Sooty shearwaters have very regular migratory and breeding patterns. Southern Maori, who have special dispensation to gather muttonbirds, will tell you that on the 13th September – and not a day sooner – muttonbirds start to arrive en masse. They will find their own burrow or hole under a rock, clean it and prepare the nest for the egg. On the 25th to the 27th November they will lay one egg per pair, which the mother and father both incubate. On the 25th to the 27th of December the eggs will hatch and for the next six months the parents will work tirelessly to feed the chicks.
The harvesting season for muttonbirds starts on the 1st of April when the young confine themselves to their burrows. Hunters insert their arm into the burrows and pull the muttonbird out by the neck.
At the end of the season any young muttonbirds the hunters have missed leave the burrows at night to shed their down, and then begin their migratory flight to the islands of north-west Alaska and elsewhere in the northern hemisphere.
During the sixties over 300,000 birds would be harvested each year, but the numbers have dropped considerably with predation by cats and rats destroying most of the mainland nesting sites.
Muttonbirds are basically huge balls of very salty and tasty fat, which is why they are so highly prized. To prepare the muttonbird for sale, the hunters will pluck them and clean away all the down with hot water. Birds are then hung and dried before splitting and cleaning. To preserve them they are either dry-salted and packed into kerosene tins or cooked and preserved in their own lard. In pre-European times birds preserved in fat were packed in kelp or sealskin pouches (usually gourds in the North Island) and traded the length and breadth of New Zealand.