BROWNSVILLE, Texas—Eleven months and 12,000 miles after leaving their home in the Yukon Territory, Ken Madsen and Wendy and Malkolm Boothroyd pulled into Brownsville on bicycles.

In June, the Canadian family began their cycling trip from one corner of North America to another. It started as Malkolm’s “Big Year”—a quest to see as many species of birds as he could in 365 days. Even before the family left on the trip, Boothroyd’s goals had grown loftier.

The Big Year, which Madsen, 16, calls “the Super Bowl of birding” has long been a staple of the American Birding Association. But Boothroyd made an important amendment to the tradition — he committed to completing the pursuit without using fossil fuel.

With that ambition in mind, three inexperienced cyclists completed the first rotations of what will soon be a 13,000-mile journey.

“We’re particularly aware of climate change ... and its affect on the birds we’re trying to see,” Madsen said.

Sandy Komito, who set the record for the most successful big year, saw 745 species of birds in 1998. Komito flew and drove more than 240,000 miles in that year.

As of May 15, Boothroyd and his parents had seen 520 species of birds.

But the birds—and the 12,000 miles of cycling—have not always come easily. In Pensacola, Fla., the family slept in a graveyard in order to avoid a local drug dealer. In Albuquerque, N.M., they biked into a 30-mph headwind when the temperature was below freezing. In Alice, another South Texas town, they were surprised by security guards while trying to sleep in a Wal-Mart parking lot.

“You realize,” Wendy Boothroyd said, “that the only people you could do this with are family members.”

Despite often-precarious situations, the high school junior has no regrets.

“When you drive through the country in a car, you’re not really a part of it,” he said. “On a bike, you can really participate in everything.”

Birders might take wildly different approaches to their Big Years, but a visit to Brownsville is a near requisite.

“There are some birds that you can only see in Brownsville,” said Madsen, a former writer and photographer who is now semiretired. Wendy, his wife, is a substitute doctor in their native city of Whitehorse.

The family is now on the last leg of the trip, which will conclude near Big Bend National Park. But before they return home, they’ll have to brave some narrow South Texas roads, where bike paths are a rarity. They’ll have to make it between Laredo and Carrizo Springs, where there are few places to stop for food, water or sleep.

If anyone can offer help, they ask to be contacted through their Web site, birdyear.com

The family spent much of their time in Brownsville at Sabal Palm Audubon Center, where Boothroyd spotted species number 520, a ringed kingfisher.

Boothroyd added the bird to his list, which he stuffed in his bike’s bursting panniers. Not long after, the family was off, slim visages on a long stretch of asphalt.