Here we report that the photochemistry of cryptochrome 4 from the night-migratory European robin (Erithacus rubecula, ErCry4) is indeed magnetically sensitive in vitro, and compare the results to those found for Cry4 from two non-migratory bird species, chicken and pigeon. Site-specific mutations of ErCry4 in combination with Electron Paramagnetic Resonance studies are used to demonstrate the roles of four successive flavin-tryptophan radical pairs in generating magnetic field effects and in stabilising potential signalling states. Reference: Xu J. et al., Nature, 2021, 594, 535-540.