Dark colours play a crucial role in regulating temperatures in many biological systems. This is particularly common for animals like reptiles, which rely on environmental sources of heat to keep themselves warm.
Darker colours absorb more heat from sunlight, and animals with these colours are more commonly found in colder climates with less sunlight. This broad pattern is known as Bogert’s rule.
Birds’ eggs are useful for studying this pattern because the developing embryo can only survive in a narrow range of temperatures. But eggs cannot regulate their own temperature and, in most cases, the parent does it by sitting atop the clutch of eggs.
In colder environments, where the risk of predators is lower and the risk of chilling in cold temperatures is greater, parents spend less time away from the nest.
We predicted that if eggshell colour does play an important role in regulating the temperature of the embryo, birds living in colder environments should have darker eggs.
The average colour of eggshells in different areas around the world. Wisocki et al. 2019 ‘The global distribution of avian eggshell colours suggests a thermoregulatory benefit of darker pigmentation’, Nature Ecology & Evolution, Author provided
To test the prediction, we measured eggshell brightness and colour for 634 species of birds. That’s more than 5% of all bird species, representing 36 of the 40 large groups of species called orders.
We mapped these within each species’ breeding range and found that eggs in the coldest environments (those with the least sunlight) were significantly darker. This was true for all nest types.
We also conducted experiments using domestic chicken eggs to confirm that darker eggshells heated up more rapidly and maintained their incubation temperatures for longer than white eggshells.
Our results show that darker eggshells are found in places with less sunlight and lower temperatures, and that these darker colours may help keep the developing embryo warm.
How future climate change will affect eggshell appearance, as well as the timing of reproduction and incubation behaviour, will be an important and fruitful avenue for future research.
This story is part of Covering Climate Now, a global collaboration of more than 250 news outlets to strengthen coverage of the climate story.
A recent report by the Greater Sydney Commission singles out urban heat as one of four priority areas given our coming climate. It identifies tree canopy as the top response for reducing city temperatures and delivering amenity. However, the public conversation about urban heat often misses the complex relationship between trees, people and the built environment, which challenges this response.
In soon-to-be-published research supported by the Landcom University Roundtable we found that responding to a more extreme climate requires new social practices and new relationships with the commons. Commons are the spaces, resources and knowledge shared by a community, who are, ideally, involved in the regeneration and care of those commons. Trees are an important social commons, but they also present multiple challenges.
For one, trees are an outdoor amenity, but we are spending more and more time indoors. For those who can afford it, air conditioning delivers cooling in the privacy of your own home or car – no need for trees.
However, staying in cool bedrooms and car rides mean less time outdoors and with others, which isn’t ideal for human health and well-being.
Air conditioning also uses more fossil-fuel-based energy, which generates more greenhouse gas emissions. The result is more climate change.
Mixed feelings about trees
As the Greater Sydney Commission report makes clear, tree canopy in Greater Sydney is roughly proportional to household wealth. The “leafy suburbs” are the wealthier ones. This means tree planting is an important investment in less wealthy parts of the city, which experience more extreme heat days.
However, research also shows people have mixed feelings about trees. In comparison to the neat shrubbery and easily maintained sunny plazas we’ve become used to in our cities, trees can be “messy” and “unpredictable”. Leaf litter can be slippery and natives like eucalypts, with their pendulous leaves, provide limited shade. People worry about large trees falling over or dropping branches.
Trees are often at the centre of disputes between neighbours. They can also be perceived as a security problem – if trees reduce visibility they might provide cover for wrongdoers.
Urban development tends to give priority to roads and delivering the maximum number of dwellings on sites. This leaves little space for trees, which need to fit into crowded footpaths with ever-changing infrastructures. For example, will larger trees interfere with 5G?
When juggling priorities in the streetscape, trees often lose out.
It’s an obvious point, but trees take time to grow. It can take many years for a planted sapling to become a shade tree. In that time there will be no shelter from the heat.
Also in that growing period, which can sometimes be unpredictable, trees need to be nurtured, especially in times of drought. And, once the tree is mature, fingers crossed that extreme weather events do not undo all those years of waiting.
So, while increasing tree canopy sounds like an obvious solution, trees are in fact a complex social challenge. In our research, we point to ways some of these tree-related tensions can be managed.
Shade in the meantime
A structure to support fast-growing vines has been built on one of Darwin’s hottest streets, but even these will take some time to grow. Darwin We Love It/Facebook
Shade is an important civic resource. Large, mature trees with spreading canopy provide the best shade, so strategic construction bans and tree preservation orders are an obvious first step.
However, if shady canopy is decades off, we need to think about other, creative ways to provide shade in the meantime to ensure, for example, that people of diverse abilities can walk their city in reasonable comfort. This might include temporary shade structures such as awnings, bus shelters and fast-growing vine-trellised walkways (if there is space to create troughs for soil and the structure doesn’t cause access problems).
And, as the Cancer Council consistently reminds us, we all need to adopt more climate-defensive clothing.
An important alternative is to follow our regional neighbours and start to populate parks and other public spaces at night. This suggests a need for removable shade, so we can take part in activities like stargazing.
Cultivating an intergenerational commons
Mature trees can die back or die altogether, so other trees should be maturing to take their place. Usually, experts design and maintain landscapes for others to enjoy.
However, users of the cooling services of parks could be invited into the process of planning and realising landscape designs. This would give them a say on the trees of which they have “shared custody”. Planting for succession can create an intergenerational sense of ownership over a shared place.
Current planning practices tend to ignore wind and solar patterns. The result is urban forms that make heat worse by prioritising comfortable private interior spaces over the commons of public space. Designing cool cities means using trees, water and buildings to create cool corridors that work with cooling breezes – or even summon these in still, heat-trapping basins like Western Sydney.
These few examples point to new ways of living with trees as social commons, but they also point to new forms of commoning – collaborative forms of care and governance that invite people to adopt new social practices better suited to living well in the coming climate.
It is a positive step that state development agencies like Landcom aim to demonstrate global standards of liveability, resilience, inclusion, affordability and environmental quality. In so doing, they initiate transitions to these more commons-based ways of living.
In addition to the authors of this article, the Cooling the Commons research team includes: Professor Katherine Gibson, Dr Louise Crabtree, Dr Stephen Healy and Dr Emma Power from the Institute for Culture and Society (ICS) at Western Sydney University (WSU), and Emeritus Professor Helen Armstrong from Queensland University of Technology (QUT).
Kevin's Walk on the Wild Side 