Gray whales begin their southward migration in mid-October, passing through Unimak Pass between late October and early January. They arrive in Baja California mainly in December and January. The most famous of these is Scammon's Lagoon, named after the whaling captain who first discovered the calving areas. Breeding occurs mainly in late November and early December on the southward migration. A single calf is born in late December to early February of the following year, after a gestation period of about 13 months.
Most females bear calves once every two years.
Growth slows as the whales reach sexual maturity at about age 8 but continues until the age of 40. They apparently feed by lying on their sides and sucking up sediment from the sea floor. Small invertebrates are captured on the baleen while sediment and other particles are expelled through the baleen fringes. Gray whales eat primarily amphipod crustaceans. Concentrations of 12,000 to 20,000 amphipods per square yard have been found in the southern Chukchi Sea and northern Bering Sea where the whales feed.