How Many Did You Say?

The Western Atlantic subspecies of Red Knot, Calidris canutus rufa, or affectionately just “rufa,” has experienced dramatic declines over the last 30 years, dropping to a shadow of its population in the 1980’s, and sending perplexed conservation biologists scrambling for answers. This stunning long-distance migrant is arguably the most studied Arctic-breeding shorebird anywhere. Long before 2015, when it was listed as threatened under the U.S. Federal Endangered Species Act, biologists throughout the Americas had been working hard for more than four decades to understand all aspects of rufa ecology. Thankfully, because of these committed souls who are still applying themselves to studying knots, we have been pulling the pieces of the rufa conservation puzzle together. We know where they nest, we know more about where they fly, we know what they like to eat, we know where they sleep, we know what eats them, we know most of the important places the birds stop during migration, and we even know where and when they molt their old feathers and grow new ones. So you might think that at any given time we would be able to tell you quickly how many rufa knots are out there. It is not so easy.

Understanding how many shorebirds exist in a population, which is a distinct group of a species that share nesting, migration paths, and non-breeding (wintering) areas, is paramount to our ability to focus efforts on the birds that need help. These are biogeographic populations of subspecies, and each one adds to the whole of a species. The rufa knots are a distinct group of the Western Atlantic Red Knots. Research is taking place at key locations for this subspecies from Arctic Canada to the southern tip of South America. Some locations with long-term monitoring include Manitoba and Quebec in Canada; Massachusetts, New Jersey, Delaware, Virginia, South Carolina, and Georgia in the U.S.; Ceará, in Brazil; Río Negro, Cubut, and Tierra del Fuego in Argentina; and Magallanes, in Chile. The habitat conditions and food availability at these key locations is important for attracting and holding the birds for days, weeks, or even months every year.

Counting knots or other shorebirds is done in a variety of ways, all using the most accurate methods available to fit the conditions and challenges of each site. None of the methods are foolproof, and all depend on honed field skills developed over many years. Marine environments can be very dynamic, and food abundance can vary according to the local conditions, making population estimates even harder. Knots away from the breeding grounds spend almost all of their lives on the coastal edge, where ocean meets the land. Of all the survey methods, the most common is still just looking, using binoculars and spotting scopes to count knots from the ground. Or, watching them fly to estimate the total number of birds in large flocks.

Along with counting the knots, the ability to see and “read” a small marker on a bird’s leg becomes a valuable aspect of observation. About ten percent of rufa knots wear small plastic markers called “flags” that have been put on their legs by researchers. The majority of these flags have been placed on knots in the Delaware Bay during their northbound migration in May.

A faithful crew of international observers has been capturing, tagging, and resighting leg-banded knots for several decades. Biologists like Nigel Clark and Phil Atkinson, who have represented the British Trust for Ornithology; the intrepid Larry Niles and Mandy Dey leading efforts in New Jersey; and Henrietta Bellman, who is currently leading shorebird conservation activities for the State of Delaware–these are just some of the experts leading the work. Many people, including volunteers, work throughout Delaware Bay each spring to find birds with bands and share those unique letter and number codes in a database for all researchers to use. Other biologists throughout knot migration paths are counting the birds and recording the band codes in far-flung corners of the Western Hemisphere, cooperating to build a broad understanding of how these birds use habitats around the globe. The collective effort is helping to steer conservation decision-making.

Yves Aubrey is another one of the dedicated surveyors of rufa knots. He has been working for the Canadian government (Environment and Climate Change Canada) for more than 40 years and is considered one of Canada’s leading shorebird scientists.

He has a unique and rare front row seat to witness Red Knots as they depart Canada. The southbound birds find abundant food resources on the tidelines of the Mingan Archipelago, a string of islands hugging the southern Quebec coast. Yves and his colleagues are there each year to greet them; the biologists carry out intensive ground surveys several times a week from July through October.

“We usually count the birds at high tide on two to three different islands known to host Red Knots, with two to three observers [simultaneously] patrolling the shoreline of those islands,” he says. They boat between islands, maneuvering through the surf to get to the roosting sites where the birds are concentrated. Yves is able to witness adults as well as juvenile birds at this important stopover site. He counts individual knots to come up with a total, and records all the observations throughout the migration season.

The 2021 highlight for him was recording some of the highest numbers of juvenile knots seen since 2006, including one flock of 950 first-time migrants. Incredibly, the one young Red Knot photographed in Paraguay in September could have been among the same juvenile knots tallied by Yves in Quebec in August.

South of Quebec, on Cape Cod, long-time knot researcher Brian Harrington, Senior Scientist Emeritus at Manomet, counts southbound knots stopping over on the Monomoy National Wildlife Refuge and nearby Pleasant Bay. Brian has spent 50 years studying knots in eastern Massachusetts, and he initiated the International Shorebird Survey (ISS) in the 1970s. The ISS, a large-scale, volunteer-based citizen science program, is still coordinated by Manomet today and it is the world’s most significant resource for long-term shorebird population analysis. Like Yves, Brian’s surveys are conducted at high tide in late summer and fall as birds are fattening up to make their next jump south. However, based on the unique leg bands they wear, we know the birds observed in Quebec are not the same ones arriving in eastern Massachusetts–they are clearly different groups within the same rufa subspecies. Like Yves, Brian says his highlight of 2021 was counting large numbers of fresh young knots. Brian was out on the beach of Monomoy with Alan Kneidel, Conservation Biologist at Manomet, and they estimate that they saw more than 300 juveniles in a flock of 600 total–a really healthy ratio because it suggests that many knot pairs nested successfully in the Arctic.

But what if someone can’t get to a place where knots are gathered to count them, or even understand where the birds might be? Getting into the air in a plane or helicopter to scan large areas can be handy and is used regularly at some sites that are just too vast or remote to count from the ground. Red Knots pile up in surprising numbers in very densely-packed sites where the food they seek is also concentrated in great abundance and available for the birds. Sometimes the best way to locate these knot hideaways is to fly large sections of coast until… bingo! A single flock of several thousand birds is suddenly found. Counting from the air, however, has its own challenges. Observers need to be highly skilled, and able to work in several dimensions at once. Factors like a moving aircraft and maintaining its flight at very low altitude, with one flock on the beach and another flock flying above the plane, the sun position and its glare off the water, plus bouts of air sickness can make identifying and estimating knot numbers very hard.

Repetitive surveys over consecutive days improves the accuracy of an estimate, and importantly, having decades of experience counting shorebirds from the air may be the best foundation for success. But experienced observers are rare.

Guy Morrison has been observing shorebirds from small aircraft over South America’s coasts since the early 1980s, joining fellow observer Ken Ross on flights that circumnavigated the entire continent. In more recent years, Guy has been applying his skills intensively within the destination of rufa knots during their nonbreeding season: Tierra del Fuego at the southern tip of South America.

In 2020 and 2021, Guy, a Canadian, worked with Chilean biologists Ricardo Matus, Carmen Espoz, and Heraldo Norambuena to census knots on the vast intertidal flats of Bahia Lomas. They conducted the survey about 150 feet (50 meters) over the wet ground by plane or helicopter. Bahia Lomas, a sprawling, sandy mudflat 26 miles (44 km) across with 40 miles (64 km) of coastline, is where the Straights of Magellan meet the Atlantic Ocean. This immense, wind-blasted mudscape hosts some of the same rufa knots that Yves Aubrey saw leaving the Canadian Atlantic, and others birds recorded by Brian Harrington on Cape Cod. In 2020, the Bahia Lomas survey was run twice, and both estimates for Red Knot counts were just under 12,000 birds. The results were nearly identical in 2021 with 12,243 knots tallied. These numbers are not unlike numbers estimated at Bahia Lomas over the last 15 years, but they are shockingly lower than the 40,000-plus knots that were typically recorded at the same location during the 1980s and 1990s.

The conservation community is embracing advances in statistical modeling in search of big-picture answers like finding the overall global population of rufa knots. Jim Lyons, a research ecologist with the Eastern Ecological Science Center of the U.S. Geological Survey–and a former Manomet research intern from 1989 to 1991–has been estimating knot populations at key stopover locations for more than a decade.

Jim is a pioneer, leveraging specific observations collected by field biologists to develop statistical computer models of knot populations. His work goes beyond just knot numbers, capable of teasing out nuanced ecological bits like turnover (how long a knot might stay at a location), how many birds use a site during one migration season, and even the ratio of knots stopping at a location from different groups of the rufa subspecies.

Working with the data generated by everyone working in Delaware Bay in the spring migration of 2021, Jim determined that 1,591 individual knots were detected from leg bands placed on the birds by researchers in five countries: Chile, Argentina, Brazil, the U.S., and Canada.

The 2020 number of individually-marked knots was 1,587, a remarkably similar tally.

Jim plugged all this relevant resight data into his computer models and then let those analyses grind away. A full day later, the model’s result, known affectionately as a “super population,” came up on Jim’s computer screen. It revealed the estimated number of rufa knots that came through Delaware Bay during the 2021 survey period to be 42,271. That’s very similar to the 40,444 knots estimated during the 2020 spring staging period in the same area.

While it’s not representative of the entire rufa subspecies of Red Knot (not all knots pass through Delaware Bay on the way to their breeding grounds in eastern Arctic Canada), it is considered a benchmark for how the population is doing. The good news, as a result of comprehensive fieldwork in both Chile and Delaware Bay, is that the long-beleaguered rufa knots appear to have maintained their numbers between 2020 and 2021. With strong counts of juvenile birds returning south this year, we may even see their numbers begin to grow.

We know quite a bit about rufa knots, and thanks to a growing group of passionate conservation biologists applying vastly-improved scientific technologies along the knots’ flyways, we are continuing to learn more about these birds all the time. We are fortunate, however, that shorebirds still have so many mysteries to keep us all intrigued and focused. A single juvenile knot that flies down to central Paraguay adds to the intricate ecological puzzle shorebirds present to us and can also reinspire our commitment to uncover more knowledge in hopes that we can respond, working across borders to maintain the conditions that support the birds throughout their migrations.