Cockatoos Are the Prettiest Little Weed-Eaters You’ll Ever See

The only thing that people must do is stop shooting or poisoning these parrots and allow them to conduct their superior weed-removal practices in peace

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NOTE: This piece was a Forbes Editors’ Pick.

A flock of sulfur-crested cockatoos (Cacatua galerita) on a lawn in Sydney, Australia. They are digging up and eating the corms of onion grass (Romulea rosea), an invasive species from South Africa. (Credit: Howard Bales / CC BY 2.0)

If you live in Australia, you probably see lots of white cockatoos — either a few individuals or even large flocks of sulphur-crested cockatoos and several species of corellas in particular — busily picking through grassy lawns, golf course greens, playing fields and open grasslands. What are they doing?

This is exactly what horticulturist Gregory Moore, a senior research associate who studies Plant Science and Arboriculture atthe University of Melbourne, wanted to know.

“While isolating in the suburbs of Melbourne, I wanted to find out why cockatoos return to the same places, and what they’re after,” Dr Moore said in email. With little else to occupy his mind and his time due to pandemic lockdowns, Dr Moore began carefully following the flocks of sulfur-crested cockatoos feeding at seven sites near Brimbank Park along the Maribyrnong River in July and August of 2020.

He observed flocks that ranged in size from nine to 63 cockatoos, and found that each feeding bout lasted from 30 minutes to two and a half hours. This suggested that whatever the cockatoos were eating was extraordinarily plentiful. Were the cockatoos eating just one thing, or a bunch of different things?

“Onion grass was the only item on their menu,” Dr Moore explained in email. “They are full of carbohydrate and the birds get to them early before the reserves in the corm are depleted by extra leaf and flower production.”

F I G U R E 1 : Onion grass (Romulea rosea) blooms are quite attractive. It is endemic to the western Cape Province in South Africa, but it has become naturalised in Europe, Australia, New Zealand, and California in the United States. It is considered to be an invasive “weed” in much of Australia. (Credit: Andrew Massyn / CC BY-SA 3.0)

Onion grass, Romulea rosea, is a small plant with long thin hollow leaves that is inconspicuous, blending easily into grassy lawns or pasture — until late winter and early spring, when it boldly announces its presence with attractive pink-and-yellow flowers (Figure 1). This delightful little flower is probably the reason that humans introduced onion grass everywhere.

Onion grass originated in South Africa but now is naturalised in many parts of the world, including much of Australia, where it is considered to be a serious invasive weed. For example, one study reported nearly 5,000 onion grass plants per square meter in some areas of Australia ( ref). This massive invasion into a prized emerald-green lawn tends to inspire expensive and environmentally-damaging control measures, such as spraying powerful poisons or even scraping away the upper layer of soil.

Like its cousins, garlic, onions, and chives, which it closely resembles, onion grass is an edible perennial that grows from an underground corm (Figure 2), which is a round subterranean portion of its stem that swells with stored carbohydrates. In addition to its corms, onion grass also produces prodigious amounts of seed, which greatly enhances its invasiveness.

F I G U R E 2 : Intact onion grass (Romulea rosea) corms extracted from the soil but not eaten by sulfur-crested cockatoo flocks. (doi:10.1111/emr.12449)

In his recently published study on the onion grass dining habits of sulphur-crested cockatoos, Cacatua galerita, Dr Moore reports that each cockatoo extracts and consumes about 200 onion grass corms per hour, and a flock of 50 or so birds can devour around 20,000 corms in just a couple of hours ( ref).

Feeding cockatoos were remarkably fast, according to Dr Moore: they could remove an onion grass corm from the soil in as little as six seconds, although sometimes this process might take up to 30 seconds.

“The speed at which corms are harvested is amazing, as is the manipulation of the plant by the tongue of the birds,” Dr Moore marvelled in email. “What they can do with their beaks and tongues is spectacular.”

Moisture affected the speed at which a cockatoo could pluck an onion grass plant from the soil: in wet soil, they could remove and consume one corm every 14 seconds on average, whereas it took them 18 seconds, on average, to remove onion grass plants from harder, dry soil. Hungry cockatoos are quite efficient; consuming more than 87% of the corms that they removed from the soil. Further, cockatoos are very sociable and are creatures of habit: flocks are known to travel many kilometers to eat onion grass corms from a particular location and are known to return for days on end.

As for their part, onion grass corms are found near the surface of the soil so their removal causes little damage: Dr Moore measured the depth of 140 corms and found that most of them were within 20 mm (4/5 of an inch) of the soil surface, and only 4 of those 140 plants had corms that were deeper than 30 mm (1 inch). Thus, the holes created by corm-feeding cockatoos were small and shallow.

Sulphur-crested cockatoos (Cacatua galerita) and long-billed, or slender-billed, corellas (Cacatua tenuirostris) feeding on onion grass corms in a grassy park in Australia. (Credit: Gregory Moore.)

This study was particularly interesting to me because it documents that a native species can become an important and devoted consumer of an invasive weed that originated on the opposite side of the planet. This is an unusual situation. As humans rapidly destroy Australia’s natural habitat and convert it into a few grassy parks randomly distributed within urban concrete-and-glass canyons, the cockatoos adapted to this loss by taking up residence in suburban and urban areas. These clever and inquisitive parrots quickly discovered that an invasive weed, which was growing in tremendous abundance in suburban lawns and parks, was a valuable source of carbohydrates that could replace the loss of their natural food sources.

After people began destroying vast tracts of native woodland for human habitation, sulpher-crested cockatoos (Cacatua galerita) have taken up residence in Australia’s cities, such as Sydney. (Credit: Chris Madeley / CC BY-NC 2.0)

At this point, it shouldn’t take a rocket scientist to realize that cockatoos are a significant factor in reducing the presence of invasive onion grass weeds in grassy areas. Further, they are efficient, persistent, environmentally-friendly and, perhaps best of all, they provide their services free of charge.

Yet, despite the fact that cockatoos are native species and have every right to live in Australia, many Australians don’t like them because they can be loud and destructive. For example, cockatoos particularly enjoy digging small shallow holes into moist, deeply-watered golf course greens, which enrages greenskeepers, who retaliate by shooting the helpful and hungry parrots. Of course, the presence of onion grass also outrages these same greenskeepers, who prefer to invest time and money into applying toxic poisons to their greens rather than working alongside the wild cockatoos, who are their natural allies in weed-control.

“The birds can remove acres of weeds if the flocks are large enough and we then don’t have to spray and there are real environmental benefits,” Dr Moore said in email. “You also want to make sure that the birds are not consuming sprayed plants.”

“In this case I see that the cockies are our partners in better environmental management. We can learn from them as they are real experts in harvesting these corms.”


Gregory M. Moore (2020). Harvesting of Onion Grass, Romulea rosea L. by Sulphur‐crested Cockatoo, Cacatua galerita Latham, Ecological Management and Restoration, published online on 15 December 2020 ahead of print | doi:10.1111/emr.12449

Originally published at on 22 February 2021.

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𝐆𝐫𝐫𝐥𝐒𝐜𝐢𝐞𝐧𝐭𝐢𝐬𝐭, scientist & writer

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PhD evolutionary ecology/ornithology. Psittacophile. scicomm Forbes, previously Guardian. always Ravenclaw. discarded scientist & writer, now an angry house elf

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𝐆𝐫𝐫𝐥𝐒𝐜𝐢𝐞𝐧𝐭𝐢𝐬𝐭, scientist & writer

Written by

PhD evolutionary ecology/ornithology. Psittacophile. scicomm Forbes, previously Guardian. always Ravenclaw. discarded scientist & writer, now an angry house elf

The Startup

Get smarter at building your thing. Follow to join The Startup’s +8 million monthly readers & +789K followers.

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