Lord Howe Island — a speck of land about 370 miles off mainland Australia, home to just a few hundred people — is the breeding ground for tens of thousands of sable shearwaters: dark brown-colored, long-winged ocean birds with strong hooked bills.

Scientists from the ocean research group Adrift Lab have been visiting for nearly two decades to monitor these birds’ exposure to plastic pollution. Every year they find more contamination, but this year was shocking, said Jennifer Lavers, a marine biologist and coordinator of Adrift Lab, who recently returned from the island.

Shearwaters were found with levels of plastic far exceeding anything the scientists had seen before. They discovered an extraordinary 778 pieces of plastic inside one chick alone, smashing the previous record of 403 pieces.

It “left us all speechless,” Lavers said. The scientists are now trying to solve the mystery of why this year was so bad. Plastic pollution is on the rise but “does that explain a doubling in 12 months? Absolutely not,” she told CNN. “So there’s something else going on.”

Seabirds are often referred to as sentinels for ocean health, and the story they’re telling is alarming.

Global populations have declined 70% over the last 50 years as they grapple with multiple threats, including from invasive species, the fishing industry and climate change.

Plastic pollution is yet another danger and a particularly “insidious” one as its impacts are so hard to detect, said Richard Phillips, a seabird ecologist at the British Antarctic Survey.

Lord Howe Island offers a unique natural laboratory for studying seabirds.

The shearwaters reliably come back to the same breeding colony each year, usually within inches of the same burrow, allowing scientists to track individual birds’ progress. “There’s nowhere else in the world that I can think of where we could do a study like this,” Lavers said.

The scientists visit every April and May, when the chicks are leaving their burrows for the first time and preparing to take their first big migration to the Sea of Japan.

Shearwaters are nocturnal, so every dawn, the scientists go to the beach to find chicks that were too weak and emaciated to make the flight. They bring them back to the lab to examine them. “We often see high levels of plastic in these birds,” said Alix de Jersey, a researcher at the University of Tasmania.

The scientists return to the beach at night to analyze the healthier birds getting ready to fly. They “lavage” them using a feeding tube, gently pumping water into their stomachs to make them vomit up the plastic.

The process may not be pleasant, de Jersey said, but “it’s just fantastic knowing that that bird is starting its migration without this huge load of plastic within its stomach.”

Most of the plastic found in the birds this year was made up of unidentifiable fragments but they also found bottle caps, tile dividers and large amounts of plastic cutlery, de Jersey told CNN.

The plastic accumulates inside the birds’ bodies and can form a kind of brick. The pollution is so crammed into some shearwaters, it’s audible. “You can hear the crunching of the bottle caps and the shards and things shifting and moving against each other,” Lavers said.

The scientists believe most plastic is ingested due to parents accidentally feeding it to their chicks, instead of the fish and squid that make up their usual diet.

Plastic may smell good to birds because of the algae that can coat it, said Matthew Savoca, a marine ecologist at the California Marine Sanctuary Foundation and Stanford University. “Other times, birds may eat something that has already eaten plastic,” he told CNN.