Tracking Devices Reduce Warblers’ Chances of Returning from Migration

CONDOR-16-180 T Boves
Geolocators like this one provide valuable data on bird migration but can also impact the birds that carry them. Photo credit: T. Boves

The tools ornithologists use to track the journeys of migrating birds provide invaluable insights that can help halt the declines of vulnerable species. However, a new study from The Condor: Ornithological Applications shows that these data come at a cost—in some cases, these tracking devices reduce the chances that the birds carrying them will ever make it back to their breeding grounds.

Geolocators are small devices attached to birds that record light levels over time, which can be used to determine location. They’re widely used to study migration patterns, but studies have suggested that some species may be negatively affected by carrying them. Douglas Raybuck of Arkansas State University and his colleagues monitored male Cerulean Warblers with and without geolocators to see how they fared, and they found that while geolocators had no effect on the birds’ nesting success in the same season following their capture, birds with geolocators were less likely to reappear on their territories after migration the next year—16% of geolocator-tagged birds returned from migration, versus 35% of the birds in the control group.

The data gained from geolocator studies are enormously useful for bird conservation, and on a global scale those benefits are likely to outweigh potential the costs. The results from this study suggest that the potential impacts of individual research projects need to be carefully evaluated, but we should remember that only a small number of birds are ever tagged relative to the total size of the population under study.

The researchers captured Cerulean Warblers in Pennsylvania, Missouri, and Arkansas by luring them into nets using call recordings and wooden decoys. Outfitting some with geolocators but others with only identifying color bands, they monitored the birds’ nests and then searched for them the following year to determine whether they’d returned. “Re-sighting males and identifying their unique color-band combinations as they moved about in the canopy was not always easy, but our dedicated and skilled field crew did a fantastic job of overcoming these obstacles, which were compounded by inclement weather and the rugged topography of the sites,” says Raybuck.

“New technologies such as geolocators and automated radiotracking arrays have led to a surge in new tagging studies of migratory songbirds,” according to York University’s Bridget Stutchbury, an expert on geolocators and the conservation biology of North America’s migratory songbirds. “Finding that tagged birds were far less likely to return the next year compared with un-tagged birds puts researchers in a serious dilemma, because despite the potential costs of tagging small birds, long-distance tracking is essential to find out which wintering and migratory stopover sites should be highest priority for conservation.”

Mixed effects of geolocators on reproduction and survival of Cerulean Warblers, a canopy-dwelling, long-distance migrant is available at

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society, which merged with the American Ornithologists’ Union in 2016 to become the American Ornithological Society.

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2 thoughts on “Tracking Devices Reduce Warblers’ Chances of Returning from Migration

  1. Who wrote this blurb? I find fault with the third paragraph. It is not that I disagree with the statement per se, but I find the opinion statement ‘on a global scale those benefits are likely to outweigh potential the costs’ troublesome in a scientific journal. It is unattributed and presented without supporting data. Maybe it is true, maybe it is not. Let us decide. Don’t instruct us that ‘we should remember only a small number of birds are ever tagged’, just state a very small number of birds are tagged annually. This comes off as a hedge in case any one gets too hung up over deploying these transmitters. If the transmitters negatively impact return rates and someone wants to ignore that and deploy them anyway, that is another discussion in my opinion. I generally do not read these introduction blurbs for advertised articles, perhaps they are all like this. But I find it a strange way to introduce an article to the community.


    1. Chris, thank you for your comment. This press release was written by the journal’s Communications Assistant and approved by the journal’s Editor-in-Chief and the corresponding author of the paper. This is a tricky subject, and we did not want to oversell the implications of the results. However, we understand your points and welcome your comments. Thank you for your interest in the study, and we hope you’ll also read the full open-access paper if you haven’t already!


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