The migration of the Short-tailed Shearwaters

The islands in Bass Strait, especially those off shore from Flinders island, are famous for their mutton birds. The proper name for mutton birds is the Short-tailed Shearwater, Ardenna tenuirostris. They are called mutton birds because, surprise, surprise, apparently they taste like mutton!

For many years mutton birding was an important industry on the islands. 300,000 are still harvested annually, for the oil from their stomachs, down for pillows etc and meat.

While I was on Flinders Island I came across a fascinating book — Patsy Adam-Smith’s “There was a ship: The story of her life at sea.” Patsy Adam-Smith lead a fascinating life, including time as a cook on the steamers that supplied the islands.

Part of her book describes the migratory habits of the shearwaters. Adam-Smith begins by retelling the scientific research of Dom Serventy. When her book was written he was the Chief Officer of the Wildlife Survey Department of the Commonwealth Scientific and Industrial Research Organisation (CSIRO). He established a shearwater  banding program in early fifties, banding every bird on the three rookeries on Fisher Island. Each band had  a serial number and request to contact the Fauna Board. 20,000 birds were banded, not an easy job, and he did much of the work himself.

“……Serventy had slept among the grass tussocks with the sky for a roof, on lonely, uninhabited Bass Strait islands, walked hundreds of kilometres over rock peaks, and pushed through scrub where two metre tiger snakes curled….”

His research solved what had been a mystery.

Previously, people in southern Australia knew that the shearwaters flew away from Australia and then, to all intents and purposes, vanished. Similarly, in the Northern Hemisphere birds also took off and then vanished. The connection, that these were the same species, wasn’t made because it seemed impossible that these birds could make the journey in the time. However, when bands were returned to Serventy from the edge of the Arctic Circle and from Japan, he knew that they were his birds.

“Several Japanese fishermen did return bands. The first no. 22208 was banded on Babel Island on March 13th 1955 and 75 days later, on 27 May was found off Tanoura, Shikoku Island, 8,050 kilometres away. This proved conclusively that the Northern and Southern Hemisphere group, originally thought to be two separate species, were one and the same.”

Adam-Smith described the life of the shearwater:

“As the bird spends most of its life on the wing, coming into land only because of its breeding obligations, it develops a great power of flight. It has been known to wander 1000 kilometres and more from the breeding islands for food.

      To accommodate its moult to its migration the bird moults head and body in the southern hemisphere then waits until the long flight is over and it is safely in the northern hemisphere before moulting its wings and tails. At this time, the Eskimos report, the waters of the icy north are covered with sooty brown feathers.

      On the long migratory flight, which roughly resembles a figure eight, the birds are assisted all the way by the strength and direction of the prevailing winds, except on the last leg of their flight back to the east coast of Australia. Then the south-east trade winds, which blow all the year across the Pacific, batter them badly. Some years the birds suffer tremendous mortality at this spot on their circuit and thousands of their bodies are washed up on the beaches of New South Wales.

         Because of their webbed feet and long wind span (over a metre) the birds cannot rise directly from the land, but must climb a rock or a cliff face to get the lift of a breeze under their wing.”

Their time spent on the islands is equally amazing.

“The birds come to the Furneaux group of islands in September, to scratch out their nest and, if possible, occupy the same burrow each year. Then suddenly, during the first week of November, they desert the islands and every burrow is empty. It stays that way until the end part of the month when they all come ashore at dusk in a big cloud, swooping down to their burrow. The next day they lay their eggs.

     The older islanders told Serventy the one- and two- year old birds didn’t return to the breeding islands. After five years of banding Serventy proved them right.

      When the mutton bird fledgling leaves it burrow it sets off on its first remarkable flight, not making landfall until three or four years later. When it reaches the age of five it comes in with the mature breeding birds. Females begin to breed from five to seven years; males from seven to eight years. For a bird this size, its period of immaturity is very long.”

The adults set off on their migratory flight in mid-April, leaving the chicks in the nest. After 14 days, the chicks are mature enough to make the flight, and they too leave the islands. The wind was very strong when we were there in May, and some of the locals referred to it as a Mutton Bird wind, as the chicks needed the high winds to give them lift. (Unfortunately they had already left the Island. It would be a sight to remember.) The chicks instinctively know where to head to start their first migration.

The explorer George Bass mentioned it in his writings from his journey with Matthew Flinders

“For several hours during the early part of the morning, a vast stream of sooty petrels (Short tailed shearwater) issued from the deep bight which had been left unexplored, and passed the vessel on their way to the westward. There must have been some millions of birds.”

The journey is very arduous and the website WIRES has this to say about the dangers:

“……thousands of birds die each year from starvation, bad weather conditions, gillnet fishing lines (150,000-280,000 yearly), ingesting plastics, oil spills and predators. Often exhausted and emaciated birds in great numbers are washed up on the beaches of Japan, the Aleutian Islands, North America and Australia every year. The mortality rate is very high in the first year that the young Shearwaters migrate with up to 50% not making the journey back.”

I love the story of the short tailed shearwaters for a couple of reasons. Firstly, they are amazing animals. Their ability to navigate around the world, to fly such distances and at such speeds and with such grace. Like all the creatures we share this planet with, their uniqueness deserves to be celebrated.

Secondly, I love the dedication of Serventy. He contributed so much knowledge to our understanding of birds and their migration. We need to celebrate the scientists in our world too. Gotta love scientific research!