Humpback whales mate in or around Hawaii during the winter months, gestation lasts 11 months and female humpback whales return to Hawaii the following year to give birth and raise their young. Birthing is still shrouded in mystery - it is yet to be reliably documented and there is only one incident to date (in over 25 years of whale watching and whale research). Based on evidence from whaling days we know that calves are, between 4- 4.6 meters in length and weight around 1500 kg at birth.
There is also some evidence to suggest that many calves are born at the end of migration, before the females reach Hawaii. Once the calves are born though, the waters around Hawaii certainly become their chosen nursery and, within these waters, humpback whale calves are easy to spot. Almost invariably, they are closely accompanied by their mothers, at least during their time in Hawaiian waters.Earliest sightings of mother and calf pairs have been reported in November and December some years, but generally calf numbers rise slowly through January, peak numbers of calves are seen in February by which time they comprise up to 10% of the population, and by the end of March mothers and their calves are by far the most predominant groups and they are frequently seen at the surface; extended sequences of breaches and other surface acrobatics are common components of the calf's behavioral repertoire.
They are also the most persistent groups; mothers with calves will stay longer in these waters than any other social group, with pairs sighted through May and even into early June. Given the lack of feeding resources in this area and the energetic surface activity of the calves, the persistence of mother and calf pairs is puzzling, however our research indicates that calf behavior is functionally driven; each different regime of behavior that we see in Hawaiian waters plays a key role in the development and future success of the young calf and the persistence of mother and calf pairs in the nursery area directly enhances the calf's future chances of survival and success.