Some of the World’s Most Abundant Birds

Leach’s storm-petrel is a small seabird that lives much of its life on the ocean (Figure 1A). Even though each storm petrel is tiny, they are some of the most common birds on the planet, with over 5 million adults in Atlantic North America alone [1]. Storm-petrels spend most of their time gliding just above the ocean waves, on the hunt for small fish and squid.

Storm-petrels only come onto land when it is time to lay eggs and raise their chicks, and when they do, it is surprisingly spooky. In fact, their calls reminded sailors of witches laughing in the dark, which is why storm-petrels were once nicknamed “Mother Carey’s chickens”, after a folk-story witch said to control storms.

To stay safe, storm-petrels choose remote islands for their nests, away from predators. They dig burrows underground and only come out at night so that predators cannot spot them. When the chicks hatch, they look like sooty cotton balls, a perfect way to stay warm (Figure 1B). Long ago, sailors collected adult birds and used them as makeshift candles (Figure 1C) [2], showing just how rich in fat these little birds are.

Today, storm-petrels face new challenges. Predators introduced from other areas can kill large numbers of these docile birds; bright coastal and offshore lights disorient birds flying at night; and other forms of pollution are causing their numbers to drop [3]. In response, Leach’s storm-petrels have become an important species for conservation.

Why are Seabirds Important in Ocean Ecosystems?

Seabirds like storm-petrels play an important role in marine ecosystems. They sit near the top of the ocean food web, so when something changes in the ocean, like a change in food or habitat availability, seabirds are often the first to show signs of this change. Scientists can study shifts in seabird numbers and behavior to understand overall changes in the whole ocean.

Seabirds also act like nutrient-delivery systems. They feed in the ocean and bring those nutrients back to land as poop, eggshells, and feathers. All of these are incredibly effective fertilizers that help plants grow and even cause coral reefs to flourish. When seabird populations shrink, ecosystems lose this important nutrient boost. Because storm petrels are both abundant and ecologically important, understanding why they are declining is an important goal for conservation biologists (scientists who study and protect wildlife).

Counting Seabirds

Animal populations grow and shrink for many reasons. Some causes are natural, such as predators, food availability, diseases, or natural climate changes. Other changes are caused by humans, like pollution, habitat destruction, and hunting. In the Atlantic Ocean, where storm-petrels live, pollution can be especially harmful. For example, pieces of plastic floating in the water can look like food. When young birds eat plastic, it can damage their organs and affect their brains [4].

Conservation biologists need many years of monitoring data to understand what is harming a population and how to best protect it. But some animals are difficult to study, which means there is little data, if any, about their population trends. Storm-petrels are one of those tricky species. They spend most of their lives flying over the ocean and only return to land at night, sneaking in and out of underground burrows on remote islands. This makes them very difficult to see and count. Instead, researchers use a method called grubbing, which means carefully reaching into burrows to feel whether a bird or egg is inside (Figure 1D). On islands with thousands of burrows, this becomes a huge job, and it takes a brave biologist to stick their hand into a dark hole without knowing what is inside! Luckily, storm-petrels do not bite … very hard… but puffins do, and they nest in similar places. For biologists, it is a bit like playing “trick or treat” with wildlife.

Collecting a Sediment Core

Because traditional surveys take so much time and effort, we decided to try a different approach to understand how storm-petrel populations have changed over thousands of years. Instead of counting birds directly, we looked for clues hidden in lake sediments using a scientific method called paleolimnology. Paleolimnology is the study of information stored in the mud (sediment) at the bottom of lakes and ponds. As sediment builds up layer by layer, it forms a natural “time capsule” that records changes in the environment.

When storm-petrels feed in the ocean, they collect certain chemicals and minerals in their bodies (Figure 2A). They bring these materials back to the island where they nest (Figure 2B). Over time, their droppings, feathers, and eggshells wash into nearby lakes when it rains (Figure 2C). This changes the lake’s chemistry and affects the tiny organisms, like algae and insects, that live there. As these organisms die, they sink to the bottom and become part of the sediment layers, which steadily accumulate over time (Figure 2D).

If the number of birds increases, more nutrients and chemicals end up in the lake. If the number of birds decreases, the lake receives less. By studying the chemistry and preserved organisms in each sediment layer, scientists can reconstruct how the bird population changed over time.

Seabird Islands

In our study, we traveled to Grand Colombier Island in St. Pierre and Miquelon, a small French territory south of Newfoundland and Labrador, Canada’s most eastern province. The island is treeless, covered in lush green ferns, and completely uninhabited, except for thousands of seabirds including storm-petrels, puffins, and gulls. Traditional surveys suggested the storm petrel population was relatively stable at around 400,000 adults since the 1980s (Figure 3A).

Once on the island, we paddled a small inflatable boat to the middle of the lake and used a long plastic tube to collect a column of mud, called a sediment core (our “time capsule”) from the lake bottom. We cannot directly count feathers or eggshells in the sediment because they are too rare. Instead, we use proxies, which are indirect clues. These include chemicals from bird waste, tiny algae called diatoms, and insect remains. Seabird waste is high in nutrients like nitrogen and phosphorus, so when it washes into the lake it changes the environment and the lake’s tiny organisms, which biologists can analyze. By combining all these clues, we reconstructed long-term changes in the lake that were linked to changing storm-petrel populations (Figure 3B).

Historical Secrets Revealed from a Sediment Core

The sediment core we collected contained more than 5,800 years of history, older than Stonehenge and the Pyramids of Giza! Over this enormous time span, the storm-petrel population on Grand Colombier Island rose and fell dramatically. We found two major population peaks: one about 2,600 years ago that lasted roughly 1,500 years, and another around 700 years ago that lasted about 1,400 years. These natural ups and downs happened long before humans arrived, likely because storm-petrels moved between islands depending on food availability and climate.

But everything changed after a surge of European settlers arrived in 1816. Although today’s seabird population appears stable, it is now less than 20% of its historical maximum (Figure 3). This sharp decline is unlikely to be natural. Human activities likely play major roles, like increased fishing that reduces the number of fish seabirds can eat or the introduction of invasive species like rats that eat the seabird eggs and baby birds. Even artificial lights from cities and ships can distract and confuse the birds when they fly at night, causing large numbers of them to crash into structures.

Future Conservation of Storm-Petrels

Our study shows why long-term data are so important for understanding and protecting wildlife. Short-term surveys might suggest that storm-petrel numbers are stable, but when we looked deeper into the past, we saw that today’s population is much smaller than it used to be. To protect storm petrels and other species, we must restore damaged habitats, protect habitats that are still healthy, and continue using long-term scientific approaches, like paleolimnology, to guide conservation decisions. These methods can help us make better choices to protect vulnerable species for the future.

Glossary

Population Trend: ↑ A pattern showing whether the number of animals in a group is going up, going down, or staying the same over time.

Grubbing: ↑ A method biologists use to check seabird nests by gently reaching into burrows to feel for birds or eggs. This does not hurt the birds!

Traditional Surveys: ↑ Manually counting birds to get an understanding of population size. They take a lot of time and are expensive.

Paleolimnology: ↑ The study of past lake conditions using clues found in mud layers. These layers act like a natural history book.

Sediment Core: ↑ A long tube of mud collected from the bottom of a lake. Each layer represents a piece of the lake’s history, with the oldest at the bottom and the newest at the top.

Proxies: ↑ Indirect clues, like chemicals, algae, or insect remains, that scientists use to learn about things they cannot measure directly, such as past bird populations.

Diatom: ↑ A type of tiny algae with glass-like shells that live in water. When they die, their shells sink and get preserved in lake mud, helping scientists learn about past environments.

Invasive Species: ↑ Animals or plants that humans bring to new places, where they do not naturally belong. They often harm native species. For example, invasive rats can eat seabird eggs.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

Thank you to Sylvie Allen-Mahé, Christophe Barbraud, Jules Blais, Amaël Boudreau, Rachel Bryant, Karine Delord, Christopher Grooms, Linda Kimpe, Bruno Letournel, Joeline Lim, Hervé Lormée, Neal Michelutti, and Frank Urtizbéréa, who all contributed to the original manuscript. This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada (grant no. RGPIN-2017-04548) and an E.G. Bauman Fellowship.

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