Worrying effects of bio-logging discovered in reintroduced Northern Bald Ibises
Newsletter 22/10/2020
Any form of tagging or marking of wild animals also entails costs and risks for the bearer. In our recently published paper (Download), we describe a particularly unexpected case of impairing consequences of biologging.
In 2014, we started using GPS-devices for tracking our birds. Initially all birds were equipped with battery-powered devices, fixed on their lower back. Since 2016 an increasing number of individuals have been equipped with solar-powered devices, fixed on the upper back just below the neck, as the more sun-exposed position.
During a regular health monitoring in 2016, we observed one bird with a noticeable opacity in the cornea of one eye. This symptom has hitherto been completely unknown to us. By 2018, a total of 25 birds were affected, with varying intensity, leading in the worst case to blindness. In all individuals only one eye was affected, which is why we call this phenomenon not previously described in literature as unilateral corneal opacity (UCO).
Extensive clinical examination revealed no clear cause for the symptoms. But we found an unexpected and remarkable correlation. Only birds carrying a biologging device on the upper back were affected. In contrast, none of the birds carrying devices on the lower back ever showed UCO symptoms. After realizing that, we immediately removed the devices attached to the upper back from the birds. In the affected individuals, the symptoms then disappeared, unless the eye was already irreversibly damaged.
The connection between UCO and biologging becomes apparent when the behaviour of the birds is observed. Northern Bald Ibises roost with their head on the back, where one eye frequently comes into close proximity to the device when positioned on the upper back. Thus, UCO seems to be caused by a cumulative proximity effect of device components on the nearby eye. Since corneal tissue is known to be amongst the most thermally vulnerable tissues of the body, we consider a repetitive slight temperature rise in this tissue as the most obvious explanation. The radiation is emitted by the GSM mobile phone module of the device when transmitting position data. However, this causal relationship is still a hypothesis which needs to be verified.
Meanwhile, cases of UCO were also found in the Spanish Northern Bald Ibis release population (Proyecto eremita). We assume that the lack of evidence in other bird species may simply be caused by the difficulty to recognize the symptoms in the wild. Our findings highlight that a comprehensive monitoring of tagged individuals is indispensably and that it is crucial to consider apparently unrelated side effects.
Currently, about 90% of the individuals in the release population carry biologging devices. All of them are attached on the lower-back. This position is clearly suboptimal for solar tags, because the solar panels are easily covered by wing feathers when the bird is not flying. But despite continuous monitoring, no bird with a device on the lower back ever showed UCO symptoms.
Fritz J, Eberhard B, Esterer C, Gönner B, Trobe D, Unsöld M, Voelkl B, Wehner H & Scope A (2020) Biologging is suspect to cause corneal opacity in two populations of wild living Northern Bald Ibises. Avian Research 11:38, 1-9; DOI: 10.1186/s40657-020-00223-8; Download
Picture:
Northern Bald Ibis with eye opacity; this symptom is difficult to observe in the wild, which is probably the reason why it has not yet been described in any other bird species; picture D Trobe
