Migratory timing of humpback whales (Megaptera novaeangliae) in the central North Pacific varies with age, sex and reproductive status

Humpback whales migrate seasonally between high-latitude summer feeding grounds and low-latitude winter breeding grounds. Identification photographs of humpback whales were collected in the Hawaiian Islands between 1977 and 1995, and sighting histories were compiled for individuals. Analyses revealed that (a) mean dates of first identification were significantly earlier for juveniles and females with no calf than for males and females with a calf off the Big Island, and significantly earlier for juveniles than for females with no calf, males and females with a calf off Maui; and (b) mean dates of last identification were significantly earlier for juveniles and females with no calf than for males and females with a calf off the Big Island, and significantly earlier for females with no calf than for males and females with a calf off Maui. A within-subjects comparison showed that the date of first identification tended to be later for individual females in the years when they had a calf than in the years during which they had no calf. It was concluded that (a) migratory timing varies as a function of age, sex and reproductive status, (b) migratory timing is intimately connected with reproductive success and (c) migratory timing has important consequences for our understanding of humpback whale behaviour on the winter grounds.