Cockatoos and rainbow lorikeets battle for nest space as the best old trees disappear


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Gregory Moore, The University of MelbourneThe housing market in most parts of Australia is notoriously competitive. You might be surprised to learn we humans are not the only ones facing such difficulties.

With spring rapidly approaching, and perhaps a little earlier due to climate change, many birds are currently on the hunt for the best nesting sites.

This can be hard enough for birds that construct nests from leaves and twigs in the canopies of shrubs and trees, but imagine how hard it must be for species that nest in tree hollows.

They are looking for hollows of just the right size, in just the right place. Competition for these prime locations is cut-throat.

Sulphur-crested cockatoos battling for spots

Sulphur-crested cockatoos, Cacatua galerita, are relatively large birds, so naturally the hollows they nest in need to be quite large.

Unfortunately, large hollows are only found in old trees.

It can take 150 years or more before the hollows in the eucalypts that many native parrot species nest in are large enough to accommodate nesting sulphur-crested cockatoos. Such old trees are becoming rarer as old trees on farms die and old trees in cities are cleared for urban growth.

In late winter, early spring you quite often find sulphur crested-cockatoos squabbling among themselves over hollows in trees.

A cockatoo sits in a hollow.
It can take 150 years or more before the hollows in the eucalypts that many native parrot species nest in are large enough to accommodate nesting sulphur-crested cockatoos.
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These squabbles can be very loud and raucous. They can last from a few minutes to over an hour, if the site is good one. Once a pair of birds takes possession and begins nesting, they defend their spot and things tend to quieten down.

The stakes are high, because sulphur-crested cockatoos cannot breed if they don’t have a nesting hollow.




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Enter the rainbow lorikeets

In parts of southeastern Australia, rainbow lorikeets, Trichoglossus moluccanus (and/or Trichoglossus haematodus), have expanded their range over the past couple of decades. It is not uncommon to see sulphur-crested cockatoos in dispute with them over a hollow.

The din can be deafening and if you watch you will see both comedy and drama unfold. The sulphur-crested cockatoos usually win and drive the lorikeets away, but all is not lost for the lorikeets.

Sometimes the hollows prove unsuitable — usually if they are too small for the cockatoos — and a few days later the lorikeets have taken up residence. Larger hollows are rarer and so more highly prized.

A rainbow lorikeet shelters in the hollow of a tree.
It is not uncommon to see sulphur-crested cockatoos in dispute with rainbow lorikeets over a hollow.
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How hollows form

Many hollows begin at the stubs of branches that have been shed either as part of the tree’s growth cycle or after storm damage. The wood at the centre of the branch often lacks protective defences and so begins to decay while the healthy tree continues to grow over and around the hollow.

Other hollows develop after damage to the trunk or on a large branch, following lightning damage or insect attack. Parrots will often peck at the hollow to expand it or stop it growing over completely. Just a bit of regular home maintenance.

Sulphur-crested cockatoos can often be seen pecking at the top of large branches on old trees, where the branch meets the trunk. They can do considerable damage. When this area begins to decay, it can provide an ideal hollow for future nesting.

Sadly, for the cockatoo, it may take another century or so and the tree might shed the limb in the interim. Cockatoos apparently play a long game and take a very long term perspective on future nesting sites.

A cockatoo sits in a hollow.
Every effort must be made to ensure old, hollow-forming trees are preserved.
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Which trees are best for hollows?

In watching the local battles for parrot nesting sites, some tree species are the scenes of many a conflict.

Sugar gums, Eucalyptus cladocalyx, were widely planted as wind breaks in southern Australia and they were often lopped to encourage a bushier habit that provided greater shade.

Poor pruning often leads to hollows and cavities, which are now proving ideal for nesting — but it also resulted in poor tree structure. Sugar gums are being removed and nesting sites lost in many country towns and peri-urban areas (usually the areas around the edges of suburbs with some remaining natural vegetation, or the areas around waterways).

A rainbow lorikeet hides in a hollow.
Many species need hollows for nests.
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Old river red gums, (Eucalyptus camaldulensis) growing along our creeks and rivers are also great nesting sites. They are so big they provide ideal sites for even the largest of birds.

These, too, are ageing and in many places are declining as riverine ecosystems suffer in general. Even the old elms, Ulmus, and London plane trees, Platanus x acerifolia — which were once lopped back to major branch stubs each year, leading hollows to develop — are disappearing as they age and old blocks are cleared for townhouses.




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Protecting tree hollows

Cavities in trees are not that common. Large cavities are especially valuable assets. They are essential to maintaining biodiversity because it is not just birds, but mammals, reptiles, insects and arachnids that rely on them for nesting and refuge.

If you have a tree with a hollow, look after it. And while some trees with hollows might be hazardous, most are not. Every effort must be made to ensure old, hollow-forming trees are preserved. Just as importantly, we must allow hollow-forming trees to persist for long enough to from hollows.

We consider our homes to be our castles. Other species value their homes just as highly, so let’s make sure there are plenty of tree hollows in future.The Conversation

Gregory Moore, Doctor of Botany, The University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Don’t disturb the cockatoos on your lawn, they’re probably doing all your weeding for free



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Gregory Moore, The University of Melbourne

Australians have a love-hate relationship with sulphur-crested cockatoos, Cacatua galerita. For some, the noisy parrots are pests that destroy crops or the garden, damage homes and pull up turf at sports ovals.

For others, they’re a bunch of larrikins who love to play and are quintessentially Australian.

Along with other scientists, I had a unique opportunity during the COVID-19 lockdowns to study things that had intrigued me closer to home, perhaps for years. While isolating in the suburbs of Melbourne, I wanted to find out why cockatoos return to the same places, and what they’re after.

The answer? Onion grass, reams of it.

Onion grass is a significant weed, and I estimated in a recent paper that one bird gorges on about 200 plants per hour. A flock of about 50 birds can consume 20,000 plants in a couple of hours.

This significantly reduces the weed level and may make expensive herbicide use unnecessary. So if you have a large amount of onion grass on your property and are regularly visited by sulphur-crested cockatoos, it would be wise to let them do their weeding first.

When play verges on vandalism

Most of us see cockies whether we live in rural communities or major cities, but how much do you really know about them?

Two sulphur-crested cockatoos sitting on a branch
Sulphur-crested cockatoos nest in old hollow trees.
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In late winter and early spring in many parts of Australia, flocks of sulphur–crested cockatoos can be seen grazing on the ground. They’re usually found close to water, nesting in woodlands with old hollow trees, such as river red gums, Eucalyptus camaldulensis.

Where these forests and trees are being cleared, the number of cockies falls. But they are resilient and adaptable birds, and have spread their range to cities and the urban fringe, where numbers are increasing.




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The birds are known to play with fruits, hang upside down on branches or perform flying cartwheels by holding a small branch or powerline with their feet, flapping their wings as they do loop after loop.

Sometimes their play verges on vandalism as they follow tree planters, deftly pulling up just-planted trees and laying them neatly beside the hole.

While cockatoos feed on the fruits and seed of native species, they’ve adapted very quickly to the introduction of exotic species, such as onion grass from South Africa, which is plentiful and easy to harvest.

I observed flocks ranging from nine to 63 cockatoos at seven sites along the Maribyrnong River in Keilor last July and August. Onion grass was the only item on their menu.

A pest for humans, a feast for birds

Onion grass (Romulea rosea) is small and usually inconspicuous with grass-like leaves. It’s typically only noticed when it flowers in spring, producing pretty, pink and yellow-throated flowers.

Conspicuous onion grass with a small purple flower
Onion grass comes from South Africa, and is a big problem for native grasslands.
Harry Rose/Wikimedia, CC BY

Onion grass can be a serious weed that’s very difficult to control. It’s not only a problem for agricultural land, but also for recreational turf and native grasslands.

In some areas, there are nearly 5,000 onion grass plants per square metre. This is a massive number requiring costly control measures, such as spraying or scraping away the upper layer of top soil.




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The river red gum is an icon of the driest continent


Onion grass gets its name from its onion-like leaves. At the base is a small bulb, which works as a modified underground stem called a “corm”. The corm is what cockatoos will travel many kilometres for, to dig up and return to for days on end.

A brown bulb with small roots coming out
When cockatoos eat onion grass corm, it prevents the weed from regenerating.
Harry Rose/Wikimedia, CC BY

Their super weeding effort

Like other native parrots, sulphur-crested cockatoos are famously left-footed. So it was interesting to observe them primarily use their powerful beaks to pull onion grass plants from the ground and dig up corms, using their left foot only occasionally to manipulate the plant.




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The cockatoos fed for between 30 minutes and two and a half hours. At each feed, one or two sentry (or sentinel) birds, depending on the flock size, would keep watch and give raucous warning should danger threaten.

The cockies could remove a plant and corm from the ground in as little as six seconds, but sometimes it could take up to 30 seconds. They then removed and consumed a corm every 14 seconds on average in wet soil and every 18 seconds from harder, dry soil.

Eight cockatoos on grass, with autumn leaves
When flocks feed, one or two sentinel birds keep watch for danger.
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This means a flock of 63 birds could remove more than 35,400 onion grass plants in a feeding session lasting two and half hours. This is a super weeding effort by any standard!

Future partnerships

My further investigation revealed most of the corms were within 20 millimetres of the soil surface, so the holes left in the soil by the birds extracting the onion grass were shallow and quite small. This shouldn’t give seeds from onion grass any great advantage.

And they’re very efficient: the birds eat over 87% of the corms they lift, which then won’t get a chance to generate in future years. So, if we’re going to try to eradicate onion grass, it may be better to let the cockies do their work first before we humans take a turn.

We have a lot to learn about how our native species interact with introduced weeds, and more research might reveal some very useful future partnerships. They might be birdbrains, but sulphur-crested cockatoos really know their onions when it comes to, well, onion grass.




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The Conversation


Gregory Moore, Doctor of Botany, The University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Australia: NSW – Sydney Residents Track Cockatoos with iPhone App


The link below is to an article reporting on how residents of Sydney are tracking Sulphur-Crested Cockatoos with an iPhone app.

For more visit:
http://www.australiangeographic.com.au/journal/public-help-track-cockatoos-with-iphones.htm