radio collar

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A collar fitted with a small radio transmitter that when attached to a wild animal can be used in tracking the animal's movements by radio telemetry.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A radio transmitter, fastened to a wild animal, (usually around the neck) so that the animal's movements may be tracked