alt Hacker NewsSmall 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.
Bird ones are easy because birds are high in the air, so there's nothing to block the signal.
They are also not implanted in the birds, but are a relatively large "backpack" or leg tag.