Scientists have attached tiny "backpack" geolocators to thousands of songbirds in recent years to study where and when birds fly during migration.
Now scientists finally think they have an idea of how the birds feel about those burdens — and, to their surprise and relief, it appears that for many of the birds the backpacks are no big deal.
Migration patterns are difficult to track any other way, but the information is crucial for the conservation of songbirds, whose populations have been plummeting in recent years.
"A lot of very, very important information has come out of their use," said Graham Fairhurst, lead author of the new study.
For example, a study in 2013 found that purple martins are now migrating at the wrong time for a warmer climate, missing the spring population boom of insects that they need to feed their young.
However, the information from the devices is limited because they only record data and don't transmit it. That means the data can only be recovered if and when a bird is recaptured at a later time, usually the following year.
Research shows the geolocators don't affect the birds' ability to breed and raise their young. However, they are less likely to return to their breeding grounds if they're carrying a geolocator, suggesting that the devices could put extra strain on the birds and cause higher mortality.
"This is sort of the conundrum that has been raised," said Fairhurst. "We need to use these things because they can provide us very good information, but at the same time we want to be sure not we're not harming birds in the process of doing that."
He added that scientists also need to be sure that the information they get using the devices isn't biased by causing the birds to behave differently.
In order to test whether the devices were harming the birds, Fairhurst decided to look at the birds' hormone levels. Corticosterone is a hormone that increases in birds' blood when they are stressed or exerting themselves more than usual. Those hormone levels are reflected in the birds' feathers, as their growth is fed by blood.
Fairhurst, a post-doctoral researcher at the University of Saskatchewan, collaborated with researchers from across Canada, the U.S. and Europe to collect feathers from barn swallows and tree swallows at 12 different breeding grounds from Prince George, B.C., to Sackville, N.B.
The feathers came from birds with or without geolocators, before and after migration to Central and South America. He then compared their hormone levels and found them to be comparable, whether the birds had migrated with geolocators or not.
The results, published this week in the journal Open Science, showed no differences in hormone levels among the different groups of birds or before or after migration while carrying a geolocator.
"I think at first it was a little bit surprising," Fairhurst said. But he said after thinking about it, he was relieved. "This is showing we might not be harming them."
It also suggests the geolocators shouldn't influence the kind of data scientists are getting.
But the results don't answer the question of why birds with geolocators are less likely to return to their northern breeding grounds.
It's possible that the ones that did return, in spite of the geolocators they carried, are "the best quality birds" and that is why they didn't show changes in their hormone levels, Fairhurst said.
Scientists also have no idea what happened to the birds that didn't return.
"Did they die outright?" Fairhurst suggests. "Did they just choose to breed somewhere else? We'll have to address that with future work.… That's still a very big unknown."