Written for Bonorong by Pink Lemonade Social

Slender-billed Shearwater or Muttonbird (Short-tailed Shearwater)

Ardenna tenuirostris; formerly Puffinus tenuirostris

The most common seabird in Australia is the Short-tailed Shearwater with a population of around 23 million!  During the breeding season, millions of these seabirds congregate on a variety of small islands in the Bass Strait and Tasmania, Australia. The adults leave the breeding islands when their chicks are large enough to fend for themselves and migrate north-east, flying through the central Pacific Ocean and spend the winter months at sea off the coasts of Japan, Siberia, and Alaska in the northern Pacific - travelling roughly 15 – 17 thousand kilometres in each direction annually!

Short-tailed Shearwater Size, Description & Appearance

The short-tailed shearwater has a one-meter wing span, a blunt tail, and a black bill. And weighs about 500 grams. Shearwaters have webbed feet and are excellent swimmers while their long, narrow wings allow them to fly at high speeds with ease. As a result of these adaptations, the shearwater finds it difficult to move around on land or to fly when there is no wind. Shearwaters have dark brown plumage, with a white patch on the underwing in the centre. Their dark grey feet trail slightly behind when flying, giving it a round tail appearance. There is a wide range in the average lifespan of shearwaters, from 15 to 38 years.

Short-tailed Shearwater Habitat & Location

The Short-tailed Shearwater is Australia's most common shearwater during the summer months and can be found in coastal areas. As they return from their wintering grounds in the North Pacific, enormous flocks of Shearwaters flock to southern coasts in Australia to breed. Over 18 million birds have made the journey, according to some estimates, passing through at a rate of 60,000 individuals per hour. As a result of sickness, exhaustion, and bad weather, many birds wash ashore at this time. Here at Bonorong, April and May is known as Shearwater season, and we regularly treat injured birds at our Wildlife Hospital, helping them to rehabilitate. Surprisingly, given the enormous volume of birds involved, the number of fatalities during this annual migration is quite low.

There are short-tailed shearwater colonies around Tasmania, on Bass Strait islands and in South-East Australia. The size of a colony can range from a few hundred individuals to over a million individuals in a single colony. 

Short-tailed Shearwater Feeding & Diet

Short-tailed Shearwaters eat krill, small fish, and a variety of other small marine life. There are times when birds dive for food, but it's more common to see them on the surface of the water. Shearwaters in Tasmania feed on a tiny crustacean called Nyctiphanes australis (Coastal krill, 1-2cm), but in the Antarctic, they eat Antarctic krill (5 cm). The lesser part of their diet includes arrow squid, fish and other crustaceans.

Following their day at sea, adult shearwaters return to their colonies at night in order to avoid land-based predators like eagles, feral cats and possums. They can travel a distance of 1,500 kilometres (930 miles) to feed and leave the chick unattended for more than a week during these trips.

To catch their prey, Short-tailed shearwaters dive to depths of up to 50 metres. However, the majority of dives are no deeper than 20 metres. To get around on the water, the birds use their wings as propulsion.

Short-tailed Shearwater Breeding

Short-tailed Shearwaters build huge colonies off the southern and southern-eastern coasts of Australia each year. More than 16 million adults can be found in colonies off the coast of Tasmania, while colonies in Victoria and New South Wales can hold an additional two million or more. During the night, birds make their way to the colonies. They nest in a leaf-lined chamber in a burrow in the ground. The burrows can be up to two metres long, and shearwaters and penguins compete for territory at the beginning of the breeding season in some colonies.

All petrels, including short-tailed shearwaters, produce only one egg per season. Both sexes incubate the egg for 52-55 days.

They have a long-life expectancy of more than 30 years and tend to breed with the same partner year after year. In January, the chicks hatch, and they leave the colonies in April. The chicks are now heavier than their parents after their final feeding from their parents. For up to three weeks, the chick can survive on the last meal before it leaves the burrow.

In order to feed their chicks, shearwaters break down their prey into an oil that has a lower mass than the prey itself. Using this oil to feed chicks is an efficient way to transport food over long distances (such as from Antarctica to Tasmania). Both parents feed their children by regurgitating food and the frequency of feeding visits decreases as the chick ages. Approximately 82-108 days after hatching, the young is abandoned by its parents.

When they are between the ages of 5-8, Shearwaters begin to breed for the first time. It is common for the same site to be used over and over again by both sexes as they excavate the burrow. This burrow's nest chamber is either completely bare or covered in grasses.  

Short-tailed Shearwater Migration

Between September and April, around 18 million Shearwaters make their way to Tasmania. There are an estimated 11.4 million burrows in Tasmania's with 209 known colonies. On Babel Island, there are three million burrows, the largest colony - landing and taking off are made easier by headlands (coastline that projects into the sea).

Early records indicate that the population was much larger than it is today. One flock in Bass Strait was estimated by Matthew Flinders to contain 100 million birds in 1798. Because they don't come ashore during the months of their migration, it has been difficult to determine their exact route. It appears that most birds fly north along the western Pacific Ocean to Arctic regions and then return southwards via the centre of ocean. Geolocators, a relatively new piece of technology, have revealed that, prior to beginning their post-breeding northward migration, these birds first fly south of the Antarctic Polar Front to feed for several weeks at highly productive cold water upwellings. These shearwaters fly between 15 and 17 thousand kilometres per year in both directions. It has been reported that they can fly this incredible distance in just six weeks, covering the Pacific Ocean Basin in just 13 days, averaging 840 kilometres per day. A flight from Tasmania to the Antarctic feeding grounds for these birds can take as little as three to four days at speeds of up to 40 knots!

Short-tailed Shearwater Distribution & Conservation Status

Short-tailed shearwaters are secure in their abundance. Short-tailed shearwaters migrate to the North Pacific Ocean every year and reach the Arctic Ocean north of Alaska before returning in September to their colonies in the south. Migrating north-south, rather than figure-eight, around the Pacific Ocean has been established as their true migratory path.

Short-tailed Shearwater Threats

In spite of the apparent abundance, short-tailed shearwaters are still at risk of harm due to overfishing and the destruction of their habitat. After a fire, the soil can be eroded, making burrowing difficult. Up to 50,000 birds are accidentally killed each year by gillnets in the North Pacific. Commercial operations in Tasmania currently harvest and sell about 200 000 chicks each year. While at sea, small plastic particles can be ingested by birds, limiting the birds' ability to maintain their physical condition and increasing the number of birds that die during their migration. Shearwater chicks are easy prey for feral cats, who are also a problem. Humans can also kill burrow dwellers by trampling on them. Similarly, burrowing habitats can be destroyed by erosion brought on by recreational vehicles. Avoiding colonies is critical. In the first migration, exhaustion and starvation lead to a high rate of natural deaths.

Short-tailed Shearwater Facts You Should Know

·       A typical lifespan is between 15 and 19 years, but it can be as long as 38 years.

·       In the spring-summer, they return to southern Australia after travelling 15,000 kilometres to the Arctic each year.

·       When a single chick hatches, each parent feeds it for 2–3 days before departing to find food for up to three weeks.

·       They are one of the few bird families to have a well-developed sense of smell, with tube-like nostrils on top of their upper beaks.

·       Members of Captain Cook's Third Expedition in the Arctic Ocean in 1778 were some of the first to observe the shearwater.

·       When migrating they can travel up to 840 kilometres per day

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