From birds to fish, how extreme heat causes wildlife to suffer - Earth - Environment - Biology - Ecology

Birds are particularly vulnerable to heat, with a body temperature of between 39C and 42C that rises further during flight or when foraging.

Like humans, wildlife is increasingly vulnerable as climate change fuels longer and more intense heat waves, disrupting feeding and breeding and, in extreme cases, proving fatal.

The human toll of heat waves is well documented, but their ecological impacts have received less attention.

A study published in March in the journal Nature Ecology and Evolution found that three-quarters of land and ocean species assessed were "negatively impacted" during a major 2021 heat wave across western North America.

Heat waves can be "brutal" on wildlife, said Gregoire Lois, an ornithologist at the National Museum of Natural History in Paris, which is withering under its second heat wave of the year.

Animals have fewer opportunities to adapt during sudden heat waves than under gradual warming, he told AFP.

Birds

Birds are particularly vulnerable to heat, with a body temperature of between 39°C and 42°C that rises further during flight or when foraging.

They also lack sweat glands, making it harder to dissipate heat when temperatures soar.

Instead, they cool down through evaporation via their respiratory tracts, a water-intensive mechanism that increases the risk of heat stress and dehydration.

Young birds, still in the nest and unable to fly during the summer heat, are particularly vulnerable.

"The young, suffocating from the heat, sometimes fall from the nest whilst searching for air," said the League for the Protection of Birds, a French wildlife NGO.

Birds that nest under eaves—such as swifts and swallows—are most at risk, it added.

Mammals

Vertebrates regulate their body temperature through panting or sweating, but that process "results in greater water loss the smaller the animal is," Lois said.

Anne-Laure Dugue, from the league's "fauna in distress" program, said the risk of hyperthermia or dehydration was particularly significant among hedgehogs and certain small rodents.

Mass bat die-offs have been recorded during heat waves as animals become disoriented and dehydrated.

In January 2026, thousands of flying foxes perished during a heat wave in southeastern Australia.

Large mammals are not spared, either: For species adapted to the cold—such as bears, bison, reindeer and moose—their thick fur becomes a hindrance when temperatures rise.

Exposure to even a week of 27°C (81°F) daytime highs greatly increases the chance of koala illness or death, concluded a study of 20 years of observational data published in Biology Letters in May.

Other species like foxes can suffer burns to their paws when they come into contact with scalding surfaces such as tarmac or sand.

Invertebrates

Most invertebrates are ectothermic, meaning their body temperature depends largely on their environment.

Lois said the consequences can be "severe" when their thermal tolerance limit is exceeded.

This is particularly true as their movement is often limited, and sometimes nonexistent.

In 2021, during a widespread heat wave in the North Pacific, more than a billion mussels, clams and starfish perished. 

by Delphine PAYSANT and Julien MIVIELLE

edited by Alexander Pol 

Source: From birds to fish, how extreme heat causes wildlife to suffer