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Migratory songbirds now molt feathers earlier each fall as climate warms, study finds

Birds regularly shed and regrow their body and wing feathers in a process, called molting, that is critical for flight, migration, insulation, breeding and survival.

A new study by University of Utah biologists examined molt phenology, or the timing of feather replacement, in response to climate change and made some startling discoveries.

Using 13 years of bird-banding data collected at the university's field station in southeastern Utah, the research team led by graduate student Kyle Kittelberger documented how molt has shifted for birds, particularly in relation to climate factors such as El Niño. Their findings suggest that molt may be becoming more flexible and climate-sensitive in the fall, with implications for avian survival, migration and reproduction.

"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 Kittelberger, who is wrapping up his doctorate in biology professor Çağan Şekercioğlu's lab. The shift is likely a response to climate-driven changes in the birds' migration and breeding.

"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," Kittelberger said.

"It allows for the replacement of old, worn and damaged feathers. 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."

Yet changes in molt phenology have not previously been closely studied in North America. Kittelberger's study, in The American Naturalist, is based on data recorded from 22,072 birds, representing 134 species, captured from 2011 to 2024 at the university's Bonderman Field Station at Rio Mesa outside Moab.

ÅžekercioÄŸlu's Biodiversity and Conservation Ecology Lab oversees a seasonal mist net program that captures mostly migratory songbirds in the spring (early April to early June) and fall (August through early November) as the birds travel between their wintering grounds in the south and summer breeding areas to the north. The station's 16 nets are up for six hours a day most days, depending on weather, starting 30 minutes before sunrise.

During capture seasons, the nets are checked every 30 minutes. Species, sex, age, molt stage, feather and body conditions and other data are collected from each bird pulled from the nets before it's released to continue its biannual journey. Bonderman posts .

"We didn't see any shift at the community level for spring body molt," Kittelberger said. "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."

Other studies have shown that many migratory songbird species are breeding sooner in response to climate change.

"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."

Changes in molt phenology could potentially undermine birds' survivability if it results in a mismatch that interferes with other energy-intensive elements of their life cycle. Normally, birds don't actively molt during migration or breeding.

"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," Kittelberger said.

The study demonstrates the value of long-term bird banding stations like the U's Bonderman Field Station at Rio Mesa for understanding birds' biology, though it offers only a snapshot of a bird's life history as they pass through this one spot along a stretch of the meandering Dolores River just upstream from where it meets the Colorado.

"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," Kittelberger said.

"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."