From the field: Why are Glossy Black Cockatoos So Rare?


In 1996 four people-Tamra Chapman, Gabriel Crowley, Stephen Garnett and Lynn Pedler worked on the birds, trying to find out what is controlling cockatoo numbers. Here are some ideas:

Not Enough Food

Though there seems to be a lot of Sheoaks (trees) on Kangaroo Island, the cockatoos are picky about which trees they will feed in and even which cones they will eat from a tree. When the research is complete, it should be possible to estimate how much food each cockatoo needs per year and the number of cockatoos that can currently be supported by the habitat available.

There is no doubt there are fewer Sheoaks on Kangaroo Island today than there were 200 years ago. Some have been cleared, others burnt. But there has also been much regeneration where fires and sheep have been excluded. From aerial photos, we can see some good stands now existing where only scattered trees stood in 1945. The former limits of Sheoak distribution can also he determined from the extent of suitable soils and other landscape features.

Food Not Good Enough Tamra Chapman has found that while the cockatoos have plenty of


time for courtship and midday siestas in the summer, they spend virtually all day feeding once they have young in the nest. At times, the quality of the food may be important. As with any food crop, the key to quality is often in the soil, so we are currently looking for links between soil fertility and seed quality. We are particularly interested in the nitrogen-fixing nodules on the roots of Sheoaks, made by the fungus Frankia, and how they are affected by levels of phosphorous or salinity. Analysis are being done by Yanco Agricultural Institute.

Too Few Hollows

The cockatoos need big hollows for nesting and there has long been a fear that many pairs would like to breed but cannot find a suitable vacant hollow or a site that is not already occupied. Hollows may be occupied by feral bees or be in use by one of the other cockatoos. Even those that do find hollows may be having to settle for hollows that point straight into the south-westerly weather or are so shallow that predatory birds can reach in and steal the egg. To alleviate a possible shortage of high quality vacant hollows, over 60 nest boxes have been erected. Two nest boxes are in use in one area, but elsewhere natural hollows may be more abundant than previously thought. However, it may simply be that the birds have not found them, particularly in areas like the Dudley Peninsula where there is no recorded tradition of nesting.

Too Many Possums

The whole world now seems to know about the surfeit of koalas on


the island but, as anyone living here knows, Brush-tailed Possums are far more common. Neither species has a significant predator on the island. Possums, however, are a significant predator themselves and include in their diet the young and eggs of Glossy Black Cockatoos. If possum numbers are higher than in the past, then the cockatoo nest failure rate may have increased. Glossy Cockatoos may be more vulnerable than other hollow users because they nest in winter. It has been suggested that, when the weather turns cold, possums may move from summer shelters beneath the yaccas to warm dry hollows in the trees. This year some nests, and all nest boxes, have been protected against possums by collaring trees with tin and by judicious pruning where nest trees touch other trees.

Other Possible Reasons

Cats, people, inbreeding and disease have all been suggested as possible threats to the cockatoo. Cats are blamed for the decline of most rare birds but seem to prefer rare mammals to birds. Nevertheless, they do take a substantial number of the larger Redtailed Black Cockatoos from their low nest hollows in Western Australia and there are certainly plenty of cats on Kangaroo Island. While it would be a clever cat that could reach a Glossy Cockatoo's nest, we are hoping the tin collars will also ensure against cat predation. Egg collectors and people trapping birds for aviaries could have a serious impact. However, the results of genetic research that are to be part of this project, would mean it will take only one feather or a scraping from inside an egg to tell whether it came from Kangaroo Island. As there are presently no Kangaroo Island birds in captivity, birds with Kangaroo Island genes must have been taken illegally.

With fewer than 180 individuals in the population, close relatives are likely to breed with each other, possibly causing problems with infertility, If inbreeding is a major problem then the focus of the program will have to shift to captive breeding.

Finally, while always mindful of the possibilities of disease, we have no evidence to suggest it is a problem.