In Bermuda, the signs of the season are changing

White-tailed tropicbirds spend most of the year at sea, out of sight of people. When the seabirds make their annual descent into Bermuda to breed, though, their long, streamer-like tails, snowy white plumage, and shrill calls are impossible to miss. To locals, the birds’ return to the islands is a reliable sign of spring.

But long-term records kept by birdwatchers hint that white-tailed tropicbirds, known locally as longtails, have been turning up in Bermuda earlier and earlier. Instead of being heralds of spring, these birds are potentially becoming harbingers of changes occurring in the nearby ocean, according to new research.

The study found that since the 1950s, the average date of the first longtail observation in Bermuda has advanced by 20 to 25 days. Seventy years ago, tropicbirds arrived in mid-March; today, they’re showing up in mid-February.

It’s a striking result, and the study's lead author believes climate change is the only factor that could cause such a dramatic shift.

McGill Department of Natural Resource Sciences Professor Kyle Elliott, who was not involved in the research, agrees that if the birdwatching data reflects a real shift in the birds’ behavior, the cause must be something related to the climate. “Presumably, they’re responding to either something physical at the nest site [in Bermuda],” he says, “or, more likely, to something happening at sea.” Elliott says earlier phytoplankton blooms would attract the fish that the tropicbirds eat, which would prompt the birds to arrive earlier, too.

Given their sensitive position at the top of the local food chain, tropicbirds may be the proverbial canary—or seabird—in the coal mine, signaling changes in the marine ecosystem. The authors hope their study will prompt scientists to pay greater attention to how the iconic species is responding.