BY DR DANIEL HOLDSWORTH

Dear Editor,

Regarding the article on the tracking of various birds using tags, there is a steadily accumulating body of evidence that shows that this practice isn’t very good for the birds thus tagged, and that the deleterious effects range from mild impairment to severely handicapping birds.

The main effects of tags are to cause aerodynamic drag (this is especially the case with wing tags), to wear away body feathers and generally cause the bird discomfort. Looking at the tagging from the point of view of a former biologist, I strongly suspect that the type of tag in use is being chosen not for minimal effects on the birds, but for the convenience of the biologists involved.

Eagles are quite big birds, so a tiny passive-resonance tag such as are used on insects would not be used, despite the fact that the eagle wouldn’t notice it at all. Indeed, leg rings seem to have zero effect on all birds.

Wing tags are severely deleterious as this paper shows here.

Other tags are also deleterious, varying by bird, tag type and so on as is demonstrated here.  

Finally, the failure modes of the tags are, as you discussed in your article, very much a subject that needs to be investigated. The RSPB generally seems to think that tags only fail when birds die; any experimental biologist will tell you that electronics generally die much more easily than that and under field conditions electronics may be considered expendable resources that if they work at all for any length of time can be considered a miracle. I would also note that solar power in Scottish winters is highly unlikely to be very reliable.

Yours faithfully,

Dr Daniel Holdsworth