Incredible feat of miniaturization. I have no idea how you get a Bluetooth transmitter down to that SWaP factor, and I'm not exactly a n00b at either RF or EE stuff in general.

Almost worth $200 to order one of the BlūMorpho trackers and take it apart to see how they did it.

I wonder if 60mg is a lot of extra weight?

Looks like monarchs weigh from 0.25 grams for smaller individuals to 0.75 grams for larger ones.