to the flavin, forming well-separated radical pairs that have been proposed to play a role in magnetoreception. Herein, we employ first-principles electronic structure methods and hybrid quantum mechanical/molecular mechanical (QM/MM) simulations to investigate the stabilization, interconversion, and recombination of these well-separated radical pairs. Reorganization of the protein environment and the aqueous solvent substantially stabilizes the long-range charge-transfer states associated with the anionic flavin radical and the cationic radical localized on the third or fourth tryptophan, rendering these radical-pair states energetically comparable to those in the charge-neutral state. Free energy analysis combined with electronic coupling calculations provides supporting evidence for the previously proposed idea of a "composite" radical pair involving both the third and fourth tryptophans, as the functional magnetoreceptor. To provide guidance in probing the potential role of the composite radical pair in magnetoreception, we identify key amino acid residues whose mutation may significantly alter the relative stabilization and chemical dynamics of these radical pairs and consequently affect magnetic field sensitivity of ErCry4a