Here was our response:
With regards to the potential impacts of the tags upon the birds: these are clearly questions we are constantly asking ourselves.
Obviously the welfare of the bird is paramount and the continued use of the tags depends entirely upon their safe use.
Additionally, it is important to us that the tags are comfortable and have no impact upon the bird as we are seeking to record normal behaviour that is unaffected by the presence of the device.
Tags are under 3% of the birds body weight. The 3% figure is a benchmark used by most ornithologist when considering the added weight of a tracking device. It is widely agreed that at this level, the tag's impact will be minimal.
Nevertheless we need to be certain that the tag (both its weight and means of attachment) poses no detrimental effect upon the bird. This is understandably hard to measure.
However, we have been able to re-capture three of our tagged birds in subsequent winters - these bird's tags had stopped transmitting, probably as the result of 'battery issues' and so were removed and re-used.
Having a bird in the hand, that had carried the tag for a year or more, allowed us to check for physical signs of wear or damage resulting from the tag. In all cases, there was no evidence of this.
Additionally, the birds were all still healthy weights, similar to those when first caught, indicating that the tag was not impacting upon fitness or the ability to feed.
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