Bird species are testing new survival tactics
More than a century ago, zoologist Joseph Grinnell launched a pioneering survey of animal life in California, a decadeslong quest — at first by Model T or, failing that, mule — to all corners and habitats of the state, from Death Valley to the High Sierra.
Ultimately Grinnell, founding director of the Museum of Vertebrate Zoology at the University of California, Berkeley, and his colleagues produced one of the richest ecological records in the world: 74,000 pages of meticulously detailed field notes, recording the numbers, habits and habitats of all vertebrate species that the team encountered.
In 2003, museum scientists decided to retrace Grinnell’s steps throughout the state to learn what changes a century had wrought. And that’s why Morgan Tingley, then an ecology graduate student at the university, found himself trekking through the Sierra for four summers.
Tingley wanted to know how birds had fared since Grinnell last took a census. Years later, the answer turned out to be a bit of a shock.
Of 32,000 birds recorded in California mountain ranges in the old and new surveys — from thumb-sized Calliope hummingbirds to the spectacular pileated woodpecker — Tingley and his colleagues discovered that most species now nest about a week earlier than they did 70 to 100 years ago.
That slight advance in timing translates into nesting temperatures about 2 degrees Fahrenheit cooler than the birds would encounter had they not moved up their breeding time — almost exactly counterbalancing the 2-degree rise in average temperatures recorded over the last century.
The scientists’ analysis, published last fall in the Proceedings of the National Academy of Sciences, showed that the birds’ temperature-rebalancing act could limit the exposure of eggs and fragile nestlings to dangerous overheating.
Early nesting has been noted in individual bird species, but such a widespread behavioral change over a very large and varied landscape was “not on the radar at all,” said Jacob Socolar, a postdoctoral scientist in Tingley’s lab.
The study of 202 species showed that most of them are adapting to rising temperatures with “overlooked flexibility,” the scientists reported — unexpected hope for wildlife in an uncertain time.
“Does it mean climate change is not bad for you? No,” said Tingley, now an assistant professor of ecology and evolutionary biology at the University of Connecticut. “But any time we find that a species has more adaptive mechanisms to cope with climate change, that’s a good thing.”
The researchers can’t say which species can benefit the most, or if the early nesting strategy can keep up with increasing temperatures in the long run.
But it seems to be working for the birds now.
“We saw nesting dates that were as much as five to 12 days early on average across all birds,” Socolar said. “When we plotted the temperatures — good God, we realized that the amount of cooling they would gain by that behavior is as much as California has warmed up over the last century.”
Tingley and Socolar collaborated on the research with Steven Beissinger, director of the Grinnell Resurvey Project and professor of conservation biology at the University of California, Berkeley, and Peter Epanchin, a scientist at the U.S. Agency for International Development.
Ecologists generally believe that birds adapt to rising temperatures by moving to higher elevations or heading north. They shift their nesting time for a different reason: to sync with food availability, like an early appearance of plump caterpillars or swarms of insects.
But in 2012, researchers found that about half of the bird species in certain regions of the Sierra essentially stayed put over the past century, not significantly extending their ranges to cooler elevations even though the climate was warming.
The new study offers a plausible explanation. If the birds lay their eggs earlier, they can stay in their centuries-old range, with no need to migrate to higher altitudes.
“Ecologists have really kept range shifts like migrating upslope separate in their minds from phenological shifts, such as nesting earlier,” said Peter Dunn, an ecologist at the University of Wisconsin-Milwaukee, who was not involved in the new analysis.
“The research makes you realize that birds can manipulate all sorts of things, not only spatially by migrating upslope but also temporally — shifting their nesting time in response to rising temperatures.”
Tingley and his colleagues also reported the results of an ingenious thought experiment devised to test a hypothesis: that the temperature advantage from early nesting means more baby birds survive.
The researchers turned to data from Project NestWatch, a nationwide survey compiled by the Cornell Lab of Ornithology that draws on amateur birders’ sightings of “active nests.”
Analyzing 47,000 nest sightings across North America, the researchers showed that when abnormally warm weather arrived at nesting time, birds in the colder, northern fringes of their range were more successful at fledging young than were birds in the hotter, southern edges of their range.
“Temperature really matters for nestling survival,” Socolar said.
“Eggs and hatchlings are stuck in one place — nowhere to go,” he added. “And unlike adults, they can’t control their internal body temperature. Hatchlings of many species are naked — very few insulating feathers for up to a week or more.”
An uptick of a few degrees Fahrenheit at nesting time can make all the difference, Socolar said. Nestlings are much more vulnerable to heat than to cold.
Climate change is having another dramatic effect on bird behavior. The study in 2012, also part of the Grinnell Resurvey, showed that about as many bird species in each region had moved downslope as upslope during the century’s warming climate.
Ecologists were puzzled at first, but the behavior turns out to make good ecological sense. It is likely a response to increasing snow and rain at upper elevations over the last century in mountain regions, Beissinger said.
Increased precipitation is a less obvious consequence of climate change than heat. But rain and snow also are greatly influencing wildlife behavior — and often in the opposite direction than does heat.
Some species among the 202 studied may not have been able to shift their nesting times in response to the warming climate.
The Grinnell Resurvey data in 2012 included 30 long-distance migrants. In spring, marathon migrators like yellow warblers — 5 inches long and weighing 9 grams preflight — travel more than 2,000 miles from Central America to nest in willows, aspens and cottonwoods in North America.
Yellow warblers’ numbers are down, but they are not among the endangered. Still, what they encounter as they arrive in California hints at the challenges tropical migrants face.
These birds are still in Mexico or even farther south when spring now arrives at their California nesting grounds. Arriving later in the season, they may miss the cues to shift their nesting time or move upslope, Beissinger said.
The jury is still out, but it may be that migrants are the least able to adapt their nesting behavior to the warming climate.
In some regions revisited by the Grinnell Resurvey, the century has not been kind. Analyzing data from California’s hot and dry southern deserts, Beissinger and graduate student Kelly Iknayan have found that most species there may be facing population collapse.
In other areas, wildlife is threatened by huge losses of natural lands because of the nearirreversible march of agriculture and urbanization. A Resurvey analysis is planned to examine the relative impact of climate change versus shifts in land use on bird numbers, ranges and diversity.
Mining the Grinnell Survey and Resurvey data on both birds and mammals will continue for years. Comparing records from the two surveys a century apart offers a brief history of change, as expected — although in the mountain range studies, not really as expected at all.
Analyses of wildlife fates in other regions will likely bring more surprises. “We have the chance to learn more and more about the past, present and our possible futures,” Beissinger said.