Why passenger pigeons went extinct a century ago



A passenger pigeon flock being hunted in Louisiana. From the ‘Illustrated Shooting and Dramatic News’, 1875.
(Wikimedia/Smith Bennett), CC BY-NC-ND

Eric Guiry, Trent University

On Sept. 1, 1914, a Cincinnati Zoological Gardens employee found the lifeless body of Martha, the world’s last living passenger pigeon, resting beneath her perch.

Forty years earlier, Martha’s ancestors numbered in the billions. Their flocks formed avian clouds across eastern North America, obstructing sunlight for days. The sight was so overwhelming that the American conservationist Aldo Leopold called them a “biological storm.”

By the early 1900s, only a handful of birds remained, and these were in captivity. How, in a few short decades, could one of the world’s most prodigious bird vanish from the sky?

As an archaeological scientist with a background in ecology and chemical analyses, I have always been fascinated by great extinction events and the disappearance of the passenger pigeon is one of the most notable in North America’s history. It’s exciting to look at the events that led to their demise.

Forests of food?

For decades, two theories have been used to explain the extinction of passenger pigeons. While it has long been understood that human activity caused their extinction, the exact mechanism wasn’t known.

A male passenger pigeon on display at the Cleveland Museum of Natural History in Ohio. The last wild bird was shot in 1901, and Martha, the last captive bird, died on Sept. 1, 1914, at the Cincinnati Zoo.
(Tim Evanson/flickr), CC BY-SA

One theory was that because the birds mostly ate a highly specialized diet of tree nuts (known as “mast”), such as acorns and beechnuts, they died off when they could no longer find enough food after the forested habitats they devoured were cut down by humans.

The other theory was that their obliteration was due mainly to humans killing staggering numbers of birds for sport and to feed growing urban populations.

The conflict between these two ideas was already evident in the early 19th century, when the almost ceaseless slaughter of passenger pigeons was well underway. After the Civil War, technological advancements, such as the telegraph and expanding rail networks, helped professional hunters, called pigeoners, to locate migrating flocks at their nesting sites and collect birds, young and old, on an industrial scale.

The great American ornithologist John James Audubon may have captured popular sentiment when he said, “… nothing but the gradual diminution of our forests can accomplish their decrease as they not infrequently quadruple their numbers yearly, and always at least double it.”

So, which was more likely: hunting or habitat destruction?

Diet clues

My colleagues and I used stable isotope analysis to study chemical markers in the bones of passenger pigeons found in archaeological deposits dating from 900-1900, in the heart of the birds’ former nesting habitat in Ontario and Québec.

An animal’s bones can tell us a lot about what ate before it died. Because bones grow and remodel slowly over the course of an animal’s lifetime, their stable isotope composition gives us information about average diet over a period of months or even years. This longer-term record of diet lets us see what a bird ate over its entire life, rather than at a single meal or in a single season.




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Our study found that passenger pigeons could live off other foods, including farmers’ crops. This suggests that an unchecked commercial pigeon industry was likely the more important driver behind the birds’ extinction.

A passenger pigeon skull collected during archaeological excavations.
(Eric Guiry)

Prior to our research, little was known about the diversity (or lack thereof) of their diet. At the time of their decline and disappearance, no one had the technology to be able to follow and document the birds throughout their full life cycle, including cross-continental migration.

Past historical research indicated that mast was the birds’ food of choice, as they roamed up and down the great forests of eastern North America searching out patches at the peak in their masting cycle. Yet there was also scattered anecdotal evidence that the birds would at times descend on farmers’ fields of corn and wheat.

Most of the birds we sampled did eat mostly mast, but a subset had chemical compositions that suggest their diet was made up largely of crops like corn that would have been available even as their traditional sources of food grew scarcer. We also tested the subset of birds to see if they belonged to a specific age category or genetic group but found that corn-based diets occurred in both young and old birds, as well as in all genetic groups, suggesting that this dietary flexibility may have been widespread.

A new mystery?

Our analysis answered our original question, but also opened up another mystery for future study.

The passenger pigeon was found across most of North America east of the Rocky Mountains, north of the Mississippi and south of Canada. But sometimes they were seen in Bermuda, Cuba or Mexico.
(Shutterstock)

We performed DNA analyses to confirm the birds we were testing were, in fact, passenger pigeons. These results suggested that there may have been more genetic diversity in these birds than previous studies revealed.

Much of the previous DNA work was concentrated on birds that died not long before the species disappeared entirely, which may have meant the genetic diversity in the birds was already waning. A sample from the earlier birds in our study suggests there may have been more internal diversity during the thousands of years these flocks dominated the skies and forests of eastern North America.




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This research reveals the amazing potential that archaeology and scientific techniques have for helping us understand major events of the past and how the actions of humans have shaped the world as we know it today.The Conversation

Eric Guiry, Post-Doctoral Fellow, Trent Environmental Archaeology Laboratory, Trent University

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

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How we discovered the conditions behind ‘slow earthquakes’ that happen over weeks or even months – new research


The world’s tectonic plates.
Naeblys/Shutterstock

Åke Fagereng, Cardiff University

You’re probably familiar with earthquakes as relatively short, sharp shocks that can shake the ground, topple buildings and tear rips in the Earth. These earthquakes, and their aftershocks, happen because although tectonic plates move at centimetres per year, this motion is seldom steady. Earthquakes result from a “stick-slip” motion, where rocks “stick” along fault planes while stress accumulates until a “slip” occurs – a bit like pulling on a stuck door until it suddenly opens. This slip also releases energy as the seismic waves that, in large magnitude earthquakes, create substantial damage.

In the last two decades another class of stick-slip motion has been discovered worldwide. These “slow slip events” last for weeks to months, compared to seconds to minutes for earthquakes. Slow slip events occur faster than average plate motion, but too slow to generate measurable seismic waves. This means they need to be studied by GPS networks rather then seismometers.

Although their motion is slow, the amount of movement that occurs in a slow slip event is substantial. Earthquake magnitude depends on the distance that rocks move and the area this movement occurs over. Using the same definition, many slow slip events would have had magnitudes above 7.0 if they slipped at earthquake speeds.

Slow slip events repeat at intervals of a year to a few years. Compared to major earthquakes, which have repeat times of hundreds of years (or more), slow slip events are actually very frequent. Even in the short time of a couple of decades that we’ve observed these types of slip, many cycles have occurred in several places – notably around the Pacific Rim.

Slow slip events generally happen next to areas where faults are locked and expected to rupture in major earthquakes. It’s therefore possible that these slow slip events can trigger earthquakes on neighbouring locked faults. It has, for example, been suggested that slow slip events preceded the 2011 magnitude 9.1 Tohoku earthquake in Japan and the 2014 magnitude 8.1 Iquique earthquake in Chile. That said, numerous slow slip events have also been observed without any immediate, subsequent major earthquakes on neighbouring faults.

Earthquakes may also trigger slow slip. In particular, the magnitude 7.8 Kaikōura earthquake in New Zealand in 2016 triggered slow slip events up to 600km away from its epicentre.

It is not known why some fault segments host slow slip and others host earthquakes. Neither is it known whether the same area can change behaviour and host either slow slip or earthquakes at different times. It’s therefore important to characterise the source of slow slip, and find out what materials help create slow slip and under what conditions.

A unique opportunity

The Hikurangi subduction zone.
Åke Fagereng composite using map data from NOAA., Author provided

The Hikurangi subduction zone (where the Pacific ocean floor is pulled underneath the New Zealand continent) offshore New Zealand’s North Island is potentially the country’s largest earthquake fault and is a unique opportunity to investigate slow slip events. This is because slow slip here happens shallower and closer to the shoreline than anywhere else in the world.

The drill.
Åke Fagereng, Author provided

The shallow slow slip events in New Zealand have been observed by onshore GPS and ocean bottom pressure sensors. Oceanic scientific drilling expeditions recently sampled sediments and installed observatories along this margin.

The subduction zone.
Stihii/Shutterstock

These International Ocean Discovery Program expeditions – which drilled to just over 1km deep in water depths of 3.5km in late 2017 and early 2018 – revealed that the seafloor rocks and sediments hosting slow slip in Hikurangi are extremely variable. The range of rocks, described in a recent Science Advances paper led by Philip Barnes of NIWA (New Zealand’s National Institute of Water and Atmospheric Research), include mudstones, sands, carbonates, and sedimentary deposits from oceanic volcanic eruptions. The seafloor samples show that the source of the slow slip is a mixture of very soft sediment and hard, solid rocks.

Different types of rock from the New Zealand seafloor.
Åke Fagereng, Author provided

The diverse seafloor sediments are not the only variability offshore of New Zealand. The seafloor itself is also very rough, including seamounts (submarine mountains rising over a kilometre above the seafloor). This seafloor roughness also makes the fault vary depending on where along it you are.

The observations are consistent with a hypothesis where slow slip events occur in rocks that are transitional between moving steadily and moving in earthquakes. One way to think of this model is as rigid rocks interacting with softer, more ductile surroundings. Researchers using numerical simulations and laboratory experiments have also suggested that variable fault rocks can cause slow slip.

But diverse fault rock isn’t the only model for the mechanics of slow slip. Another possibility is that pressurised fluids decrease frictional resistance and slip speed along faults. It is also possible that some rocks become stronger when they move faster – so that faults start accelerating but slow down before reaching earthquake speeds.

The recent discoveries in New Zealand may be applicable to other depths and locations around the world. However, future studies will undoubtedly lead to further insights and complexities – including in the relationship between slow slip events and earthquakes.The Conversation

Åke Fagereng, Reader, School of Earth and Ocean Sciences, Cardiff University

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