Migrations and Murmurations

Lichen Songs, #2

It was low tide and the vast mudflats and saline marshes of Shepody Bay were teeming with shorebirds foraging for tiny worms and crustaceans. After breeding in Canada’s Arctic, the diminutive but long-legged wader makes a single stop here in Shepody Bay at the head of the Bay of Fundy, to feed before flying non-stop to South America, and they come in the tens of thousands.

This area, near Dorchester Cape, New Brunswick—referred to as a staging ground in bird migration lingo—is also famous for having the highest tides in the world. When the tide is all the way out, the mudflats can be more than 2 km wide in some locations, and chalk full of macro-invertebrates, the preferred food of the species. For about six weeks in August and September, it’s estimated that more that nearly 270,000 semipalmated sandpipers, as well as short-billed dowitchers, sanderlings, and semipalmated plovers, stop here and we had come to witness the phenomenon.

Semipalmated sandpipers near the Nature Conservancy of Canada Shorebird Reserve and Interpretive Centre (Source)

Diana Hamilton is a biology professor and field ecologist at Mount Allison University specializing in mudflat and shorebird ecology. The university is located in Sackville, New Brunswick, about a 20 minute drive from the Shorebird Reserve and Interpretive Centre at Johnson’s Mills. In recent years Hamilton has been focused on tracking shorebird movements and habitat use, as well as the effects of severe weather on the species. In collaboration with others, Hamilton is also studying the risks posed by microplastics and mercury and the effects of other “forever chemicals” on the species.

The bird that Hamilton’s team has collected the most data for is the semipalmated sandpiper. In an interview, Hamilton tells me something I find astonishing. The species breeds in the Arctic and the parents and offspring make their way to Shepody Bay, and eventually to various destinations in South America, separately.

“The parents leave the breeding grounds first and the juveniles stay behind and will get a little bit bigger, and then they set off. So, [the juveniles] are often 2 to 4 weeks behind the adults,” explains Hamilton.

Hamilton says her research shows that juvenile birds also tend to move around a bit more while they're in Shepody Bay. “It's like they're sampling the habitat because they're new foragers and not as experienced,” and she adds, “because they’ve never been here.”

This is the part of the story that kind of blows my mind. How do these birds know where to go if they’ve never been there before?

Diana Hamilton is a biology professor and field ecologist at Mount Allison University specializing in mudflat and shorebird ecology. (Source)

According to Hamilton, there are a number of theories on migration and lots of different ways that birds navigate. She says it depends on the species and that they probably use a “combined set of cues,” but she says “it’s got to be hard wired into them because they’ve got nobody leading them. Basically, they have evolved to follow this migratory path.”

Hamilton:

I'm not going to say they use this or they use that because I don't think we can 100% know that, but there have been numerous studies on lots of species looking at how birds orient themselves. There are some that use the stars if they migrate at night, the orientation of constellations. There are some that use geomagnetic fields. They can detect those. They can tell time a little bit. They can kind of get where they are, based on the sun if they're migrating during the day. There are so many different strategies that birds will use to migrate, and I think it’s safe to say they’re using all of these mechanisms. I'm aware of studies where they kidnap birds and translocate them from one area of the continent to another and look at how they re-orient and things like that. But it's not been done with the shorebirds.

Mudflats at Shepody Bay, New Brunswick. If you look carefully, all the small dots are sandpipers. Photo: Linda Pannozzo

Hamilton says that while the two-month-old juvenile birds are fully independent, they are lighter than adult birds, which “introduces some challenges for them in this region because one of the things we're working on now is the effects of storms and severe wind on birds in their habitat.”

“We know that the migration season for shorebirds overlaps quite heavily with the Atlantic hurricane season, and we also know that it's probably even a bigger deal for the juveniles because they arrive a little bit later—they're into September a lot of them. We've done some studies on that and we found that that it has a very substantial effect on them. We did a kind of a semi-planned study with Hurricane Fiona and the effect was quite large.”

The storm forced them to stay longer and they lost a lot of weight, she explains.

Did it result in them not making it to where they needed to go, I ask?

“Who knows?” says Hamilton. “We have tracking tags on these birds—it’s basically a huge network—we have receivers all over the world, and the birds carry these little radio tags—they're very small, they weigh half a gram or a little more than half a gram—and when they fly past the tower they get picked up. It is dependent on them flying past a tower, and the towers are more sparse in the south, so we can't be sure. If they get picked up [by the towers], we know they arrived. If they don't get picked up, we don't know they didn't arrive.”

Hamilton says that a couple of the tagged birds probably died as a result of the storm because they stopped transmitting and were never seen again anywhere, while others went missing for a while.

“After the storm happened, a bunch of birds disappeared for about a week and then they came back and we think they probably got blown away and then they reappeared a week later and then they started from scratch gaining weight. So [these storms] are a big deal. Juvenile birds are vulnerable and mortality at the juvenile stage is quite a bit higher than it is at the adult stage. That's normal.”

Semipalmated sandpiper murmuration. Photo displayed inside the Interpretive Centre.

If there was one thing we were all hoping to see while we stood on the observation deck at the Interpretive Centre, it was a murmuration—the swirling, undulating movement of thousands of birds in flight, the spontaneous changes in direction, astonishing precision, and sudden shifts between light and dark, much like the movement of a school of fish. The Danish call it sort sol, translated to black sun, because the grouping of birds, often at dusk, can be so large—numbering into the hundreds of thousands—that it blocks out the sunset.

But for the hour or so that we watched, there was only one brief uplift and quick descent far in the distance.

In her 2020 book of essays, Vesper Flights, author Helen Macdonald reflects on a starling murmuration:

“The changing shape of starling flocks comes from each bird copying the motions of the six or seven others around it with extreme rapidity; their reaction time is less than a tenth of a second. Turns can propagate through a cloud of birds at speeds approaching ninety miles per hour, making murmurations look from a distance like a single pulsing, living organism.”

I ask Hamilton what the “purpose” might be of the phenomenon. “Somehow they're coordinating it. If you're dealing with predators and you have a big take off like that, it's going to confuse predators. They do occasionally get attacked by peregrines and things like that. So that's a possibility.”

Hamilton says there are many reasons why sandpipers flock together and she says these probably also help explain the murmurations.

There are lots of advantages to being in large groups if you're a bird that wants to avoid getting eaten because there's something called the dilution hypothesis, which basically says your individual chance of getting eaten is probably lower because [a predator] is not going to eat 10,000 sandpipers. There's another thing called the “many eyes hypothesis,” which basically means shared vigilance. So, you don't have to work as hard to look for predators because somebody else is going to notice.

Folks gathered on the observation deck of the NCC Shorebird Reserve and Interpretive Centre. Photo: Linda Pannozzo

The vast majority of shorebird populations are declining, says Hamilton. “We certainly see many fewer sandpipers in Atlantic Canada in the Bay of Fundy in particular today than we did, say, 40 or 50 years ago. There's probably half as many.”

A migrating species like that has a breeding area and a non-breeding area and a stopover area. If something goes wrong at any of those spots, it's a problem. So, conserving a migrating species is harder because let's say something goes horribly wrong at the breeding grounds. Well, the population's going to decline and it doesn't matter if the wintering grounds or the staging grounds are great, you're still going to lose them. So, you can lose them at any point. Migratory species in general are more vulnerable because they need different habitats and we have to think about their whole range.

Hamilton says that in the 1980s and 1990s individual sandpipers would stay in the Bay of Fundy for roughly two weeks, whereas now they are staying for three. They are also leaving the Arctic later than they used to, something she says may be connected to climate and mismatches taking place there. “For example, if food becomes available at a time different than when birds are breeding, that's a problem,” explains Hamilton.

Mudflats near the Shorebird Reserve and Interpretive Centre at Johnson’s Mills. Photo: Linda Pannozzo

On top of all that, Hamilton says the semipalmated sandpipers depend on prey populations that can be “quite variable.”

“We used to think that these little amphipods called Corophium were their main prey and why they were here,” says Hamilton, but Corophium populations are “pretty volatile” and in some areas have been replaced by polychaete worms, which are also great food for the sandpipers.

“The change in the prey base, coupled with a later arrival, coupled with more predators, coupled with exposure to storms—all of these things are probably joint challenges that they face right now. But when it comes right down to it, I think they're still doing reasonably well here. Our habitat is pretty good, and if I had to guess, I would say we may not be the biggest source of the challenges for them. In South America there's illegal hunting of some species, and that's an issue. There's habitat degradation there too.”

When we visited the Interpretive Centre, roughly 260 shorebird biologists from twenty countries, many in Central and South America were also gathering at Mount Allison University for the 10th Western Hemisphere Shorebird Group meeting, which happens every second year—the next meeting in 2026 is in La Paz, Mexico. The Shorebird Group, which includes researchers, conservationists, students, and enthusiasts, formed in 2006 with the stated goal to “promote pan-hemispheric discussions and actions for the conservation of shorebirds and of coastal ecosystems.”

The Sackville event was obviously timed to coincide with the shorebird migration, and Hamilton was involved in co-chairing the week of meetings and workshops. She says there were working groups for numerous shorebird species and that eventually all the abstracts presented will be collated in an online publication.

Hamilton says there is something captivating about the sandpipers epic journey from the Arctic to the Bay of Funday, and then to various destinations in eastern South America including Surinam, French Guiana, and Brazil.

It’s an annual phenomenon that attracts thousands of visitors a year to the Interpretive Centre’s observation deck. Hamilton says that’s because the spectacle is nothing short of “remarkable.”

“These sandpipers arrive here at 20g, and roughly double their weight and then migrate south nonstop. It's been equated to running a four-minute mile for 90 hours. The physiological preparations are something we as humans could never in a million years do. These are little superstar athletes and I think people are captured by that.”

I know I am.

[This is part of an ongoing and occasional series titled Lichen Songs]