A mysterious visitor is haunting a coastal Maine island. Like most clandestine strangers, he prefers to avoid the limelight and is seldom seen. But he is faithful to his summer residence, returning every season for the last nine years.

He is a red-billed tropicbird.

As the name implies, he should not be here. Tropicbirds are tropical, mostly. There are three species within the tropicbird family, but none ventures too far from the equator. The red-billed tropicbird breeds in scattered colonies throughout the equatorial belt. There are significant nesting sites in the Galapagos off the coast of Ecuador. The Cape Verde Islands in the South Atlantic are also home to many. Other colonies are scattered across the globe in the Caribbean, the Pacific, and even the Persian Gulf. It’s hard to estimate how many there are in the world, but there aren’t many — perhaps a mere 7,500.

Tropicbirds are white and resemble terns, but they are not closely related. In fact, science has had a hard time figuring out if they have any close relatives at all. They were once lumped with pelicans and cormorants. More recently, taxonomists have decided that their descent from a common bird ancestor occurred so long ago that they are a unique family with no other relatives.

Tropicbirds are larger than our local terns and have long, streaming tail feathers. The red, down-curved bill gives the red-billed tropicbird its name. I will leave you to guess how the white-tailed tropicbird and the red-tailed tropicbird got their names.

Tropicbirds are awkward on land, like loons. Their feet are located so far back on the body that they cannot walk. All they can do is push forward on their bellies. However, they are aerial acrobats, often hovering over the ocean like a tern and plunging beneath the surface to snatch a squid or fish. Flying fish are their favorite food, which they sometimes snap out of the air while gliding over the ocean. Maine waters are too cold for flying fish.

It doesn’t add up. Why would a rare bird with a taste for flying fish and an aversion to cold water keep returning to Maine every summer? He has a favorite roosting spot on Seal Island, which is a major seabird nesting colony just a few miles southwest of Isle au Haut. Perhaps he enjoys the company of the other seabirds there, but they don’t seem to like him. He harasses the terns regularly and they respond in kind.

A couple of years ago, a tropicbird wood carving was placed near his favorite roosting spot. He romanced the decoy through aerial displays and attempted copulation, so we know he is a male and he is interested in breeding. But if he is looking for love, he is not going to find it here. The mystery deepens.

The chance of viewing this enigmatic vagrant has added extra pizzazz to puffin tours that visit Seal Island. In the last month, I’ve been out twice with the Isle au Haut Ferry, volunteering as spotter for the crew. I missed him both times, which was not a surprise. The tropicbird spends much of the day wandering the ocean in search of food. He tends to be around the island only in the morning and evening. I am also aware that he usually roosts out of sight. My odds of seeing him are miniscule.

No matter. A visit to Seal Island is rich in puffins, razorbills, black guillemots, eiders, common and arctic terns, double-crested and great cormorants, and the potential for other wandering seabirds. It’s also hard to beat the scenery. Once the ferry leaves the dock in Stonington, it skirts multiple islands, brushes past Isle au Haut, veers near several ledges loaded with seals, and eventually charges out to sea. Every moment is a Kodak moment. (Remember Kodak? I think they used to make cameras.)

The Isle au Haut Ferry is planning two more visits to Seal Island this summer. For tickets and information, visit www.isleauhaut.com. Local naturalist Kathie Fiveash will be on board for the ferry’s Aug. 3 visit. I’ll be the spotter on the upcoming July 20 voyage. This will give me one more chance to fail at seeing the red-billed tropicbird. I realize that failure often builds character. To be honest, I think my character is strong enough already. I’m willing to risk feeling successful for a change.

Bob Duchesne serves as a Maine Audubon trustee and vice president of its Penobscot Valley Chapter. Bob developed the Maine Birding Trail, with information at mainebirdingtrail.com. Bob can be reached at duchesne@midmaine.com.