Tracking birds may stop them coming home

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A Cerulean Warbler with attached geolocation device.
Credit: T. Boves

Tracking devices provide enormous insight into the behaviour of migratory birds throughout the world, with positive knock-on effects for conservation – but a new study warns this process could be negatively impacting the species it aims to protect.

Geolocators – small, light-sensitive devices – are regularly attached to birds by researchers keen to record their movements. The process helps track what migratory birds are doing, so efforts can be made to protect their nesting environments, and help protect endangered populations.

A research team led by Douglas W. Raybuck at Arkansas State University in the US wanted to gauge the impact of these geolocators on songbirds in particular.

“During 2014 and 2015, we deployed geolocators on 49 adult male Cerulean Warblers (Setophaga cerulea) in Pennsylvania, Missouri, and Arkansas,” the researchers write in The Condor: Ornithological Applications.

The team monitored the effects of geolocators across a one-year period by comparing the survival rates, nestling provisioning rates, nest survival and return rates after migration, between geolocator-tagged adult males, and a control group with coloured bands for identification.

“We found no negative effects of geolocators during the breeding season … but the return rate of geolocator-tagged birds was lower than that of control birds.”

Following their migration, only 16% of geolocator-tagged birds returned to their territories, compared with 35% of birds with coloured bands.

Bridget Stutchbury, an expert on geolocators and the conservation of North American migratory songbirds at York University, says the finding puts researchers in an awkward position.

“New technologies such as geolocators and automated radiotracking arrays have led to a surge in new tagging studies of migratory songbirds,” says Stutchbury.

“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.”

Geolocators are used on a very small number of birds per species, and the researchers suggest this cost is likely outweighed by the benefit to the species as a whole, and to our understanding of bird conservation more broadly.

“Although finding no effect of geolocators during the breeding season is encouraging, the lower return rate of geolocator-tagged birds warrants further investigation in the field,” say researchers.

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