There are a surprising number of Australian bird species that rely on hollows for nesting sites. Cockatoos only nest in tree-hollows, and most other species of parrots do too; so do most owls. In many regions there is a shortage of available hollows suitable for building nests in. In some areas these shortages are the result of clearing. Hollows only form in the trunks or branches of old trees. Introduced or feral species are at the heart of several different conservation issues. Some are active predators of native wildlife. Black Rats, which were accidentally introduced onto Lord Howe Island in 1918, for example, caused havoc by preying upon many species of birds which were previously common on the island, and directly caused the extinction of at least half a dozen species within two years. Foxes and cats are also species introduced to Australia which take a great toll on our native birds. Cats are widespread in all habitats throughout the country, and were even reputed to have reached Uluru (Ayers Rock) long before white men did. They kill untold numbers of birds every year, and have been recorded taking at least 212 species of birds, as well as many small mammals, reptiles and amphibians. Foxes, too, are voracious predators of native wildlife. They have even been recorded digging out the nesting burrows of birds such as Spotted Pardalotes and White-backed Swallows to eat their eggs or young.
Other invasive species compete directly with native birds, either for food or nesting sites. Large flocks of introduced European Goldfinches and European Greenfinches are often seen feeding on the seeds of low saltmarsh plants in south-eastern coastal mainland Australia, in competition with endangered Orange-bellied Parrots. Competition for nesting sites is one of the major problems posed by many introduced invasive species. In many areas of eastern Australia, introduced Common Starlings and Common Mynas have proliferated and expanded their range since they were introduced from Europe and Asia in the 19th century. They actively and aggressively compete with native species for nesting hollows, sometimes even building their nests on top of eggs or nestlings of other birds. Exotic species do not necessarily have to come from overseas. Laughing Kookaburras were introduced into Western Australia in 1896 and Tasmania in the early 20th century, and now successfully compete with native hollow-nesting species, as well as preying on native wildlife. Competitors for nesting hollows are not always birds, either. Feral honeybees are increasingly seen as serious competitors for spouts and other hollows suitable for breeding by parrots.
Other invasive species simply ruin the habitats used by other species. In the Northern Territory, introduced Water Buffalo caused tremendous damage to waterholes by trampling the surrounding vegetation and reducing the wetlands to muddy wallows, unsuitable for use by most other animals, especially waterbirds. Feral pigs may have the same effect in other parts of Australia.
The presence of weeds in the environment may pose various threats to Australia's birds. If left unchecked, some weeds may grow so prolifically that they choke habitats, making them uninhabitable for our birdlife. The invasive Water Hyacinth is an exotic floating waterweed from South America which chokes many wetlands in New South Wales and Queensland. Its shiny green leaves grow so densely on the water's surface that they prevent sunlight from reaching submerged aquatic plants, adversely affecting aquatic invertebrates and fish (important food items of many waterbirds) and the dense foliage and the tangle of submerged stems make it physically impossible for most waterbirds to inhabit any infested wetland.
African Boneseed, an introduced shrub with prolific yellow flowers, grows so densely that in some areas it dominates the understorey of woodlands, and forms a nearly impenetrable cover in previously open areas. This mass of vegetation is readily inhabited by some of the more common species which require shrub cover, such as Brown Thornbills, but some threatened woodland species, such as the Diamond Firetail, are directly affected by the loss of areas of open understorey. BirdLife Australia has been actively removing Boneseed from a heavily infested area of the You Yangs, south-west of Melbourne, since the early 1990s.
The saltmarsh of Lake Connewarre, a wetland in the lower reaches of the Barwon River in southern Victoria, was formerly a major wintering area of the Orange-bellied Parrot, particularly soon after the birds' arrival on the mainland after crossing Bass Strait. In recent years, however, the number of Parrots recorded at Lake Connewarre has declined dramatically, coinciding with the rapid spread of the native grass Australian Salt-grass, great hummocks of which have smothered the saltmarsh in which the Parrots forage. The swamping effect of invasive weeds is also felt on our offshore islands. Nesting burrows of the Flesh-footed Shearwater on Lord Howe Island are often blocked by excessive growth of Kikuyu grass, which reduces the number of burrows available to the birds.
Clearly, the removal of such invasive weeds should be an important part of the conservation of a wide range of birds. But the manner in which it is done is equally important. For example, in the Wheatbelt of Western Australia, attempts to eradicate the agricultural weed Double Gee or Spiny Emex by using herbicides has poisoned remnant roadside patches of native vegetation which support Carnaby's Black-Cockatoos.