"We learned that if you're going to look at plants along the tracks, look at them briefly, always have a lookout and be ready to run into the woods," Miller-Rushing says.
In the spring of 2004, they began reprising Thoreau's work in earnest. With the help of several undergraduates, Primack and Miller-Rushing combed the warmest places in town. As they navigated crowds of tourists at Minute Man National Historical Park or stepped around the sunbathers at Walden Pond, they found they had a lot in common with their quirky collaborator. "We'd come out of the woods, sometimes covered with mud, and start asking people if they would move their towels so we could see the flowers," Miller-Rushing remembers. "That's when we realized that we weren't normal people."
What they discovered wasn't quite normal, either. Primack and Miller-Rushing compared three years of their results with those of Thoreau and Hosmer, focusing on the 43 plant species with the most complete records. They learned that some common plants, such as the highbush blueberry and a species of sorrel, were flowering at least three weeks earlier than in Thoreau's time. On average, they found, spring flowers in Concord were blooming a full seven days earlier than in the 1850s—and their statistics clearly showed a close relationship between flowering times and rising winter and spring temperatures.
Primack and Miller-Rushing also found other naturalists who had carried on Thoreau's tradition of obsessive observation. Robert Stymeist, a retired accountant and devoted birder, frequents the trails of Mount Auburn Cemetery in Cambridge, a shady, arboretum-like spot that attracts colorful waves of migrating birds each spring. Stymeist, 59, has been watching and recording them for almost as long as he can remember: when he was just 10 years old, too young to be trusted with a key to the cemetery gates, he began sneaking into the grounds, binoculars and bird guide in hand. "It's just always been my spot," he says.
The ecologists' quest also led them to Kathleen Anderson, a great-grandmother and lifelong birder, who has lived on a wooded property south of Boston for nearly six decades. Born in rural Montana, she remembers that her mother rewarded her and her siblings for spotting the first bluebird or daffodil, inspiring a record-keeping habit that Anderson, now 84, continues to this day. Her elaborate daily diaries, shelved in her low-ceilinged farmhouse, detail not only family weddings, births and the news of the day but also natural phenomena ranging from bird arrivals to frog choruses to the newest blooms in her yard. "I guess I'm an old-fashioned naturalist—I'm curious about everything," she says. "But I never in my wildest dreams thought that these records would be of any significance. I even wondered if my children would be interested in them."
Like Thoreau's data, the records of these naturalists were idiosyncratic and tricky to analyze. Amateurs don't usually record exactly how long they searched for an animal, or how many people were looking, or how certain they were about what they saw—and these gaps make professional scientists nervous. "Scientists are used to analyzing other scientists' data," says Miller-Rushing. "We're not so comfortable venturing into the world of personal journals."
But Primack and Miller-Rushing found that the bird records from Mount Auburn, Anderson's diaries and data collected by trained researchers at the Manomet Center for Conservation Sciences on the Massachusetts coast all told a similar story. On average, migratory birds are turning up earlier every year in eastern Massachusetts. And as with the precocious blooms in Concord, the shifts in schedule are best explained by warming temperatures.
Even in the mythic American landscape of Concord, global warming is disrupting the natural world. Since Thoreau's time, average temperatures have risen more than four degrees Fahrenheit because of local urban development as well as global climatic warming. Concord, once a farming community, is now a busy suburb—Boston is just a half-hour drive from Walden Pond—and expanses of warmth-absorbing concrete and blacktop have created a "heat island" of higher temperatures in the greater metropolitan area.
Seasonal routines such as migration, blooming and breeding are the pulse of the planet, and everything from agriculture to allergy outbreaks depend on their timing—and, often, their precise coordination. "Pollinators have to be around when plants are flowering, seed dispersers have to be around when seeds are available, leaves have to be around for herbivores to eat them," says Miller-Rushing. "There are endless numbers of these relationships, and we don't have a lot of good information about what happens when their timing gets jumbled up."
While some flowers in Concord, like the bluets in Sleepy Hollow Cemetery, are blooming weeks earlier than in Thoreau's time, others haven't changed their schedules. Observations from Thoreau and other naturalists reveal that plants are reacting to temperature changes more dramatically than short-distance migratory birds, suggesting that climate change could divide plants from their pollinators. Spring's acceleration is far from orderly.