Trackers may tip a warbler’s odds of returning to its nest

Strapping tiny trackers called geolocators to the backs of birds can reveal a lot about where the birds go when they migrate, how they get there and what happens along the way. But ornithologists are finding that these cool backpacks could have not-so-cool consequences.

Douglas Raybuck of Arkansas State University and his colleagues outfitted some Cerulean warblers (Setophaga cerulea) with geolocators and some with simple color tags to test the effects the locators might have on breeding and reproduction. This particular species globe-trots from its nesting grounds in the eastern United States to wintering grounds in South America and back each year. While the backpacks didn’t affect reproduction, birds wearing the devices were less likely than those wearing tags to return to the same breeding grounds the next year. The birds may have gotten off track, cut their trips short or died, possibly due to extra weight or drag from the backpack, the team reports May 3 in The Condor.

The study adds to conflicting evidence that geolocators affect some birds in negative ways, such as altering their breeding biology. At best, potential downsides vary from bird to bird and backpack to backpack. But that shouldn’t stop researchers from using geolocators to study migrating birds, the researchers argue, because the devices pinpoint areas crucial to migrating birds and can aid in conservation efforts.