In the course of a LIFE+ project, a migratory Northern Bald Ibis (NBI) population is reintroduced in Europe (Fritz et al., 2019; www.waldrapp.eu). Since 2014, all birds are equipped with standard GPS transmitters. Remote monitoring is an essential element of the reintroduction due to the large-scale movement patterns of the released birds. At first, all birds were equipped with battery-powered devices attached to their lower back via leg-loop harnesses. Since 216, we successively switched to solar-powered devices which are usually attached to the sun-exposed upper back area via wing-loop harnesses. In 2016, we observed a first case of unilateral corneal opacity (UCO) in one individual. From 2017, further cases were detected; up to 2018, a total of 25 birds was affected by UCO with varying degrees of intensity, including blindness. So far, all involved experts were unfamiliar with the symptoms. However, we found a striking correlation in 2017: Only birds carrying a solar-powered GPS transmitter that was attached via wing-loop harness were affected by a UCO. In 2017, this corresponded to 70% of the released birds carrying this transmitter type! Furthermore, not a single bird with a GPS transmitter attached to the lower back showed UCO symptoms.