1983 | Scrubby Creek, Australia
Palm cockatoo (Probosciger aterrimus) stick drumming
Animals that use tools grab headlines, and give us plenty of fodder for catchy videos on Youtube. Birds are perennial favourites—think of wild New Caledonian crows making hooked tools to extract insect larvae from candlewood trees. But birds also routinely perform astonishing engineering feats that far surpass poking a stick into a hole: they build nests.
There’s a question of why tool-use and nest building are typically separated by people studying tool-using animals. It’s one that has occupied zoologist Mike Hansell for a long time, in his role as the Emeritus Professor of Animal Architecture at the University of Glasgow. Today, we examine one instance of bird tool-use that sits close to that line, while also giving us a look at the natural rhythms of bird life. Introducing the palm cockatoo.
Stripping and dancing
Just about every tale of animal technology should begin with: ‘Local people have known about it for a long time, but scientists only recently caught up’. The palm cockatoo is no exception. Indigenous communities in northeastern Australia and New Guinea are no stranger to this black bird, which is the world’s largest cockatoo.
It was only in the 1980s, though, that the first reports were published in the scientific literature of male cockatoos using sticks to drum out rhythmic patterns. These came from the aptly-named Graeme Wood, an entomologist and naturalist who spent 30 months living in the Iron Range area of Cape York Peninsula. This region is part of Kutini-Payamu National Park, home to Australia’s largest remaining area of lowland rainforest.
From his base at Scrubby Creek, Wood could observe several mating pairs of palm cockatoos as they fed, communicated and maintained their territories. These monogamous birds nest only in hollows formed when termites eat out the centre of mature trees, which allowed Wood to set up observation hides around these focal points on the landscape.
Palm cockatoos build their nests by gathering sticks from local trees, using their powerful beaks to slice and twist off the ends of growing branches. Wood noted that pieces up to 3cm thick and 20cm long were ‘cut by the birds rocking back and forth with their bills clamped around the branch, not unlike the action of a pipe-cutter’. Once carried to the nest, each piece is then further stripped into long fragments and dropped within the nest hollow. These nest-strips form a thick but permeable layer that likely helps insulate the nest, while allowing it to drain.
To a certain extent, stick drumming starts the same way. A male cockatoo—or only rarely a female—gathers a suitable stick by detaching it from a nearby branch, and removing any attached leaves. They then position themselves next to the nest opening. Holding the stick in one foot, they repeatedly strike it against the edge of the nest or the branch on which they’re standing, producing a loud banging sound that carries through the forest. The number of strikes varies from a handful up to around 100 in a row.
Have a look a this 3-minute video from Australia’s ABC TV, which shows the nest-building/twig-stripping activity, followed by drumming:
That video draws on more recent work from the Palm Cockatoo Project, led by Christine Zdenek. Zdenek and her academic supervisor Robert Heinsohn spent years in the rough terrain of Cape York trying to film and record palm cockatoo behaviour, including their elusive drumming. Over a six-year period they managed to record 131 drumming sequences, by 18 individual male cockatoos.
They found that most of the tool-use was likely a display by the males towards a nearby female. Wood reached the same conclusion, although he also saw the same activity immediately after a male had chased a rival cockatoo away from his territory. In the two encounters he saw, Wood reports that:
Both performances commenced upon the departure of the encroaching pair. The resident male immediately removed a section from a branch, flew to the top of a hollow limb and pounded it loudly, calling intermittently. This performance lasted many minutes, the female perching nearby in silence.
Unlike tool-use by wild crows or the woodpecker finches of the Galápagos Islands, the cockatoo drumming appears to be entirely social, not a tactic to get food. It is essentially a broadcast, amplifying an existing behaviour—foot tapping on a branch—into a signal that can be heard up to 100m away.
Advertising a quality beat
It’s possible that this cockatoo broadcasting system advertises not only the location of the drumming bird, but information about itself and the quality of its nest. The first of these was discovered when Zdenek and her colleagues examined the patterning of drumbeats by those 18 wild birds.
They found that these were no random sequences. Across the group, the cockatoos reliably drummed out beats at a regular interval, averaging around eight tenths of a second between strikes. However, individual birds had their own preferred tempo, indicating that this was driven by an internal clock that may act as a signal for a specific cockatoo. This same kind of internal tempo is what allows human drummers to keep a beat, and the research team concluded that:
Our study of tool-assisted drumming in palm cockatoos shows that they use abilities seen separately in other nonhuman species in a combination that has, to our knowledge, been recorded only in humans when performing percussive musical rhythms.
Other non-human animals tap or strike things round them during displays (for example, wild chimpanzees), but none make a tool just for that purpose, and few produce that same non-random, toe-tapping beat.
I should note that it’s not only drumsticks that the cockatoos use—they also use modified seedpods from the Grevillea glauca tree. This short video from National Geographic shows these two kinds of tool use in action (first stick, then seedpod, then stick again). You’ll hear that while the strikes are not always completely metronomic, they do have a certain repetitive quality:
And what about the idea that the broadcast not only draws attention to the bird, but also might signal the qualities of the nesting site?
A turn-of-the-mellennium study by another duo from the Australian National University examined trees in and around the Kutini-Payamu National Park. Specifically, they tracked palm cockatoo nesting trees and their survival through severe natural events like bushfires and cyclones. They found that two or three of the nesting sites were destroyed by these events each year, especially through fire. However, the trees that were used specifically to raise chicks during the study period were much more likely to remain standing.
The study’s authors—Stephen Murphy and Sarah Legge—gave this tentative reason for the difference:
Our results show that the inactive nest-trees were more likely to be destroyed than active nest-trees. This suggests that Palm Cockatoos breed in nest-trees that are more durable. Although speculative, it is possible that the Palm Cockatoo’s renowned drumming display…may provide acoustic clues about the density and durability of the nest-tree.
In other words, not only is the drumming peculiar to individuals, but the sound itself may carry resonant properties that help identify a solid, dependable tree. Certainly, this blurs the line that Mike Hansell tracks between nest construction and tool use even further. Could the drumming, at least in cases of mate attraction, actually just be a logical extension of nest building? Nest-strips and drumming tools are gathered in the same way, and palm cockatoo pairs return to the same nests year-on-year, such that any clue as to the future reliability of their nursery would have clear survival value.
Hansell and his colleague Graeme Ruxton believe that people trying to study tool use have been:
unable to come up with a biologically satisfying definition of a tool. We have argued that striving for such a definition might not be fruitful, because there is rarely any advantage to be gained from separating tool use from animal construction more generally.
Whether or not you see a use for splitting these behaviours, the palm cockatoo offers an example where both sides of the divide appear to be working together.
Cyclones are not purely destructive: they also help create more potential nesting sites, by splitting open hollow trees and giving the birds access. However, the entire process from initial tree decay to final hollow availability takes decades, and human disturbance to the landscape can easily break the long chain of events that lead to new nesting sites. For their determination, we should be extremely grateful to every scientist that ventures into remote forests and comes back with information that can help us avoid inadvertently dooming yet more species to a slow decline.
The palm cockatoo is currently considered vulnerable in Australia. There are only a few thousand in the wild across Cape York Peninsula, and their specific nest requirements and slow reproduction rate make them susceptible to sudden environmental changes. However, they are more fortunate than some birds in their natural affinity for PR: they’re the logo of the World Parrot Trust, and more than any other YouTube animal favourite, they really know how to drum up support.
Further viewing: For a more complete picture of the palm cockatoo in its local landscape, along with the threats it faces as a slow-breeding species, this 10-minute video from Christina Zdenek and the Palm Cockatoo Project offers an excellent introduction:
Sources: Wood, G. (1984) Tool use by the palm cockatoo Probosciger aterrimus during display. Corella 8: 94–95. || Wood, G. (1988) Further field observations of the palm cockatoo Probosciger aterrimus in the Cape York Peninsula, Queensland. Corella 12: 48–52. || Heinsohn, R. et al. (2017) Tool-assisted rhythmic drumming in palm cockatoos shares key elements of human instrumental music. Science Advances 3: e1602399. || Murphy, S. & S. Legge (2007) The gradual loss and episodic creation of Palm Cockatoo (Probosciger aterrimus) nest-trees in a fire- and cyclone-prone habitat. Emu 107: 1-6. || Hansell, M. & G. Ruxton (2008) Setting tool use within the context of animal construction behaviour. Trends in Ecology and Evolution 23: 73-78.
Main image credit: New York Times, https://www.nytimes.com/2017/06/28/science/drumming-palm-cockatoos.html || First video credit: ABC TV, https://www.youtube.com/watch?v=xvzeKCq5nvg || Second video credit: R. Heinsohn & National Geographic, https://www.youtube.com/watch?v=nmk2CYbz9To