I think it is a practical experience value. The electronics itself have relatively small weight. The problem is with the weight of the battery. If there is a low weight harvesting device with for example the supercapacitors storage, one may come up with even smaller weight than the 3 percent of the birds weight.

I think the weight carried by the bird can be estimated to be equal with the weight of the food in its somatic not to load it during the flight.

Best wishes

ASAB/ABS Guidelines do not indicate explicitly this threshold for example:

https://www.sciencedirect.com/science/article/pii/S0003347219303628

Hi Michelangelo, it's the so called "Kenward's rule", should have been mentioned for the first time in Kenward 1987, Wildlife Radio Tagging: Equipment, Field Techniques and Data Analysis, Academic Press, London. It's a practical rule stating that the weight of radio tags for a bird should not exceed the 3% of its body weight.

I think it is a practical experience value. The electronics itself have relatively small weight. The problem is with the weight of the battery. If there is a low weight harvesting device with for example the supercapacitors storage, one may come up with even smaller weight than the 3 percent of the birds weight.

I think the weight carried by the bird can be estimated to be equal with the weight of the food in its somatic not to load it during the flight.

Best wishes

I apologize, I specialize in producing field crops

Hi Michelangelo,

Maybe this paper is helpful for you:

Barron, D.G., Brawn, J.D. & Weatherhead, P.J. (2010) Meta-analysis oftransmitter effects on avian behaviour and ecology. Methods in Ecology and Evolution,1, 180–187.

It depends on the species. The classic resource that started this unofficial cut-off has already been mentioned in the comments (Kenward 1987), but there are a few recent meta-analyses that show its likely very state and species dependent. For example, Bodey et al 2017 (Methods in Ecology and Evolution; doi: 10.1111/2041-210X.12934) suggests 1% of body mass is the threshold for negative effects, while Geen et al 2019 (J Avian Biology, doi: 10.1111/jav.01823) finds negative effects increase with proportional weight but there seems to be below average effects when the tag is less than 2% of body mass.

Like I said though, it depends on the species. Highly aerial species are more sensitive to extra weight than more terrestrial species for example. For that reason, the 3% rule by Kenward tends to work out on average but it is always good to know the ecology of the bird and be conservative.

I am reading with interest this discussion about the question of transmitter weights and relationships to bird weight with special focus on 1%-5% of a bird's body mass. Bob Kenward and I were contemporaries at Oxford in the 1970's while we were each working on our D.Phil's. Bob was working on Kestrels using radio-tracking, and I was working on a project involving sleep in Herring gulls using radio-tracking and biotelemetry. He and I had numerous conversations about how much transmitter weight we could attach to a bird before it adversely affected behavior and breeding success measures. In 1976-1977, I and two other Oxford colleagues (Richard Sibly and Robin McCleary) performed an experiment on Herring gulls by placing dummy transmitters weighing 10, 30, and 50 gms in male and female birds, all who were nesting. We also had control groups that were caught and handled like the experimental's but no weight was attached, as well as controls for next/clutch checking that were not caught or handled in any way. We measured breeding success for all experimental and control groups throughout the breeding season at Walney Island, UK, and also into a second year as some dummy transmitters were retained throughout the next year. We published this research in the Journal Biotelemetry and Patient Monitoring in 1978 and I have placed a full copy of this paper on my ResearchGate site for your use. Bob Kenward and I discussed this work many times in ensuing years over coffee and tea at the Department of Zoology and Experimental Psychology at Oxford. In short, I found that transmitters weighing over 1% of the gulls mass (male and female) adversely influenced breeding success. First-week effects were the most significant. After the first week, and while controlling for first week effects of catching, handling, etc, birds with 30gm or 50gm dummy transmitters lost over 50% of their clutches. After that, we broadly recommended to all of our colleagues using radio-tracking and biotelemetry on birds to attempt to use 1% transmitter weight maximums for free ranging birds. In laboratory studies using biotelemetry transmitters we have used transmitters weighing upwards of 5% of body mass in non-breeding conditions. I think our published work predates by nearly 10 years what colleagues in this string are referencing as the earliest reference to the so called "1%" or "5%" cutoff levels.