The effects of geolocator drag and weight on the flight ranges of small migrants

1. Researchers are currently placing hundreds of geolocators on migratory animals. Return rates for some small birds carrying these devices have been lower than expected, potentially because geolocators increase drag during flight. 2. We measured the drag of three different geolocators (1.2 gBAS-MK10, 1.0 g SOI-GL10.09 and 0.5 g SOI-GL05.10) in backpack-style harnesses on two preserved bird bodies in a wind tunnel. We then used these measurements to estimate the effects of this drag on the flight ranges of several small migratory birds. 3. Both the BAS-MK10 and SOI-GL05.10 significantly increased drag; the drag was also considerably higher when a geolocator was attached between the wings (wing harness) than on the rump (leg-loop harness). 4. The effects of the increased drag of these devices on the predicted flight ranges of birds were similar to the effects of their weight and may thus explain the results of previous studies that showed decreased return rates when using geolocators and other tracking devices. 5. We recommend that researchers and manufacturers work to minimize the drag of geolocators and other externally attached tracking or data collection devices on flying and swimming animals. This can be accomplished with geolocators by attaching devices above birds' rumps instead of between their wings and flattening the devices to reduce their height.