When Anne Charmantier set out to check her great tits—a songbird native to Europe—on June 28, she expected to find healthy, spry chicks.
As she slowly opened the doors to the wooden nest boxes—a trick to study these birds—the quiet at the nest disturbed her. Peering in, she encountered a grim scene: All chicks lay dead in their nests. An evolutionary ecologist at the Center of National Scientific Research in France, Charmantier has studied great tits for 15 years—long enough to know that this was not normal.
The culprit was a heat wave that had swept through Europe in late June. In Montpelier, where she checked the nest boxes, temperatures exceeded 110 degrees Fahrenheit, a record by more than 10 degrees.
“The heat was so much over anything we’ve ever experienced,” Charmantier says. “It was creepy.”
Though this is just one anecdote, scientists predict that extreme heat waves will become more common with climate change, carrying huge consequences for the survival of some populations.
The question plaguing scientists is this: Is climate change happening too fast for animals to save themselves—and their future offspring—by adapting quickly?
“Climate change is one of the key threats to biodiversity and to human society in the coming century,” says Thomas Reed, an evolutionary ecologist at University College Cork. “Ultimately, populations must evolve” to survive.
Early to breed
Scientists like Charmantier and Reed are studying how animals across the world will respond to a new environment shaped by climate change. Warming air and water temperatures, rising sea levels, increasing storm intensity, and disappearing ice create a drastically different world for species that evolved to live in specific conditions.
Multiple studies have looked at how individual species will fare in the face of climate change, but a study published in Nature Communications in July performed a meta-analysis, pulling all the data together to try to provide a broader conclusion.
Viktoriia Radchuk, lead author of the study, scanned over 10,000 abstracts from published studies to search for bits of data that she could include in the analysis.
Researchers focused on research from taxa beyond plants—amphibians, mammals, birds, reptiles, and insects—allowing researchers to establish a trend of warming temperatures in those study locations.
A warmer world unleashes cascading effects in a terrestrial environment: Temperatures stay warmer at night, trees shoot out leafy-green tendrils, and insects start their frantic mating dance. The abundance of new, yellow-green leaves and budding flowers supplies a smorgasbord for hungry caterpillars. These small caterpillars make their way into the begging mouths of baby birds, delivered by active parents.
Spring comes earlier with climate change, with consequences for animals. Radchuk’s team found a strong relationship between timing of key life cycle events, like breeding, and the warming pattern: Animals generally shift their breeding earlier to match the new timing.
On average, the window of time when birds lay their eggs has gotten earlier by almost two weeks over half a century. Since many small songbirds can raise their young in roughly one month, two weeks is a big shift in their timing.
“If [birds] don’t adjust, then the chicks will arrive way after the caterpillars are gone. And so, they starve,” Charmantier says.
Knowing that warming occurred in the studies and that animals were able to breed earlier, the team then asked if animals can evolve with the changing climate. The pool of studies with those detailed data dropped to 13 species—and almost all were birds.
Researchers were interested in understanding cases where they could “witness evolution in action…that happens at the speed of a few generations,” Charmantier says.
Evolution can happen very fast—over a few years—or it can be a slower process. In insects that breed much more quickly, evolution can occur faster, whereas longer-lived birds and mammals usually see these processes occur at a slower pace.
Natural selection is just one force that drives evolution. For natural selection to occur, explained Charmantier, some individuals with a particular ability—like starting breeding earlier—need to be favored. If breeding earlier means parents are able to raise more offspring—and this is linked to a gene—that means natural selection is at work to breed earlier.
The team found evidence of selection for most species, toward breeding even earlier with the warming climate. “But [evolution is] never going to be fast enough if we match this with the climate prediction,” Charmantier says.
Birds would have to shift even earlier to ensure a population can remain stable.
Those animals that can’t match the rate of warming and delay breeding, like the roe deer and Columb