Timing of the migratory cycle ensures that pregnant females and mothers with new-born calves spend the majority of their time in relatively warm water. Some movement of individual humpback whales between breeding areas has been documented. Whales photographed in Hawaii in one year have been observed in Mexico and south of Japan in other years. Although sightings of calves are common during the winter, no well-documented evidence of an actual birth exists. After a calf is born, its mother will remain close to shore, resting and nursing her newborn.
Calves survive on their mother's fat-rich milk for six to eight months.
They grow at an astounding rate, nearly doubling in length during their first year. Often mothers and calves are accompanied by a third whale called an escort. The escort whale, assumed to be a sexually active male, remains with the mother and calf for less than a day, with most associations lasting a few hours. Males and females do not form long-term pair bonds. Competition for the female involves a variety of intensely aggressive behaviors that may occasionally escalate and result in death.