Spring has arrived in Canberra! Gardens and parks are bursting with the glorious colours of wattle, blossom and bulbs – and are every so often topped with a rainbow halo after much-needed seasonal showers.
The large eucalypt outside my living room is also full of colour and frenzied activity while it plays host to families of parrots and cockatoos (parrots with crests) for the breeding season.
A pair of galahs (rose-breasted cockatoos) has moved in and is busily preparing their nest in a tree hollow. In the fork of another branch, a pair of large sulphur-crested cockatoos is doing the same.
I can’t be sure but they are probably the same pairs as last year as these birds are relatively long-lived, partner for life, and tend to return to the same tree year after year.
Some eastern rosellas (a brightly-coloured parrot with a long tail and distinctive pattern of little black scales on their back) seem keen to move in to a smaller hollow quite close to the cockatoos, while a pair of crimson rosellas has been checking out the view from a newish penthouse hollow a few metres higher up.
Daily nesting habits & home renovations
All of these birds will spend the next few weeks cleaning out their nests and lining the base with wood chips and shavings from the hollow. Cockatoos also bring in eucalyptus leaves and twigs that provide a soft bed for the eggs and fledglings, help to regulate the temperature and humidity, and act as a natural pest repellent.
Galahs are particularly house proud and may chew away bark and rotted wood surrounding the nest entrance and rub it with their beaks, macerated gum leaves and feather dust until it becomes very smooth and slippery – possibly to prevent snakes and lizards from reaching the inside of the nest hole. (No wonder they return each year after going to all of this effort!)
Each day our feathered neighbours wake up with the sun and have a morning stretch before heading off to feast on seeds, nuts, berries, flowers, fruit, roots, green shoots and leaf buds (and sometimes protein-rich insects and their larvae depending on the species and time of year).
During this period, breeding pairs will often meet up with the rest of the flock to forage as there is safety in numbers.
Once full, the birds may spend some time sharpening their beaks by stripping the leaves and bark from trees or find a shady spot to preen and enjoy a siesta in the middle of the day while they digest their food.
After another feed in the afternoon and a drink (and occasional bath) at a birdbath or other watering hole, the breeding pairs return to their respective nests to continue their renovations (while the non-breeders roost in trees and patches of woodland close to their foraging sites).
Parenting – Nature or nurture?
Cockatoo pairs will generally take turns incubating the eggs, rearing their young and finding food, while females rosellas incubate their eggs alone but share caring for the young with their mate.
The nesting period varies by species size, seasonal and environmental factors and clutch size but is usually around 20-30 days.
The nurturing period also varies by species.
Some cockatoos remain in the nest for many months and may travel with their parents for long periods after leaving the nest.
Whereas, baby galahs are packed off to creche as soon as they can fly (at approximately 7 weeks of age) while the parents continue to look after any unfledged nestlings.
(A galah creche is a protected group of trees that forms a communal nursery where many juvenile galahs of the same age congregate and are fed by their parents or by galahs without mates and older adolescent birds.)
When all of the galah offspring can fly, the family is reunited and moves nearer to food supplies that has trees for resting. After two to three weeks the juveniles begin to accompany their parents to the ground where the parents continue to feed them – but within a month they are generally eating food on their own.
The parent galahs often become quite hostile when their young are fully weaned (generally within two months of leaving the nest) and are likely to disassociate from their offspring.
The juvenile galahs then join a flock of other juvenile birds until they reach breeding maturity and have developed the skills to be parents themselves (in about 4 years). The galahs will then pair off and seek out suitable nesting sites of their own.
Turf battles & other risks
It is fairly common to have a number of different species occupy a tree as each pair selects a hollow limb or hole that suits their particular requirements. However, there is sometimes inter- and intra- species competition for the limited number of hollows and ongoing conflict within the ‘neighbourhood’.
Territorial bird calls, chatter and posturing are regularly used by our resident birds to stake their claims and warn others to keep their distance. The birds are also willing to actively defend their homes and chase away challengers with bill jabbing, foot biting or full on attacks using their powerful bills and claws, if required.
If any birds are gazumped* and end up homeless, then they will need to find another hollow and start the process all over again.
In some areas, displaced birds have been known to delay breeding and wait for the interlopers to complete their breeding cycle and leave. However, conditions may not be as conducive to nesting later in the year.
Alternatively, some species (such as galahs) may nest in rock crevices, cliff tunnels or vertical concrete pipes when suitable tree hollows are in short supply.
A far more serious risk is posed by predators that try to steal the eggs and nestlings.
Until a few years ago a ruthless (feral) Common Myna kept the whole arboreal neighbourhood on its toes (as they have a reputation for throwing eggs and fledglings out of nests), but now the risk is more likely to come from a large currawong that shares the garden and the occasional possum, kookaburra, hawk or eagle.
Crimson rosellas should also look out for other crimson rosellas as females have been reported as destroying eggs in other nests during the breeding season.
Other hazards, such as cats, power lines, foxes, rats and cars, may also endanger the lives of young inexperienced birds.
All of the birds in ‘Eucalypt Hollow’ have chosen homes on the south side of the tree to shade them from the sun’s radiant heat (that will be much stronger by the time the nestlings hatch). Other key variables include height above the ground, an entrance that approximates body size, and suitable cavity dimensions (depending on the species and clutch size).
These birds appear to have an innate housing rating system that ensures that they also consider solar orientation (aspect), thermal insulation, natural disaster resilience (such as flood and fire risk), protection from predators and other unwelcome visitors, likelihood of rainwater intrusion, and proximity to reliable sources of food and water. (Marble benchtops and European kitchen appliances seem to be low priorities.)
Tree hollows don’t just grow on trees
Around 15% of Australian land birds rely on tree hollows – mostly for seasonal nesting but some for year round roosting (e.g. some owls). They compete for these spaces with other native mammals, reptiles and amphibians that use hollows, as well as introduced species such as common mynas and starlings and sparrows.
All of these vertebrate fauna are secondary hole-nesters as none of them can initiate the excavation of large cavities. However, some species, such as parrots and cockatoos, are able to excavate both decayed and sound wood to enlarge and refine cavities.
Tree hollows suitable for vertebrate fauna are typically found in mature and old-growth trees as they often take 80-250 years to form.
Cavities result from the interaction of fungi, termites, borers and other decay-causing organisms that excavate the more vulnerable heartwood of species such as eucalypts and are exposed when injury, wind, water stress or other factors cause large branches to break off.
The sapwood grows callus tissue around the injury but in large diameter branches it cannot cover the whole lesion and simply grows around the hollow rim.
(Some hollows occur in the main stem but most occur in branches. Fire scars can form hollows but generally at the base of trees and are of less value to birds. Cracks in branches or the main stem may also occur and provide homes for smaller species but this is more common in dead trees.)
Tree hollows have a tendency to change over time as branches break, callus tissue narrows entrances, and internal decay of the heartwood continues. Such changes may render a hollow unsuitable for one species but suitable for another. And as some hollows are lost, others are formed.
Although some of Australia’s parrots and cockatoos are threatened and have already become extremely rare or extinct, others have adapted well to life in the urban environment and have even taken advantage of grain crops, fruit trees and other introduced food sources (to the point of being considered pests by some farmers).
But all of these birds are very dependent on the ongoing availability of suitable nest cavities.
Unfortunately, the distribution and abundance of hollow-bearing trees has reduced and been fragmented by extensive clearing of native vegetation over the past two centuries for agriculture (especially in more fertile areas), timber production, firewood collection, dieback in isolated and stressed communities, urban expansion and road upgrades.
What can we do to help?
It is critical that we retain and protect both live and dead trees with hollows and ensure that we have new generations of trees coming on that will provide important habitat for birds, insects and other native species in the future.
We also need to make our gardens and parks more bird-friendly – especially near remnant bushland.
This includes the provision of mature trees that are suitable for both open and cavity nesters, as well as for day or night roosting, and a thick understorey of ferns, tall grasses, and prickly shrubs that offers shelter for smaller birds.
It also means minimising harmful chemical use, managing pest species, and ensuring reliable access to clean water and a variety of foods throughout the year.
Nest boxes and other artificial cavities can play a role but they are no substitute for the real thing. They may also be taken over by common mynas and starlings and feral bees.
Similarly, bird feeders may be useful in supplementing, but not replacing, native food species such as acacia, banksia, eucalypts, grevillea, hakea, leptospermum, melaleuca, correa, hardenbergia, xanthorrhoea, casuarina and native grasses – as long as the birds do not come to rely upon them and predators do not decide that they offer easy targets at meal time.
[Ed 17/7/14: Make sure you check out my short film on this topic ‘A Tale of Two Hollows’ – here.]
* Gazump is a real estate term used when a seller accepts a higher bid after they have already accepted an offer from a different buyer.
Sources: (Accessed 5-7 Sept 2014)
- ‘Tree hollows and wildlife conservation in Australia‘ by Philip Gibbons & David Lindenmayer, CSIRO Publishing (Australia 2002)
- ‘Loss of Hollow-bearing Trees – key threatening process determination‘ at the NSW Government Department of Environment & Heritage website.
- The Australian Galah website (Introduction & Description of Galah pages)
- Wikipedia (Cockatoo & Crimson rosella pages)
- Birdlife Australia education sheets (‘Tree hollows‘ & ‘Attracting birds to your garden‘)
- 80s home – Len Williamson (R.I.P.)
- All other photos – Pip Marks