Earlier quoted context omitted.
What sort of size do you think that would be?
Small and low energy enough that tiny migratory birds can wear them for months. Externally worn of course (e.g. attached to the ear, for a wolf). You could adjust the firmware of a wildlife tag to start transmitting location every 10 minutes when the animal leaves a geo-fence.
They are also not implanted in the birds, but are a relatively large "backpack" or leg tag.