“It represents a true population increase in wintering hummingbirds here,” says Remsen. “You have to go one thousand miles into Mexico to get the next wintering population.” For him, the question is one of provenance: where are these birds coming from? A number of hypotheses have been proposed. Some biologists think that hummingbirds might be moving to new environments because of deforestation and habitat loss at their old wintering grounds in Mexico. (Remsen doubts this. “In general, wintering birds’ habitat requirements aren’t as rigid as breeding birds,” he says. “As long as there are flowers and bugs, they’re fine.”) Or, climate change might be responsible in some way: winter temperatures in the southeast have risen almost 2 °F in recent years, so the region is not so prohibitively cold; and climatically-driven range shifts are well-documented in many species. Or, more intriguing still, the hummingbirds might have been buzzing about in low numbers all along, and people are only now starting to notice them. But no one is certain.
Whatever the case, Remsen sees a cycle at work. When people started seeing more hummingbirds in the winter, they started leaving their feeders out year-round. This led to more birds, eager to take advantage of the food supplement, which led to more feeders. Now, during the winter, hummingbirds turn up in the Washington, D.C. area, or as far north as Massachusetts. With more birds surviving, Remsen thinks, more are going north. “A hummingbird’s life is geared towards ephemeral resources,” he says. “They’re built to wander. And they’re tough as nails.”
For Newfield, what was intended as a five-year study has extended by almost three decades, but she has kept and will continue to keep busy tracking hummingbirds. “What’s really going on after 35 years, God only knows,” she says. “But come July and August, we start to wait for the first rufous to show up, and I’m having way too much fun to throw in the towel.” She watches for news of the first migrants from Colorado, from Arizona. Who knows? she says. Maybe one of the hummers she catches next winter will be another of Dan Harville’s birds.
Back in Washington, after a few hours at Lunemann’s, Harville has had enough banding for one morning. “I usually run out of energy before I run out of birds,” he says. Still, he has time for one more. He withdraws another female—most of the adult males have left the area by now, so almost all the birds today have been females or juveniles—and takes her vitals. When he has finished, he holds out his hand, fingers stiff, palm flat. The hummingbird doesn’t move, its wings pressed to its side.
“She doesn’t know she can go,” Harville says softly. “Sometimes we have to give her a little push.” He jounces his hand just a little. The hummingbird twitches, and its wings flick out from its body like little spring-loaded blades, and in a flash it is off, whipping away through the trees.