Every year, the rufous hummingbird – a tiny fire-colored ball of feathers that weighs just three grams – flies up to 3,900 miles from its winter home in Mexico all the way to Alaska. At about three inches long, the rufous takes one of the longest migratory journeys of any bird its size. Over the past several decades, however, the feisty hummingbird has suffered: its numbers are declining at a rate of 3 percent per year, and now, scientists have a new theory as to why.
Most scientific studies have focused on the fact that warmer temperatures push some birds into higher, cooler elevations and latitudes, but there’s no consensus yet about how climate-driven changes in rain and snowfall will impact them. Yet newly published research that looked at climate impacts to 132 bird species across five western states and British Columbia, suggests that precipitation plays a surprisingly important role for many bird species, and particularly, for the rufous’ long-term survival.
“The findings suggest that many birds tune into things that are controlled not by temperature but (mostly by) moisture availability, which carries over from the winter snowpack,” said Julia Jones, a geoscience professor at Oregon State University who participated in the study. According to the report, rain and snow trends had major impacts on the population patterns of 60 percent of the bird species studied.
The researchers examined long-term data on bird populations and then tied the changes to concurrent climate events. In other words, if a species’ population took a dip, the scientists looked to see what had changed in the regional climate at that time – temperature, precipitation or other variables. Precipitation was most frequently the best match to reflect shifts in bird populations over a 32-year period.
Matt Betts, an ecologist at the university and lead investigator for the study said: “We think that the (rufous) population is declining due to a general drying trend,” referring to less precipitation in Western states seen over the last 40 years or so.
According to Betts, there are two possible reasons for the birds’ dramatic decline. First, it could be that winters in the Pacific Northwest, where the birds arrive in May, have been receiving less snow, which means less moisture carried into the spring and summer, and thus fewer flowers for the birds to feed on. Plus, many hummingbirds nest in grassy mountain meadows, which rely mostly on snow melt for spring growth. Or the crux of the decline, he said, could be that “the snow is melting earlier each spring, which means flowers bloom early and the hummingbirds, which are migratory, miss that peak in the blooming."
Although the precipitation impacts are bad news for the hummingbirds, some species, like the drought-tolerant California towhee, will apparently be just fine; the study showed the desert-loving bird’s population has remained stable.
The next step, said Betts, will be to look at other habitat variables, like disturbance from forest fires and land use changes, like logging or other human development.
For scientists, the challenge will be coming up with a ways to predict how climate change will alter rain and snowfall as well as temperature – no easy task given the high natural variability seen in the West.
Sarah Tory is an editorial intern at High Country News. She tweets @tory_sarah