I tested a few in the chicken boxes with coils as on my web page. They work well if close to the wood. (The key fobs worked 5cm above the wood) I expect they will be OK since the birds spend over 30 minutes in the box per egg,
However it is now cold and we had no eggs yesterday so they will not visit the boxes anyway......
Also - the fobs have their numbers printed on them - I will have to mark the rings in some way - perhaps record the last 3 numbers on the ring with a soldering iron tip.
My main problem at present is to get some reliable leg rings.
I use cheap key fob tags at present. The birds seemed not to notice them and still laid OK but those shown on the web page eventually failed because the hens pecked through the copper wires. I aim to try a steel wire or all nylon design if commercial rings are too expensive.
I have a home built incubator that works very well - details on request.