Clouds of corellas are filling the skies across parts of Victoria, South Australia, and southern New South Wales, creating a cacophony of sound and more than a few headaches for farmers and councils.
They damage sporting grounds, strip seals and fittings from buildings, dig up crops, and generally make a nuisance of themselves.
Councils have tried various programs to deal with the problem, ranging from replanting ovals with different grasses to firing off "sound cannons".
Some individuals have sometimes applied more lethal control measures.
But nature writer and ABC wildlife expert Tanya Loos says the birds cannot be blamed for their prevalence.
"As a wildlife person, I don't like using the word 'plague' for really any animal because it's often implying immediately it is the fault of the animal," Ms Loos said.
"'Plague' implies something really bad.
"I would ask 'are they over-abundant?' Which is a more scientific question, [and that] really depends on your acceptance and tolerance of these animals.
"Nobody would say magpies are in plague proportions, but they don't make the same noise.
"We've created the perfect conditions for [corellas and white cockatoos] in terms of providing so much food for them in our lawns and our farms, and they have just responded in kind."
The Arthur Rylah Institute for Environmental Research estimates there are 7.7 million sulphur-crested cockatoos, 5.2 million long-billed corellas, and 2.9 million little corellas in Victoria alone.
The populations of all three species have been stable or increasing over the past 10 years, although there has been a slight recent decline in sulphur-crested cockatoos.
According to the Victorian Department of Energy, Environment and Climate Action (DEECA) all three species are protected under the Wildlife Act 1975, which means it is illegal to "wilfully disturb or destroy" them without approval.
However, the department will grant permission for councils and organisations to deal with problematic flocks using non-lethal control options before being able to apply for approval to use lethal control.
"Cockatoo or corella damage can be a significant problem for many Victorians, with impacts experienced across many different farming ventures, community assets and private residences," a DEECA spokesperson explained.
"DEECA is working with the community to manage the impacts of the birds by providing advice about how to reduce cockatoo damage."
Little corellas and long-billed corellas dominate agricultural areas of Australia, but have also established populations along the coast and in metropolitan centres including Melbourne and Sydney.
Wildlife photographer Peter Rowland, co-author of Australian Birdwatching Megaspots, said corellas moved into built-up areas during droughts and never left.
"I can remember back in the early 1980s you would occasionally see the odd pair. Now you see good numbers," Mr Rowland said.
"Their habitat has increased. Little corellas have always been very common, forming huge flocks like galahs, but you now see long-billed corellas in huge numbers along the Murray River and towards the coast.
"As areas producing grain have expanded, so has their range.
"They have spread into areas which have been dominated by sulphur-crested cockatoos but they aren't necessarily competing."
Ms Loos described seeing at least one council corella management plan that catalogued an array of complaints including chewing holes in sails, damaging cricket pitches and grounds, and defoliating trees.
"They are super intelligent and they have to exercise their beaks because their beaks grow continuously, so they chew on a lot of things," Ms Loos said.
However, the news is not all bad.
Ms Loos said corellas and cockies can do good when allowed to settle away from those areas that cause the most disturbance.
Research from the University of Melbourne which measured the rate at which sulphur-crested cockatoos remove onion weed from turf found each cockatoo can remove 200 plants each hour.
"To put that in perspective, 50 cockatoos — and you could extrapolate that to corellas — can remove 20,000 onion weed bulbs, which is an amazing free weeding service," Ms Loos said.
Spotters guide
Three corellas and sulphur-crested cockatoos often share the same habitat and are all medium-to-large white birds.
Identification is not difficult if you are close enough or have binoculars.
Little corella (Cacatua sanguinea)
Also called short-billed corella or little cockatoo, they are up to 39 centimetres long with a wingspan of up to 90cm. They have grey skin around the eyes and a tall crest, compared with the long-billed corella. There is a dash of pink above the upper beak which is only slightly longer than the lower beak, and yellow feathers under the wing. Little corellas occur throughout Australia with three subspecies.
Long-billed corella (Cacatua tenuirostris)
Body length can be up to 42cm with a wingspan of up to 93cm. Light blue to grey skin around the eyes, with pink markings on the forehead, face, and neck. The top part of the beak is significantly longer than the bottom part and ends in a sharp point, and its white crest is small compared with the little and western corellas. The underside is mostly pale compared with some yellow on the little corella. Naturally, occurs in south-eastern Australia, and introduced into Western Australia.
Western corella (Cacatua pastinator)
Occurs only in south-western Western Australia. A separate species to the long-billed corella but closely related enough to interbreed with it. It has a slightly shorter top bill than the long-billed corella and is more heavily built — 45cm in length with a wingspan of up to 95cm.
Sulphur-crested cockatoo (Cacatua galerita)
Largest of the Australian white cockatoos with a length of up to 50cm and a wingspan of 100cm or more. Its prominent yellow crest distinguishes the sulphur-crested cockatoo from the corellas, but it also has yellow colouring beneath its wing and tail. Occurs in northern and eastern Australia.