A muttonbird is, to be precise, a sooty shearwater
(Puffinus griseus). Globally, this is a highly populous species of seabird. It nests on cliffs, generally on small islands. More generally, the term may, historically, have been applied to a number of species which were taken by sailors to restock their ship's larder
It is legal for certain tribes of Maori to "collect" the young of the muttonbird and approximately a quarter of a million squabs are taken each year from islands around the south of New Zealand (principally in the Foveaux Strait). The parents go to sea a week or so before the young bird takes flight. As a result, the chicks are extremely high in fat so that they can survive the period before they fledge. They also taste strongly of the fish which is their whole diet. A common method of preserving the catch is to keep them in brine making the meat salty. The meat is also very dark. This makes muttonbird - or Titi in Maori - something of an acquired taste.