Abstract: Protandry refers the earlier arrival at breeding sites or reproductive stages of males than females. Focusing on the general phenomenon of males arriving at breeding sites earlier than females in migratory birds, we introduced six hypotheses for explaining the protandrous arrival, including rank advantage, susceptibility, constraint, mate opportunity, waiting cost, and mate choice hypothesis. We also discussed the applicability of these hypotheses to explain the protandry in migratory birds. In addition, we put forward the issues which might be important in the future research, including 1) occurrences of protandry in species of sexually homomorphism, 2) effects of global warming on protandry, and 3) effects of intrasexual competition on protandry.