Big birds are having fewer chicks as climate changes

In a time of crisis, evolution favors the little guy.

Smaller birds are more successful in raising chicks in a hotter world than large birds, according to a study in the Proceedings of the National Academy of Sciences (PNAS) published Monday.

“It’s not good to be big in a warming world,” Lucyna Halupka, a biologist at Poland’s University of Wroclaw told The Hill.

The global study surveyed 100 species, identifying changes over the past half-century in how many fledglings a typical bird pair got out of the nest.

Compared to small birds like warblers, bigger birds like hawks and storks are raising fewer surviving fledglings, the PNAS study found.

Big birds — over about 2 pounds if they are sedentary and 2 ounces if they migrate — laid smaller “clutches” of eggs at a time and lost more chicks from those nests. 

Those drops were particularly dramatic among large migratory birds — which on average, saw 17 percent fewer fledglings survive to leave the nest. 

A larger proportion of bigger-bird nests also entirely “fail” for some reason, Halupka said. (A “nest failure” refers to a nest where no chicks survive, whether they were killed by extreme weather or predators.)

This drop coincides with a mass extinction of birds — with well-documented drops in bird populations worldwide. 

But the declines observed weren’t enough to explain why those populations are falling.

In part, that’s because a sizable minority of species — about 46 percent — weren’t experiencing offspring declines at all. (About 56 of total species did experience declines.)

Instead, when raising the next generation, some birds have proven able to exploit the new opportunities created by climate change — or expanding human settlement.

Many species of smaller birds evolve more quickly and have an average of 10 percent more successful chicks, particularly if they can breed quickly enough to pack in an additional brood during the lengthening growing season.

That’s the case with the Eurasian reed warbler, a migratory bird that Halupka studies in the marshes of central Europe.

In 2021, Halupka published a paper showing that as the world heated up and the spring lengthened, reed warblers were getting more successful fledglings out of the nest — not fewer.

Climate change had worked in the birds’ favor. In the colder springs of the 20th century, the little birds arrived in the Polish marshes to find brown sticks and dead reeds — poor nest material that provided a little cover from their predators. But as Europe’s summer growing season expands, spring has started earlier. Today’s reed warblers arrive at the same time of year to find lush banks of reeds.

“The reed warblers can build among new reeds, so they are much better hidden than in the past,” Halupka said. “And the reeds grow more quickly now, because spring is much warmer.”

But even where hotter summers shrink breeding seasons on the back end, several characteristics of smaller birds — both migratory and not — make them better suited to rising temperatures.

In general, the bigger an animal’s body, the less efficiently it can shed heat into its environment. In a warming world, that’s bad news for large warm-blooded animals, which struggle more to cool their bodies.

But there’s a more fundamental evolutionary reason why smaller can be better. Small birds tend to live fast and die young, and the short handoff between generations gives them better odds of beneficial mutations.

“Of course, maybe such a combination will be promoted by natural selection,” Halupka said. “But when a new combination happens rarely — because an organism reproduces rarely — not enough combinations are available.” 

Small birds are also more likely to rear multiple clutches of eggs in a breeding season.

The pressure toward being small is something we can see in birds’ own deep history. Modern “avian” birds are the sole surviving part of a much larger family of feathered dinosaurs — including the 11,000-pound therizinosaurus. 

When the climate changed rapidly 65 million years ago after the asteroid impacts that ended the Cretaceous Era, only the smallest birds eked through — surviving to become the only kind we know today.