Birds are molting earlier in the fall as global seasons continue shifting

Birds don’t just fly and sing. They also molt – the process of shedding and regrowing feathers – which is as vital to survival as migration and breeding.

From insulation to attraction during mating season, the quality and timing of feather replacement can make or break a bird’s chance of survival.

A new study from the University of Utah highlights just how sensitive this molting process has become to climate change.

Researchers tracked changes in molt timing – known as molt phenology – using 13 years of detailed bird-banding records gathered at the university’s Bonderman Field Station at Rio Mesa in southeastern Utah.

Bird feathers tell a new climate story

Over 22,000 birds representing 134 species were monitored between 2011 and 2024. Mist nets, set up near sunrise and checked every 30 minutes, temporarily captured birds during their spring and fall migrations.

Each bird’s age, sex, species, feather and body condition, and molt stage were recorded before release. These seasonal captures formed the foundation for an important discovery.

“In the fall, we found that birds are shifting both their body and their flight feather molt earlier over time across the 13 years at a rate of about one day earlier per year,” said lead researcher Kyle Kittelberger.

That shift suggests a response to changing climate patterns, especially related to events like El Niño. These patterns are increasingly influencing the timing of birds’ migration and breeding, indirectly prompting earlier feather molts.

Why bird molting matters

“Molt is a really fundamental component of a bird’s lifecycle. It’s one of the main elements that a bird does, one of the main activities in addition to breeding and migrating,” said Kittelberger.

Molting replaces old, damaged feathers, ensuring better flight performance and even helping with mate attraction.

“If you have poor feather quality that could impact, for example, your migration. You might not be able to fly as well. It could also in the spring impact your ability to attract a mate,” he added.

But molting isn’t something birds usually do while breeding or migrating. Those are energy-intensive periods, and feather regrowth is equally demanding. Doing both at once would stretch a bird’s resources dangerously thin.

“Birds don’t want to be actively replacing feathers in their wings when they’re migrating because they’re going to be putting energy into replacing those feathers that will compete with the energy that they need for those long-distance flights,” said Kittelberger.

Not all seasons are equal

Interestingly, no overall shift was observed in spring molt across species. “We didn’t see any shift at the community level for spring body molt,” said Kittelberger.

“Some of the reasons for that might be birds tend to migrate much faster in the spring because it’s more of a direct shot getting back to their breeding grounds so that they can start preparing for the breeding season, whereas in the fall, it’s a slower and more meandering process.”

That slower fall pace might explain why birds now molt earlier. Other studies have also found that many migratory birds are breeding sooner, which could lead to earlier molting after the breeding season ends.

“A lot of molting occurs after breeding. That might be why we’re seeing an earlier shift in time with molt in the fall, because they could be breeding earlier,” Kittelberger said.

“But we’re also seeing that some species are delaying their migration in the fall. So that could alternatively mean that they have an opportunity to molt more of their feathers earlier before a later migration.”

Following the bird’s full journey

The study demonstrates the importance of long-term field stations in understanding how birds adapt – or struggle to adapt – to changing environmental cues. However, this research offers just a snapshot in a single location along the Dolores River.

“To really get a clearer picture of changes in molt, we need to go to their breeding or post-breeding grounds and assess birds there,” said Kittelberger.

“That is the next element of this kind of research, to not only look at what’s going on in the summer when they’re beginning to molt but to see if that overlaps with anything like breeding.”

As the climate continues to shift, so too do the rhythms of bird life. Feather by feather, birds are adapting – but how long they can keep up remains a question worth watching.

The full study was published in the journal The American Naturalist.

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