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Background In this paper, we present evidence that biologging is strongly correlated with eye irritation, suggesting a causal relationship with obvious impairing effects for the affected individuals. A migratory population of Northern Bald Ibises ( Geronticus eremita ) is reintroduced in Europe, in the course of a LIFE+ project (LIFE Northern Bald Ibis). Since 2014, all individuals are equipped with GPS-devices. Remote monitoring of the whole population allows the implementation of focussed measures against major mortality causes. Initially all birds carried battery-powered devices, fixed on the lower back of the birds. Since 2016, an increasing amount of birds were equipped with solar-powered devices, fixed on the upper back, as this is the more sun-exposed position. In 2016, we observed for the first time an opacity in the cornea of one eye (unilateral corneal opacity; UCO). From 2016 to 2018, a total of 25 birds were affected by UCO, with varying intensity up to blindness. However, only birds carrying a device on the upper back were affected (2017 up to 70% of this group). In contrasts, none of the birds which carried devices on the lower back ever showed UCO symptoms. This unexpected relationship between tagging and UCO was discovered in 2017. Since then, we started countermeasures by removing the device or repositioning it on the lower back. Results Northern Bald Ibises roost with their head on the back, one eye closely placed to the device. Thus, we conclude that the most parsimonious explanation for the symptomatology is either a repetitive slight temperature rise in the corneal tissue due to electromagnetic radiation by the GSM module of the device or a repetitive slight mechanical irritation of the corneal surface. Concrete evidence is missing so far. On our advice, UCO was also found in Northern Bald Ibises of a Spanish reintroduction project, in a completely consistent context. Conclusion Our results indicate that further research in the fast-growing field of biologging is urgently needed. The findings question the positioning of devices on the upper back in birds roosting with the head on the back.