The story of bird migration is written in poop

The story of bird migration is written in poop, according to a study published in Molecular Ecology.

Bird poop is filled with gut bacteria – called the microbiome – that help digest food and fight off disease. There are trillions of types of bacteria unique to each animal and region of the world. Based on this, a team of researchers traced the bacteria in bird poop to map their migratory routes.

A bird on its back in a persons hand. It has a small blue square on its belly.
A Kirtland’s Warbler with a tiny radio tracking device on its back. Credit: Heather Skeen

The researchers put tiny geolocators on the rare Kirtland’s warbler (Setophaga kirtlandii) as they migrated between The Bahamas and Michigan, US, and found that they had different gut microbiomes when in each location.

“We’ve seen in other animals that microbiomes can be influenced by the places their hosts live,” says Heather Skeen, a PhD student at the Field Museum and University of Chicago and the study’s lead author.

“Lots of birds migrate, and they experience different environments at different points of their migratory cycle. We didn’t know how these different environments affected the birds’ microbiomes.

“In our study, we found that there are some groups of bacteria that are probably transient – the birds acquire the bacteria from their food, they poop it out, and it’s gone.

“These bacteria don’t colonise the bird; they go in and out.”

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Tiny birds, tiny geolocators

The tiny, yellow-breasted Kirtland’s warbler is incredibly rare; populations dwindeled to only167 males in 1987 but conservation and breeding programs have since stabilised them. They are extremely picky about their breeding grounds, which made them ideal for the project.

“We picked 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.

After all, trying to track common birds that don’t have specific breeding grounds is like trying to find a needle in a haystack.

Instead, the researchers went to known Kirtland’s warbler breeding grounds and lured them in with a recorded bird call. When the bird came to have a look, the researchers put a miniature geolocator on their bellies. It weighed less than half a gram (individuals weigh, on average, about 12–16g).

The researchers then scooped up the birds and put them in individual wax paper bags. Surprisingly, warblers promptly pooped.

The researchers repeated the experiment after the birds migrated – but this time the geolocators easily identified the same individuals.

Bird poop match migration routes

All in all, the researchers collected nearly 200 poop samples between the two locations. They analysed the poop and found that the microbiomes present in each location was different – even when pooped out by the same bird.

“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.”

The new microbiome data suggests that climate change may impact the gut microbiome of birds as they migrate.

“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.”

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