Look up! A powerful owl could be sleeping in your backyard after a night surveying kilometres of territory



Nick Bradsworth, Author provided

Nick Bradsworth, Deakin University; John White, Deakin University, and Raylene Cooke, Deakin University

Picture this: you’re in your backyard gardening when you get that strange, ominous feeling of being watched. You find a grey oval-shaped ball about the size of a thumb, filled with bones and fur — a pellet, or “owl vomit”.

You look up and see the bright “surprised” eyes of a powerful owl staring back at you, with half a possum in its talons.

This may be becoming a familiar story for many Australians. We strapped tracking devices to 20 powerful owls in Melbourne for our new research, and learned these apex predators are increasingly choosing to sleep in urban areas, from backyard trees to city parks.

These respite areas are critical for species to survive in challenging urban environments because, just like for humans, rest is an essential behaviour to conserve energy for the day (or night) ahead.

Our research highlights the importance of trees on both public and private land for wild animals. Without an understanding of where urban wildlife rests, we risk damaging these urban habitats with encroaching development.

One owl, one year, 300 possums

Powerful owls are Australia’s largest, measuring 65 centimetres from head to tail and weighing a hefty 1.6 kilograms. They’re found in Australia’s eastern states, except for Tasmania.

Powerful owl with half a common ringtail possum
Powerful owl at roost with half a common ringtail possum (probably saving it for later).
Nick Bradsworth

These owls have traditionally been thought to live only in large old-growth forested areas. However, Victoria has lost over 65% of forest cover since European settlement, and because of this habitat loss, the owls are listed as threatened in Victoria.

Their remaining habitat is extremely fragmented. This means we’re finding owls in interesting places — from dry, open woodland to our major east coast cities. This is likely due to the high numbers of prey, such as possums, that thrive alongside exotic garden trees and house roofs.




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Powerful owls usually eat one possum per night, or 250-300 possums per year — mostly common ringtail and brushtail possums in Melbourne. They’re often seen holding prey at their roosting spots, where they’ll finish eating in the evening for breakfast.

This has ecosystem-wide benefits, as powerful owls can help keep overabundant possums in check. Too many possums can strip away vegetation, causing it to die back, which stops other wildlife from nesting or finding shelter.

Tracking their nocturnal haunts

But powerful owls are extremely elusive. With low populations, locating owls and researching their requirements is very difficult.

So, to help narrow down the general areas where powerful owls live in Melbourne, we used species distribution models and sought help from land management agencies and citizen scientists.

Over five years, we deployed GPS devices on 20 Melburnian owls to find how they use urban environments. These devices automatically record where the owls move at night and rest during the day.

We learned they fly, on average, 4.4 kilometers per night through golf courses, farms, reserves and backyards looking for dinner and defending their territory. One owl along the Mornington Peninsula travelled 47 km over two nights (possibly in search of a mate). Another urban owl called several golf courses in the Melbourne suburb of Alphington home.

Choosing where to sleep

After their nightly adventures, the owls usually return to a number of regular roosting (resting) spots, sometimes on the exact same branch. The powerful owl chooses roosts that protect them against being mobbed by aggressive daytime birds, such as the noisy miner and pied currawong.

A powerful owl showing defensive behaviour towards nearby pied currawongs trying to mob it.

We found the owls used 32 different tree species to roost in: 23 were native, and nine were exotic, including pine and willow trees. This shows powerful owls can adapt to use a range of species to fit their roosting requirements, such as thick foliage to hide in during the day.

Owls will generally roost in damp, dark areas during summer, and in open roosts in full or dappled sunlight during winter to help regulate their body temperature.




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Our research also shows rivers in urban environments are just as important as trees for roosting habitat.

Rivers are naturally home to a diverse range of wildlife. Using trees near rivers to rest in may be a strategic decision to reduce time and energy when travelling at night to find other resources, such as prey, mates and nests.

Rivers that constantly flow, such as the Yarra River, are a particular favourite for the owls.

A powerful owl surrounded by leaves
Powerful owl at roost among dense Kunzea vegetation.
Nick Bradsworth

The urban roost risk

These resting habitats, however, are under constant pressure by urban expansion and agriculture. Suitable roosting habitat is either removed, or degraded in quality and converted to housing, roads, grass cover or bare soil.

We found potentially suitable roosting habitat in Melbourne is extremely fragmented, covering just 10% of the landscape because owls are very selective about where they sleep.

Although there might be the odd suitable patch (or tree) to roost in urban environments, what’s often lacking is natural connectivity between patches. While owls are nocturnal, they still need places to rest in the night before they settle down in another spot to sleep for the day.

A pair of powerful owls with beady eyes sitting at their roost
The classic ‘surprised’ powerful owl expression at a roost.
Nick Bradsworth

Supplementing habitat with more trees on private property and enhancing the quality of habitat along river systems may encourage owls to roost in other areas of Melbourne.

Powerful owls don’t discriminate between private land and reserves for roosting. So conserving and enhancing resting habitats on public and private land will enable urban wildlife to persist alongside expanding and intensifying urbanisation.

So what can you do to help?

If you want powerful owls to roost in your backyard, visit your local indigenous nursery and ask about trees local to your area.

Several favourite roost trees in Melbourne include many Eucalyptus species and wattles. If you don’t have the space for a large tree, they will also roost in the shorter, dense Kunzea and swamp paperbark (Melaleuca ericifolia).

Planting them will provide additional habitat and, if you are lucky, your neighbourhood owls may even decide to settle in for the day and have a snooze.




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The Conversation


Nick Bradsworth, PhD Candidate, Deakin University; John White, Associate Professor in Wildlife and Conservation Biology, Deakin University, and Raylene Cooke, Associate Professor, Deakin University

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

Melting ocean mud helps prevent major earthquakes — and may show where quake risk is highest



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Kate Selway, Macquarie University

The largest and most destructive earthquakes on the planet happen in places where two tectonic plates collide. In our new research, published today in Nature Communications, we have produced new models of where and how rocks melt in these collision zones in the deep Earth.

This improved knowledge about the distribution of melted rock will help us to understand where to expect destructive earthquakes to occur.

What causes earthquakes?

Giant earthquakes, such as the magnitude-9.0 quake in 2011 that caused the Fukushima nuclear disaster, or the magnitude-9.1 event in 2004 that caused the Boxing Day tsunami, occur at the collision zones between two tectonic plates. In these so-called subduction zones, one plate slides beneath the other.




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The sinking plate acts as an enormous conveyor belt, carrying material from the surface down into the deep Earth. Earthquakes occur where the sinking plate gets stuck; strain builds up until it eventually quickly releases. Fluids and molten rocks in the system lubricate the plates, helping them slide past each other and stopping big earthquakes from happening.

When happens when ocean mud ends up inside Earth?

My colleague Michael Förster and I were interested in what happens to sediments when they are carried down into the deep Earth at a subduction zone. These sediments start out as thick layers of mud on the ocean floor but get carried down into the deep Earth as part of the sinking plate.

Michael took a sample of mud collected from the ocean floor and heated it up to the high temperatures and pressures it would experience in a subduction zone. He found the sediments melt and then react with the surrounding rocks, forming the mineral phlogopite and also saline fluids.

A puzzle solved

Geophysical models of subduction zones allow us to map out exactly where the molten rocks and fluids are. These measurements are like x-rays of Earth’s interior, helping us peer into places we cannot otherwise see.

We were particularly interested in models of the electrical conductivity of subduction zones. This is because the fluids and molten rock we were looking at are more electrically conductive than the surrounding rock. Models of subduction zones have long been enigmatic, because they show Earth is very conductive in regions where people did not expect to see a lot of fluids and molten rock.

Melting sediment from the seafloor helps tectonic plates slide over one another without creating major earthquakes.
Selway & Forster, Author provided

I calculated the electrical conductivity of the phlogopite, molten sediments and fluids that were produced in the experiments and found they matched extremely well with the geophysical models. This provides good evidence that what we see in the experiments is happening in the real Earth, and allows us to calculate where the molten rock and fluids are in subduction zones around the world.

Understanding where big earthquakes are likely to occur

Giant earthquakes are not likely to occur in the parts of the subduction zone where the sediments melt. All of the products of the melting — the molten rock itself, the saline fluids, and even the mineral phlogopite — help the two plates slide past each other easily without causing large earthquakes.

We compared our models with locations of earthquakes in subduction zones along the west coast of the United States. We found there were no large earthquakes where sediments were melting, but the movement of fluids from the melted sediments could explain some small, non-destructive earthquakes and very faint signals of tremor where the two plates easily slide past each other.




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Earthquakes are a tangible reminder that we live on an active planet and that, deep beneath our feet, huge forces are making rocks flow and melt and collide. Accurately predicting earthquakes will be an ongoing goal of geoscientists for decades to come.

It requires intricate detective work to weave together all the tiny threads of information we have about processes that occur so deep in the Earth that we will never be able to see or sample them. Our results are one new thread in this puzzle. We hope it will contribute to one day being able to keep people safe from the risk of earthquakes.




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The Conversation


Kate Selway, , Macquarie University

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