Rufous Hummingbirds Turning Up in Unusual Places
Is climate change the reason, or are birders just paying closer attention?
A quantifiable shift in migration offers an exciting opportunity, says Scott Weidensaul. The boyish-faced bird bander and author is taking a break at the same Alabama site where Bassett is gathering spring migrants. Crouching on the sandy ground, Weidensaul says the potential shift of the rufous and others "shows the surprising degree of plasticity in bird migration--that's a hopeful thing given the decades ahead of us."
Weidensaul notes that the blackcap warbler traditionally migrated from central Europe to the Iberian peninsula, then south across Gibraltar to Africa. But since the 1950s British birdwatchers have observed an increase in blackcaps wintering in the British Isles--something rarely observed before. Today an estimated 10,000 blackcaps swarm to Britain rather than Africa. For years researchers wondered whether this shift was due to chance, environmental changes, or an alteration in the birds' genetic code itself. German and U.K. scientists finally resolved the issue when they caught 40 "British" birds and bred them with one another, while doing the same for birds that wintered in Africa. When given the chance to fly, the "British" blackcaps headed for London, while the others went in the direction of Madrid. Today Britain's milder climate, increased food sources, and lack of competition make it a smart destination for blackcaps, which passed on that information encoded in their genes.
Weidensaul suspects something similar may be taking place with western hummingbirds. Several centuries ago any that strayed too far east in the winter encountered dense forest cover and colder winter temperatures. But those conditions have been superseded by a plethora of gardens and feeders that increase the probability that errant birds will survive. "Once a death trap for hummingbirds, the suburban Southeast is now a land of milk and honey," he writes in his 1999 book Living on the Wind. That's true, however, only for the tougher western species, like the rufous, which can put on a gram of fat in a day. Before migrating, from 25 percent to 40 percent of their body weight is typically fat, which they will burn over the vast distances they cover. They can survive temperatures below zero degrees Fahrenheit by dramatically slowing down their metabolism. Such freezing temperatures, however, are likely to kill off any ruby-throated hummingbirds, which don't have to live through extreme temperature as consistently as some of the western hummingbirds do.
Back in the Owens yard, Newfield files down a band that is too large for her latest captive, as it lets out a plaintive cry from a cloth bag. "This is really all bookkeeping," she says, gesturing at her data notebook. "It's important that I like to catch them--but not worth a damn if you don't do something with it."
Despite decades of meticulous data gathering, Newfield expresses some intrigue. "We are finding questions, not answers," she says, slipping the custom-altered band on the tiny leg and frowning as she adjusts it. "It's complicated." Both Newfield and Remsen are skeptical of those who want to point the finger at human-induced climate change to account for the potential shift in hummingbird migration.
"Everyone wants to blame everything on global warming," says Remsen later that day in his cluttered office at LSU behind the Museum of Natural History. "It is tempting to believe, but the link is tenuous." He notes that in the past few years there has been a drop in counts among the more western species in the area, despite relatively mild winters. That may have to do with Katrina effects or specific conditions--drought, for instance--at a breeding ground thousands of miles away rather than a long-term global trend. And despite the increasing numbers of western species counted in the Southeast during winters in the 1980s and 1990s, the overwhelming majority of the birds remain true to their ancient migration path, which stretches from Mexico to Alaska. "It is
really hard to make any inference in a population when you are focusing on a peripheral area like the Gulf," he adds.
There also is a lack of hard data on the status of the Mexican wintering grounds. Though habitat destruction there is frequently cited, Remsen says it remains unclear whether that could be forcing some birds north and east. "You could go either way--there is a loss of habitat or they are doing so well they need to migrate to find greener pastures," he says. And areas considered degraded by humans--that is, fallen trees and debris--can be rich habitats for many hummingbird species.
It may be surprising that despite the intense focus on hummingbirds in recent decades, there is not even a consensus on what they eat. Some argue that the bulk of their calories come from insects; others say from nectar. Such a prosaic detail matters if you want to pinpoint the role of feeders in hummingbird migration. "It's not what we know that is so striking," says Sargent, who is busy at work with Bassett and Weidensaul gathering and banding migrating birds in Alabama. "It is what we don't know."
That mystery continues to power this small group of hummer lovers. "None of us would have been doing this if it wasn't for Nan Newfield," says Weidensaul. Here in the Owens yard, the banding is done for the morning. Three decades after starting her five-year-project, Newfield is grayer and more solid but no less passionate. She recalls a recent trip to Cuba, where she spotted the rare bee hummingbird. "It was one of the high points of my life," she says. Then, shyly, Newfield pulls out two pictures. The first is of her two-year-old grandson. And the second? A rare hybrid hummingbird.
This story originally ran as "The Drifter" in the March-April 2010 issue of Audubon magazine.