Authors: Voelkl, Bernhard; Fritz, Johannes;

A migratory population of Northern Bald Ibises (Geronticus eremita) is currently reintroduced in Europe, in the course of a LIFE+ project (LIFE NBI). GPS-tracking of the whole population is essential for overall monitoring and management. Since 2016, we observed more than 30 birds with an opacity of the cornea in one eye, with varying intensity up to blindness. While this condition could not be linked to infections, we found a strong correlation between one-eye opacity and the use of solar powered GPS-devices fitted on the upper back of the birds. We also found that birds with mild opacity recovered fast after we removed their tags or changed the tag position to the lower back. Since birds sleep with the head on the back and one eye placed close to the device, the most parsimonious explanation for the symptomatic is a permanent slight irritation of one eye during resting and sleeping. The position of the devices on the upper back is also disadvantageous from an aerodynamic perspective and released birds equipped with such devices show reduced flight performance. We conclude that the decision how to fit the tracking devices on the animals can impact the health and welfare of the birds.