• 1The Short-tailed Shearwater, Puffinus tenuirostris, migrates annually with remarkable constancy between its southern Australian breeding islands and the north Pacific and Arctic Oceans. The regularity of its southern landfall (in late September) and egg-laying (during the period November 19 to 21 and the following twelve days) facilitates its commercial exploitation in the Tasmanian “mutton-birding” industry.
  • 2The species is highly exceptional in that it breeds in the warmer of the two regions between which it oscillates. It arrives in the breeding area and remains there during the period of surface swarming of the euphausian plankton on which it mainly feeds.
  • 3Gametogenesis almost certainly begins in the northern hemisphere and this and the nuptial migrations are perhaps initiated by decreasing daylengths. The males possess bunched spermatozoa on arrival at the breeding islands. Individuals “home” to the same nesting site each year. Spermatogenesis then proceeds slowly, and oogenesis steadily, whilst nest-burrows are renovated and a noisy nocturnal display occurs. Fertilization perhaps takes place ashore after which there is a pre-laying exodus lasting about three weeks. During this absence at sea the single large egg (about 16 per cent of the female's body weight) matures and the flocks then return for the laying period.
  • 4The males take the first incubation shift and fast of eleven to fourteen days while the females go back to sea. At initial landfall, and at the time of egg-laying, both sexes contained abundant depot fat. After their incubation fast the males are relatively lean. The incoming females, on the other hand, are fat. This alternation of fat deposition at sea, and utilization ashore continues during the incubation period of fifty-three to fifty-five days.
  • 5Only one egg is laid per year. Out of season, or even out of phase, breeding is unknown.
  • 6The seminiferous tubules are already in a state of lipoidal and cholesterol-positive metamorphosis at the time of egg-laying when the males take the first incubation shift. The possibility that the luteinized tubules may be an endocrine organ of the same general nature of the mammalian corpus luteum is discussed.
  • 7Both parents feed the young but desert them while they are still in the burrows at an age of about three months. The mean desertion date is about mid-April. Both parents, which are now lean, disappear from the breeding area. About a fortnight after their desertion the fat young leave the burrows and disappear.
  • 8Migration north by both adults and young occurs while abundant food still remains in the breeding area. Sea temperatures are still higher than at the time of ovulation. The cessation of parental responsibility may be a major factor in the stimulus to contra-nuptial migration in the adults. This movement takes place with both young and adults, probably as in all birds, in accordance with an innate and traditional pattern of behaviour. The post-nuptial journey of 5,500 miles between Tasmania and the colder Japanese waters has been covered (as proved by a ringed fledgling) within about a month.
  • 9Breeding does not begin until the birds are six years of age and so a very considerable non-breeding population exists. These immature birds also undergo annual migrations and partial gametogenesis, some individuals producing a few spermatozoa.
  • 10A remarkable adaptation to the exacting demands of lengthy migration is shown in the division of the post-nuptial moult into two phases. The moult of head and body takes place at the breeding site but that of the wings and tail is delayed until the relatively sedentary phase in northern waters.