The island rookeries of the Bass Strait are windswept, isolated places used intensively by seabirds seeking protected areas to raise their chicks. The islands, seabirds, sea and winds combine to form a special habitat where few plant species are able to survive, resulting in distinctive plant life including the shore loving island purple grass and the rare coast lily.
Every year during breeding season millions of seabirds come to live on the islands of the Bass Strait. They forage in the sea and return to burrows or ground nests where they raise their young. Over time, rich nutrient is transferred to the land in the form of bird poo, or guano, and other biologically derived material such as dead bird carcasses. More than thirty islands in Tayaritja/the Furneaux Group have large seabird rookeries.
Common rookery birds are short-tailed shearwater (yula, or muttonbirds), little penguin, white-faced storm-petrel, fairy prion, common diving-petrel, crested tern, black-faced cormorant (shags) and Pacific and silver gulls.
Every year short-tailed shearwaters, also known as yula, moonbird and muttonbird, migrate from the northern hemisphere to the Bass Strait to breed. Photograph by Ed Dunens. © Ed Dunens CC BY 2.0.
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A muttonbird in its underground burrow with egg. Courtesy of the Mary Gillham Archive Project. © Mary Gillham CC BY 2.0.
Crested terns on Tucks Reef, Vansittart Island Group, which lies in the eastern Bass Strait in Tayaritja/the Furneaux Island Group. From a 1950s slide collection of Dr Mary Gillham. Courtesy of the Mary Gillham Archive Project. © Mary Gillham CC BY 2.0.
Rookeries are on offshore islands for good reasons. First, seabirds need to be close to the sea where they forage and feed. Second, islands provide protection. Though rookeries can also be found on the coastal mainland, particularly on cliffs and more inaccessible areas, burrows and land nests are susceptible to predation by mammals, reptiles and hunting birds. Large muttonbird rookeries, for example, don’t tend to be found on mainland Lutruwita/Tasmania where the birds make easy prey for Tasmanian devils and quolls.
Sometimes, even on offshore islands, escape from predators is not always possible. Mt Chappell Island in Tayaritja/the Furneaux Group has a large muttonbird rookery. The island is also home to a subspecies of very large tiger snake which likes descending into burrows to take young chicks.
We don’t know how long it took seabirds to colonise the islands after the sea levels began to rise approximately 15,000 years ago, but the speed with which seabirds can spread from other areas and form colonies is probably very quick. Night Island, to the west of Preservation Island in Tayaritja/the Furneaux Group, did not have a rookery when the first Europeans arrived, because it was occupied by seals. In 1828 a seafarer called Campbell reported there was “…..nothing alive upon the island excepting seal which abound all over it.. There is no water to be procured, neither do any trees grow upon this rocky islet”. After the seals were hunted out, probably by the 1830s, birds and plants arrived. The island is now home to an extensive seabird rookery and twenty species of plants. Bright green mats of bower spinach (Tetragonia implexicoma) and silvery patches of seaberry saltbush (Rhagodia candolleana) cover the centre.
Only special kinds of plants can grow in rookeries. They need to tolerate the sandy or loamy soil which is easy to burrow into and is continually being dug over by the birds. And because of the guano, the soil is highly alkaline and high in nutrients such as nitrogen (N), phosphorus (P) and potassium (K). These soil nutrient conditions are unusual in Australia. Neighbouring islands without rookeries have very different soil conditions: they are acidic and notoriously low in nutrients. The rookery soils are so rich in phosphorus they are lethal to many Australian native plants that have adapted to low phosphorus soils.
Unlike many Australian native plants, introduced plants and vegetables grow very well in nutrient rich soils. Guano deposited in seabird rookeries can build up over thousands of years and has been mined and sold as plant fertiliser. In 1861 in the Bass Strait islands a surveyor was dispatched to report on land use, land allocation and the economic prospects for guano mining. He found that sometime prior to his survey, a businesswoman from Hobart Town removed 400 tons of guano from Cat Island.
Sealers and their Aboriginal families who settled some of the islands in Franklin Sound in the early nineteenth century found these rich rookery soils were also ideal places to plant their vegetable gardens, once the seabirds were removed.
Species diversity is not high in native plants typical of eastern Bass Strait seabird rookeries. Vegetation is usually low-lying and open. Salty sea winds shape island plants, preventing them from growing too large, and conditions are inhospitable for scrub and forest development. Rookeries under scrub are rare and more likely to be found on mainland coasts or large islands.
The most common rookery vegetation is dominated by tussock grasses, mainly coast tussock-grass (Poa poiformis). In between the tussocks there are herbs like coast groundsel (Senecio pinnatifolius), leek lily (Bulbine semibarbata), sea celery (Apium prostratum), clustered everlasting (Xerochrysum papillosum), leafy peppercress (Lepidium foliosum), and austral stork’s bill (Pelargonium australe) as well as exotic weeds such as black nightshade (Solanum nigrum), great brome (Bromus diandrus) and common sow-thistle (Sonchus oleraceus). Other rookeries are dominated by succulent plants like bower spinach (Tetragonia implexicoma) that forms mats over much of the co-occurring species. Bower spinach is often found with seaberry saltbush (Rhagodia candolleana). The third common type is rookery vegetation dominated by shrubs such as Australian hollyhock (Lavatera) or grey saltbush (Atriplex cinerea).
A distinctive plant of the rookeries is island purple grass (Poa poiformis var. ramifer). Island purple grass occurs around the coasts of southern Victoria and in Lutruwita/Tasmania. In Lutruwita/Tasmania, island purple grass has a particular affinity for shore and offshore islands. It is noted in a few seabird rookeries. It has distinctive purple nodes and occurs in rookeries of ground nesting seabirds such as shags.
Muttonbird burrows in a bright green patch of Tetragonia on Little Dog Island, which lies in the eastern Bass Strait in Tayaritja/the Furneaux Island Group. From a 1950s slide collection of Dr Mary Gillham. Courtesy of the Mary Gillham Archive Project. © Mary Gillham CC BY 2.0.
Silver gulls amidst granite boulders and coast tussock grass on Tucks Reef, Vansittart Island Group, part of Tayaritja/the Furneaux Island Group. From a 1950s slide collection of Dr Mary Gillham. Courtesy of the Mary Gillham Archive Project. © Mary Gillham CC BY 2.0.
The extremely rare coast lily is found only in the eastern Bass Strait and nearby coastal regions. This specimen was observed in Yiruk/Wamoon/Wilsons Promontory, on the Victorian side of the former land bridge area. Photograph by mlefoe. © mlefoe CC BY-NC 4.0.
Rookeries also boast at least one extremely rare plant. The coast lily (Bulbine crassa) grows almost exclusively on the islands of eastern Bass Strait. Coast lily is an attractive plant with bright yellow flowers and thick fleshy leaves. It thrives on very thin soils on areas frequented by seabirds. The plant is so rare that its seeds were sent by Tasmania to be part of the Millenium Seed Bank Project, a collection of billions of seeds from around the world, including rare and threatened species, stored in the United Kingdom for conservation purposes.
The offshore seabird islands of the Bass Strait are unique places. They represent important places for Palawa people, particularly the muttonbird rookeries which are special and culturally significant. Filled with distinctive vegetation and a haven for bird life, island rookeries provide one of the best examples of the links between sea and land. Here, birds, islands, people, plants and water interconnect in a relationship formed in the 15,000 or so years since the Bassian Plain began to be inundated by the sea.
Seaberry saltbush (Rhagodia candolleana) on Truwana/Cape Barren Island. Photograph by Brendan Kays. © Brendan Kays CC BY-NC-SA 4.0.
Rum Island, a rookery area for little penguins and muttonbirds, covered in grey saltbush (Atriplex cinerea). From a scientific slide collection of Stephen Harris. © Stephen Harris CC BY-NC-ND 4.0.
A dried specimen of island purple grass (Poa poiformis var. ramifer), also called dune grass, which was picked in Barwon Heads, Victoria. Photograph by the National Herbarium of Victoria. © National Herbarium of Victoria CC BY-NC-ND 4.0.
Badger Island is managed by the Palawa community. In the left background is Mt Chappell Island where seasonal harvest of muttonbirds has long been a culturally important activity. From a scientific slide collection of Stephen Harris. © Stephen Harris CC BY-NC-ND 4.0.
Crested terns on a reef off Little Dog Island. From a 1950s slide collection of Dr Mary Gillham. Courtesy of the Mary Gillham Archive Project. © Mary Gillham CC BY 2.0.
Xerochrysum papillosum (white) and Senecio Sp. (Yellow) occurring with Poa poiformis (grass) in the muttonbird rookery on Big Dog Island. From a scientific slide collection of Stephen Harris. © Stephen Harris CC BY-NC-ND 4.0.
Australian hollyhock (Malva preissiana) occasionally occurs in rookery vegetation. This specimen was photographed on Little Dog Island. From a scientific slide collection of Stephen Harris. © Stephen Harris CC BY-NC-ND 4.0.
Little Chalky Island, a seabird rookery area for muttonbirds, white-faced storm petrel and little penguins, densely vegetated by grey saltbush (Atriplex cinerea). From a scientific slide collection of Stephen Harris. © Stephen Harris CC BY-NC-ND 4.0.
Grey saltbush (Atriplex cinerea). From a scientific slide collection of Stephen Harris. © Stephen Harris CC BY-NC-ND 4.0.
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Released under a Creative Commons Attribution Non Commercial No Derivatives 4.0 International (CC BY-NC-ND 4.0) license.
This story is subject to disclaimers, copyright restrictions, and cultural clearances. Copyright © Stephen Harris, 2024.
Further reading
Atlas of Living Australia (2024) Bulbine crassa, accessed 14 February 2024.
Brothers N, Pemberton D, Pryor H and Halley V (2001) Tasmania’s Offshore Islands: Seabirds and Other Natural Features. Tasmanian Museum and Art Gallery. Tasmanian Printing Authority, Hobart.
Harris S, Buchanan A, and Connolly A (2001) One Hundred Islands: The Flora of the Outer Furneaux Islands. Department of Primary Industries, Water and Environment. Tasmanian Printing Authority, Hobart.
Dr Mary Gillham Archive Project (2016) collection of scientific images from the 1950s to the 1990s which includes slides of Bass Strait islands, flickr, accessed 15 March 2024.
