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 heatwave 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.

Though this is just one anecdote, scientists predict that extreme heatwaves will become more common with climate change, carrying huge consequences for the survival of some populations.

“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.

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.

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.

blackbird with chicks

On average, the window of time when birds lay their eggs has got 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.

Those animals that can’t match the rate of warming and delay breeding, like the roe deer and Columbian ground squirrels, don’t raise as many young. If an animal can’t adapt, it’s possible a local population would go extinct, said Reed, a co-author on the Nature Communications study.

Reed admits that the study is a small snapshot to generalize for all populations and animals. Generalists that eat different types of food or inhabit a wide range of habitats might be more resilient to climate change. (Read how a generalist, the raccoon, spread widely across the globe.)

Jeremy Cohen, a postdoctoral fellow at the University of Wisconsin who was involved in the study, doubts it is a clear case of an evolutionary response. “Birds may be quickly reacting to what’s going on in the weather, rather than evolving these responses across generations,” Cohen says.

Because previous studies have noticed that changes in the timing of migration and breeding are linked to warming temperatures, the changes suggest more of a behavioural response. Scientists would need high-quality genetic data from related individuals in a population to show that genes that tell a bird to breed earlier are actually appearing in the next generation. Cohen noted that changes in morphological characteristics, such as body size, would have been more convincing evidence of an evolutionary response to climate change.

How animals adapt to warming temperatures is just one piece of the puzzle. The impact of extreme events on animals, like France’s heatwave, needs to be further studied.

An added factor in the extreme heatwave event that killed the great tit chicks, Charmantier says, is the “heat island effect,” where cities tend to be hotter than surrounding areas. In a nearby oak forest in Southern France, where temperatures remained cooler, fewer chicks died.

These extreme heat events could have a huge impact on selection. But some offspring need to survive during an extreme event for those beneficial genes to be passed into the population. If an extreme heatwave kills everyone, natural selection can’t occur.

We do know that these “extreme climatic events are creating a new selection pressure,” Charmantier says. “We hope that this will trigger new evolution, but we know that any evolution will be too slow to catch up with climate change.”

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