They were meant to die, just not like this.

The chickens were 19 days away from a date at the slaughterhouse when temperatures climbed disastrously inside the long shed where they spent their nights. Over the course of several hours on June 23, during France’s hottest stretch on record, 1,500 keeled over. By the time Isabelle Renaudier opened the door, one-third of the flock was dead.

“It broke my heart,” said Ms. Renaudier, who had been raising the birds at her family farm in northwestern France for supermarkets, butchers and rotisseries.

Extreme heat poses a risk to nearly all living things, but chickens, unable to sweat and cloaked in feathers, are far more vulnerable than most. If temperatures become climb too high, they start to pant, a survival technique that can leave them dehydrated and sapped of energy.

Even slow-growing chickens that roam outdoors, like those kept by the Renaudiers, are susceptible. But the broiler chickens developed by large-scale industry, which can reach full size in just five weeks, have an additional disadvantage because their fast-burning metabolism leaves them running hot. The end result: An animal that has been perfectly engineered to satisfy the demand for lean, affordable meat is poorly suited for a warming world.

Rising temperatures from climate change pose significant challenges for agriculture. For the poultry industry, it’s a “large-scale issue,” as one study put it, and it reflects how food systems that most take for granted can be jeopardized by extreme heat. In 2015, a stretch of scorching days in India killed an estimated 17 million chickens and caused prices to rise at grocery stores. A 2021 heat wave in the Pacific Northwest led to 400,000 reported deaths among chickens and 60,000 among turkeys.

And then came the blistering late June stretch in France.

As temperatures of 105 degrees Fahrenheit singed the country’s western poultry-producing regions, officials here described “massive die offs.” A company that collects and renders dead animals said it had seen more pigs and cows than usual. But the amount of poultry had risen even more sharply, roughly 10-fold.

Overwhelmed by the volume, workers temporarily halted large poultry pickups and farmers were left with mounds of dead birds rotting in the heat. Yann Nédélec, the director of Anvol, an organization representing the poultry industry, estimated that 2.5 to 3 million birds had died, mostly chickens.

“We’re dealing with an episode that has caused significant damage,” he said. “One of historic intensity.”

Even as temperatures fell, animals tired from fighting the heat were dying at elevated rates. And farmers were exhausted, too, counting up their financial losses, dealing with insurance companies and troubled by their failure to do what they had pledged: to make sure the birds survived and grew well during the brief window until they become food.

“The goal of the game is to make sure these chickens are good and to deliver them,” said Christian Delavaud, 58, a poultry farmer south of Nantes.

Rising prices are changing the way the French eat, he said. They want chicken more than ever.

Mr. Delavaud said he had spent years preparing for extreme heat, equipping his poultry sheds with high-strength fans and misting systems, both common strategies. His chickens, living only 34 to 37 days, didn’t go outside and depended entirely on his decisions.

But during the heat wave, he noticed that system alarms meant to signal problems stopped working properly. Meantime, a water management company reduced pressure in the local network, leaving his misting system unable to replenish. In the shed where he kept his turkeys, which suffered the greatest toll at his farm, indoor temperatures reached 107 degrees Fahrenheit.

So rather than preparing the birds for market, Mr. Delavaud found himself lugging the carcasses into cold storage, only to then watch with dread as their warm bodies raised the temperature of the enclosure and rendered it useless. A foul smell spread beyond the farm, and he hired an excavator to dig an improvised burial pit.

He then called in a favor from two retiree friends, who, along with some employees and one of his sons, helped lug the carcasses into the pit. It was so hot that they shucked most of their protective gear.

“It was unbearable,” Mr. Delavaud said.

Of future heat waves he said, “I don’t know what to do anymore.”

France had dealt with a huge mortality spike among poultry at least once before, in 2003, when a heat wave killed between 4 million to 5 million chickens and turkeys, representing about 2 percent of the flock nationwide, according to a French government report. Gilles Cogny, the executive vice president of Akiolis, which collects and processes animal carcasses, noted that France had made more progress over the past two decades in reducing the heat wave-related deaths among humans than among vulnerable livestock.

“A farmer doesn’t want their animals to die. They need to adapt.” Mr. Cogny said. “We cannot change poultry. So the only thing is, how do we prepare for the temperatures that will be coming in the next 10 years?”

The misters and fans can provide at least a bit of relief, according to experts. But no solution is a “magic bullet,” said Sami Dridi, a professor of poultry science at the University of Arkansas. Experts say giving birds certain vitamins can help. So, too, can putting reflective material on the top of sheds. Certain kinds of chickens, like the so-called naked neck, can more easily cope with heat. Giving the animals more space would allow for better airflow, studies suggest, though such a change would cut into profits.

Air-conditioning is among the ideas that are “not cost effective,” Mr. Dridi said.

As France’s heat wave dissipated, poultry growers tried to figure out whether there were any lessons from which birds had survived. The tiniest chicks, which need warmer temperatures, tended to far best. But otherwise there weren’t many clear trends. Some farms were unscathed; others decimated. Mr. Nédélec said that both free-range chickens and those confined indoors faced big losses.

Sylvia Goisbault, 47, who lost only 14 of her 700 chickens, said she nonetheless felt “guilty,” because she “didn’t raise them to have this kind of death.” She said that next year, she would adjust the timing of when she grows her chickens, to avoid having slaughter-ready birds in the summer.

Ms. Renaudier and her husband, Florent, had said they would make changes, too, but they haven’t yet decided which ones.

“It can only happen once, not again,” Mr. Renaudier said.

Then he asked: “Do you want to go see the survivors?”

He and Ms. Renaudier walked across the farmland they’d built up over 30 years, where they kept cows and made their own yogurt, arriving at a low-profile shed with green doors and a grain bin. Ms. Renaudier knocked on the door, to alert the chickens, and stepped inside. The birds squawked and fluttered wavelike away from the people. Some hopped through little openings into the outdoors, collecting under the shade of fruit trees. The shed was 81 degrees, compared with 100 a week earlier.

The 2,800 surviving chickens had 10 days to go until slaughterhouse trucks arrived, and they seemed to have stabilized.

They had not gained weight at the usual pace during the heat wave, Ms. Renaudier, but could still make up the difference. “Unless there is another heat wave,” she said.