Western sandpipers stop at this B.C. mudflat to fuel up during their 10,000-km migration

Western sandpipers are tiny shorebirds that weigh about as much as a slice of bread. Every spring, they migrate from their wintering grounds as far south as Peru to breeding sites in Alaska and Siberia. They stop to refuel at a few key places on the journey (which can be more than 10,000 kilometres long), including the intertidal mudflats of Roberts Bank near Vancouver.

Roberts Bank provides one of the last meals for the birds on their journey north — the next major stopover is the Stikine River estuary, more than 850 kilometres away. 

The documentary Sandpipers' Last Supper, written and directed by Isabelle Groc, captures tens of thousands of western sandpipers as they blanket Roberts Bank. It explores why this place — a mudflat next to GCT Deltaport, a large shipping-container terminal — is vitally important to their survival. 

"If this mudflat wasn't here and it wasn't in this condition, then the birds wouldn't be here," Adam Ross, a 14-year-old birdwatcher, says in the film.

It's estimated that 42 to 64 per cent of the global western sandpiper population relies on Roberts Bank to rest and refuel, and that almost all western sandpipers will use the site at least once in their life.

Why the western sandpipers stop at Roberts Bank

For decades, scientists believed western sandpipers were snacking on small marine invertebrates found in the mud. 

But in the 1990s, Bob Elner, a scientist emeritus at Environment and Climate Change Canada, started questioning the theory.

"I spent many, many hours out on Roberts Bank, just watching western sandpipers, realizing that we had a huge enigma here," he says in the film. 

The mud at Roberts Bank has little oxygen in it and is, for the most part, devoid of visible invertebrate food for birds. So Elner and his colleagues began to investigate what they could possibly be eating.

When they looked inside western sandpipers' stomachs, they were virtually empty. 

"All I could see in the stomachs was some sand particles and a fair amount of liquid," Elner says. "So the mystery deepened."

Next, they looked more closely inside the sandpipers' bills and, to their surprise, found the birds had a very hairy tongue — "like a mop," Elner says.

Using an electron microscope to look between the hairs, they found microscopic organisms called diatoms.

It turns out the birds were stopping at Roberts Bank to slurp up a special snack: an intertidal biofilm that coats the surface of the mudflat every spring, just as thousands of shorebirds arrive at the stopover site. The biofilm is packed with omega-3 fatty acids produced by the diatoms. 

This is the energy drink the western sandpipers need to complete their journey to breeding sites in Alaska and Siberia. "No other bird we know of feeds on biofilm," Elner says.

Roberts Bank at risk

Roberts Bank is the last remaining large intertidal mudflat in the Fraser River estuary that provides this biofilm.

However, the site is targeted for a major port expansion, which could compromise the biofilm essential to millions of migrating shorebirds.

"With the threats from the port development, this bank will be extinguished ecologically," Elner says. "There's no known way to recreate biofilm this productive … If it goes, that's the end of this as a major stopover. The birds will suffer very negative consequences. That species would be headed for extinction."

The loss of Roberts Bank would affect the broader ecosystem — including orcas, chinook salmon and other species — and residents in nearby Tsawwassen First Nation.

"It's got the largest crabbing grounds that is readily available, closest to us," says Tsawwassen band member Steven Stark. "This is historically our fishing and crabbing and our harvest grounds.… The estuary, it's like a living, breathing organism out there. It kind of works together to survive and protect."

In the documentary, The Nature of Things host Sarika Cullis-Suzuki joins Elner and Ross on the Roberts Bank mudflats to learn more about the western sandpiper migration and the importance of intertidal biofilm to the birds' survival.

Watch Sandpipers' Last Supper on CBC Gem and The Nature of Things YouTube channel.