Kirtland's Warbler

Bird poop shows that when birds migrate, their gut bacteria change

Heather Skeen, PhD candidate, Committee on Evolutionary Biology (CEB) & lead author

In a new study in the journal Molecular Ecology, researchers used tiny radio trackers to follow the movements of birds that migrated between The Bahamas and Michigan, and they found that the same individual birds’ gut bacteria were different in the two locations. But to figure that out, they had to examine a lot of bird poop.

Heather Skeen, a PhD candidate at UChicago's Committee on Evolutionary Biology, is the lead author of the study, with the Field Museum’s John Bates and Shannon Hackett, Nathan Cooper at the Smithsonian Conservation Biology Institute, and Peter Marra at Georgetown University. Skeen's project built on Nathan Cooper's long-standing work researching the Kirtland’s Warbler in its winter home in The Bahamas and its breeding ground in northern Michigan. She and her colleagues selected the Kirtland’s Warbler "because there are very, very few species of birds where you would have been able to track individual birds from their non-breeding grounds and then capture them on their breeding grounds,” says Skeen. Kirtland’s Warbler is one of the rarest birds in the world, nearly going extinct in the 20th century; their populations have stabilized as a result of intensive conservation efforts. This rarity, combined with the birds' extreme pickiness in their breeding grounds, made them ideal subjects for Skeen's study.

Skeen and her colleagues began their fieldwork in The Bahamas, where they lured Kirtland’s Warblers with recorded bird calls and fitted them with tiny radio tracking devices. After attaching the trackers, the researchers put the birds inside of wax paper bags for a few minutes, so they could take care of "business." The warblers were then released, leaving Skeen to go into the bags and collect fecal samples.

The second stage of the research took place at the other end of the migration in Michigan. There, Skeen and her team would wait for pings from the area’s radio towers, and then fan out with handheld radio antenna to track the warblers they’d tagged in The Bahamas. The birds were recaptured to gather a second round of poop. Initially basing her hypotheses on findings from mammals, whose microbiome is tied to their identity, Skeen assumed the warbler’s microbiome would remain consistent in its breeding and winter grounds. Instead, her analyses revealed the opposite: the bird’s gut bacteria was different in the two locations, influenced more by environment and diet than species identity. These results suggest the warbler’s digestive system has adapted to varying conditions in terms of weather and available food sources. 

“One of the most important parts about this study is that we were able to recapture birds at different portions of the annual cycle in different locations, and we have this one-to-one comparison of the same population and the same individuals and how their microbiomes changed,” says Skeen. “If we’d tested different individual birds, we wouldn’t have been able to say for sure if the changes we saw were due to location or if they were just differences between populations. Since we were looking at the exact same birds, these results are much more supported.”

Another key realization here is that the climate crisis might make gut microbiomes especially important as animals attempt to survive in changing environments. “An animal’s gut microbiome is an additional level of molecular diversity, and as global climate change alters ecosystems, the gut microbiome might be one of the avenues in which animals can adapt to the changing environment,” says Skeen. “The gut microbiome has its own unique ecosystem, and it’s ripe for discoveries.”