Crested pigeons are a common sight in many Australian backyards, and are noted for the rapid trilling sound they make when they take flight.
In our research, published today in Current Biology, we show that these sounds are produced by feathers in the wings that have evolved to communicate about danger.
These sounds were long suspected of being produced by the wings, so we used high-speed video and acoustic recording to investigate this.
The key finding is that a pair of unusual primary flight feathers, half the width of the others, produce a high-pitched sound as the wing pushes down through the air. This sound is a critical part of the alarm signal that warns the flock.
No other related pigeon species has these unusually narrow wing feathers. The narrowness of these feathers, known as the the 8th primary feathers, suggests that they have evolved to produce this high-pitched sound.
Feathers to the test
We discovered that the 8th primary feathers produce the high-pitched sound, by trimming pairs of feathers and recording the sounds of pigeons fleeing. But we were also interested in how trimming the 8th primary feathers would affect the response of other crested pigeons that heard these modified fleeing sounds.
We developed a speaker set up that let us broadcast the recorded fleeing sounds of birds from the side of a vehicle. We would then drive next to pigeons foraging in parks and on nature strips around Canberra, and broadcast one of the recordings at random.
When we played recordings of crested pigeons with both the 8th primary feathers removed, the birds looked up, but almost never flew off.
Our suspicions were correct – this unusual pair of 8th primary feathers and their high-pitched sound were necessary for warning other pigeons to flee.
So what about the neighbouring 9th primary feathers? We discovered that these produce a low-pitched sound during take-off.
When we played recordings of crested pigeons with both the 9th primary feathers removed, the birds flew off just as often as if they’d heard an alarm. These feathers were not necessary for warning flock mates.
All in a flap
Signals evolve when the creature producing the signal and the one responding to it both benefit.
Crested pigeons that see predators can benefit by inciting the rest of the flock to flee with them, much like redshanks and Belding’s ground squirrels, whose coordinated fleeing confuses predators.
The benefit from communication to every individual could explain why the 8th primary is so unique – it has evolved to exaggerate the sounds of fleeing to trigger a coordinated response.
While these pigeons always produce some sound when they take off, the sounds produced when fleeing danger are distinct. They are louder and have a higher tempo.
Footage from high-speed video cameras showed that fleeing pigeons flap faster when trying to escape a threat. Flapping faster speeds up their escape and it also increases the tempo of the sound, thus transforming the take-off sound into an alarm.
While these differences in the sound can be quite subtle to the human ear, crested pigeons are able to respond to them in about a third of a second. (Can you tell the difference between these two sounds?)
All animals produce some sound as they move, and these sounds are intrinsically informative. They tell the listener where the individual is and how it is moving. This is in stark contrast to the sounds produced by our voices, which are not the result of specific actions.
The same is true for animals: vocal calls are notoriously unreliable, even being used deceptively to steal food. Africa’s fork-tailed drogos, for example, often use their own, and even other species, vocal alarms to scare competitors away and steal their food.
Non-vocal sounds, by comparison, are unable to be faked. If a crested pigeon flees it cannot help but warn its flock mates, just as it cannot warn its flock mates without fleeing.
This means that these non-vocal alarms are a very reliable indicator of danger – and, more broadly, that non-vocal sounds produced by movement are intrinsically informative and so are likely to evolve for communication.
Other feathered messages
Crested pigeons aren’t the only birds that use their feathers to communicate. We know of several other pigeon species that produce conspicuous sounds with their wings that are just waiting to be investigated, such as the Spinifex pigeon.
Mourning doves and zenaida doves from the Americas also make sounds, that appear to be produced by their wings, that warn their flock mates of danger.
Non-vocal acoustic signals are used in many other groups to maintain territories and attract mates – manakins and hummingbirds being the most widespread examples.
It appears that non-vocal acoustic communication is much more common than most people thought, and so we hope a new generation of curious people are inspired to listen closely to the animals around them and discover even more of these unusual signals.
Australian environment groups this week found an unexpected supporter in BHP, the world’s largest mining company.
BHP has defended green groups’ right to receive tax-deductable donations, in the face of a concerted push from both the federal government and the Minerals Council of Australia.
Given the influential role of the environment movement in Australia, and the legal precedent that NGOs and charities can be political, the big Australian evidently sees value in defending them.
Environment groups’ tax status
Environmental organisations in Australia have traditionally been able to claim tax-deductible status under both the Income Tax Act and the Charities Act, in recognition of the fact that the work these groups do has a clear public benefit. But this status has now come under threat.
The federal government issued a report in 2016 entitled Tax Deductible Gift Recipient Reform Opportunities, examining the administration and transparency of the environment groups. The ostensible aim of this report was to ensure that tax-deductible donations to environmental organisations were being used properly.
Among its key recommendations was that environmental organisations would be required to seek tax-deductible status directly from the Australian Taxation Office (ATO), and that they be registered as environmental charities in order to qualify. The report also recommended removing the list of environmental groups set out under the Income Tax Act.
Controversially, the report also recommended that the ATO require environmental charities to spend at least 25% of their donation income on “environmental remediation work”, as opposed to campaigning or other activities. The government has subsequently indicated that it is considering increasing this percentage to 50%.
But the Minerals Council of Australia argues that environmental charities should be forced to commit 90% of their resources to on-the-ground environmental remediation, education and research, leaving only 10% for political advocacy.
Support within the LNP
…there are a large minority [of environmental groups] who are clearly engaged primarily in trying to stop fossil fuel development in Australia and I don’t think it’s right that Australian taxpayers, including people who work in the mining industry, be asked to fund those activities.
The Minerals Council of Australia has also backed the removal of tax-deductible status from environmental organisations, claiming that many of these groups are “not environmental organisations but rather professional activist groups whose objective is to disrupt and hamper the resources sector”.
The Minerals Council issued its own report documenting environmental organisations that is claims have committed or encouraged unlawful or unsafe activities or sought tax-deductible donations to support politically partisan activities.
The report specifically refers to activities by organisations including Greenpeace, the Australian Conservation Foundation, the Nature Conservation Foundation of NSW, the Australian Youth Climate Coalition, and Australian Marine Conservation, arguing that their activities are against federal law.
It is also important to note that environmental organisations are not the only groups to receive tax-deductible status. Other groups, such as the Institute for Public Affairs, which often campaigns on behalf of large organisations to remove environmental protections, also has this status.
Environment groups can be political
Legally speaking, there is no doubt that environmental charities and other NGOs do engage in political activities in addition to their focus on public welfare and the environment. This does not prevent them from being treated as charities.
Indeed, in the landmark High Court decision of Aid/Watch in 2011, the court specifically stated that where it is clear that public welfare is a primary motivation, the fact that the organisation also has political purposes is irrelevant.
On this basis, an environmental organisation can engage in activities to promote political change while still maintaining as its principal purpose the conservation or improvement of the natural environment.
Even BHP agrees. In response to the Minerals Council report, BHP announced that it holds a different view. It argued that environmental organisations should not be stripped of their tax-deductible status, because these organisations perform important advocacy roles for policy development in a democratic society.
Subsequently, 100 BHP shareholders have put forward a shareholder resolution through the Australasian Centre for Corporate Responsibility calling on the company to leave the Minerals Council of Australia. They argued that the Minerals Council’s position is directly at odds with “our company’s long-term financial and strategic interests”.
BHP has agreed to review its membership of the Minerals Council of Australia. It is not alone. In 2016, one of Australia’s largest emitters of greenhouse gas, AGL, left the Minerals Council, citing material differences in their respective policies on climate change and energy.
Environment groups should be allowed to do their work
At a time when we are facing a rapidly transitioning energy landscape – with the acceleration of climate change, renewable energy production, new technologies for unconventional gas extraction, and increasing concerns regarding groundwater depletion and contamination – environmental protection is a major public concern.
It’s hardly surprising that in a democratic framework, environmental organisations have become more politically active. They are striving to ensure that the research and education they conduct with respect to the environment is appropriately reflected within the Australian legal framework.
This work ultimately benefits all Australians. These organistions are constantly seeking to improve and protect the natural habitat in which we all live. In a democracy like ours, the work of these groups should not be drained of funding through changes to the taxation system.
Pep Canadell, CSIRO; Corinne Le Quéré, University of East Anglia; Glen Peters, Center for International Climate and Environment Research – Oslo; Robbie Andrew, Center for International Climate and Environment Research – Oslo; Rob Jackson, Stanford University, and Vanessa Haverd, CSIRO
Global greenhouse emissions from fossil fuels and industry are on track to grow by 2% in 2017, reaching a new record high of 37 billion tonnes of carbon dioxide, according to the 2017 Global Carbon Budget, released today.
The rise follows a remarkable three-year period during which global CO₂ emissions barely grew, despite strong global economic growth.
But this year’s figures suggest that the keenly anticipated global peak in emissions – after which greenhouse emissions would ultimately begin to decline – has yet to arrive.
The Global Carbon Budget, now in its 12th year, brings together scientists and climate data from around the world to develop the most complete picture available of global greenhouse gas emissions.
In a series of three papers, the Global Carbon Project’s 2017 report card assesses changes in Earth’s sources and sinks of CO₂, both natural and human-induced. All excess CO₂ remaining in the atmosphere leads to global warming.
We believe society is unlikely to return to the high emissions growth rates of recent decades, given continued improvements in energy efficiency and rapid growth in low-carbon energies. Nevertheless, our results are a reminder that there is no room for complacency if we are to meet the goals of the Paris Agreement, which calls for temperatures to be stabilised at “well below 2℃ above pre-industrial levels”. This requires net zero global emissions soon after 2050.
The most significant factor in the resumption of global emissions growth is the projected 3.5% increase in China’s emissions. This is the result of higher energy demand, particularly from the industrial sector, along with a decline in hydro power use because of below-average rainfall. China’s coal consumption grew by 3%, while oil (5%) and gas (12%) continued rising. The 2017 growth may result from economic stimulus from the Chinese government, and may not continue in the years ahead.
The United States and Europe, the second and third top emitters, continued their decade-long decline in emissions, but at a reduced pace in 2017.
For the US, the slowdown comes from a decline in the use of natural gas because of higher prices, with the loss of its market share taken by renewables and to a lesser extent coal. Importantly, 2017 will be the first time in five years that US coal consumption is projected to rise slightly (by about 0.5%).
The EU has now had three years (including 2017) with little or no decline in emissions, as declines in coal consumption have been offset by growth in oil and gas.
Unexpectedly, India’s CO₂ emissions will grow only about 2% this year, compared with an average 6% per year over the past decade. This reduced growth rate is likely to be short-lived, as it was linked to reduced exports, lower consumer demand, and a temporary fall in currency circulation attributable to demonetisation late in 2016.
Yet despite this year’s uptick, economies are now decarbonising with a momentum that was difficult to imagine just a decade ago. There are now 22 countries, for example, for which CO₂ emissions have declined over the past decade while their economies have continued to grow.
Concerns have been raised in the past about countries simply moving their emissions outside their borders. But since 2007, the total emissions outsourced by countries with emissions targets under the Kyoto Protocol (that is, developed countries, including the US) has declined.
This suggests that the downward trends in emissions of the past decade are driven by real changes to economies and energy systems, and not just to offshoring emissions.
Other countries, such as Russia, Mexico, Japan, and Australia have shown more recent signs of slowdowns, flat growth, and somewhat volatile emissions trajectories as they pursue a range of different climate and energy policies in recent years.
Still, the pressure is on. In 101 countries, representing 50% of global CO₂ emissions, emissions increased as economies grew. Many of these countries will be pursuing economic development for years to come.
A peek into the future
During the three-year emissions “plateau” – and specifically in 2015-16 – the accumulation of CO₂ in the atmosphere grew at a record high that had not previously been observed in the half-century for which measurements exist.
It is well known that during El Niño years such as 2015-16, when global temperatures are higher, the capacity of terrestrial ecosystems to take up CO₂ (the “land sink”) diminishes, and atmospheric CO₂ growth increases as a result.
The El Niño boosted temperatures by roughly a further 0.2℃. Combined with record high levels of fossil fuel emissions, the atmospheric CO₂ concentration grew at a record rate of nearly 3 parts per million per year.
This event illustrates the sensitivity of natural systems to global warming. Although a hot El Niño might not be the same as a sustained warmer climate, it nevertheless serves as a warning of the global warming in store, and underscores the importance of continuing to monitor changes in the Earth system.
No room for complacency
There is no doubt that progress has been made in decoupling economic activity from CO₂ emissions. A number of central and northern European countries and the US have shown how it is indeed possible to grow an economy while reducing emissions.
Other positive signs from our analysis include the 14% per year growth of global renewable energy (largely solar and wind) – albeit from a low base – and the fact that global coal consumption is still below its 2014 peak.
These trends, and the resolute commitment of many countries to make the Paris Agreement a success, suggest that CO₂ emissions may not return to the high-growth rates experienced in the 2000s. However, an actual decline in global emissions might still be beyond our immediate reach, especially given projections for stronger economic growth in 2018.
To stabilise our climate at well below 2℃ of global warming, the elusive peak in global emissions needs to be reached as soon as possible, before quickly setting into motion the great decline in emissions needed to reach zero net emissions by around 2050.
Pep Canadell, CSIRO Scientist, and Executive Director of the Global Carbon Project, CSIRO; Corinne Le Quéré, Professor, Tyndall Centre for Climate Change Research, University of East Anglia; Glen Peters, Research Director, Center for International Climate and Environment Research – Oslo; Robbie Andrew, Senior Researcher, Center for International Climate and Environment Research – Oslo; Rob Jackson, Chair, Department of Earth System Science, and Chair of the Global Carbon Project, globalcarbonproject.org, Stanford University, and Vanessa Haverd, Senior research scientist, CSIRO