PIT tags are a useful tool to identify and follow individuals within a large population to monitor movement and behavior.
Tags transmit a unique identifying number that can be read at a short distance that depends on the tag size and antenna design.
Passive RFID tags are inductively charged by the reader and do not have a battery. Tags can remain operational for decades.
An antenna can be constructed at a point of passage (stream, trail, culvert, underpass, etc.) and a reader will record the time and unique identifying number of each tag detected.
When multiple tags are in the magnetic charge field they all talk at the same time and cause tag collision where none or perhaps only the loudest tag will be heard. A tag monitoring system needs to be designed to limit this effect.