When multiple PIT tags are in the detection zone they will all send their messages at the same time. Their signals interfere with each other which garbles the message. Sometimes the closest tag can be read but if all tags are the same distance from the antenna then none can be read.
It is important to consider tag collision when designing a PIT tag detection site so that only one tag at a time will be in the field. There are different methods for doing this including narrowing the passage or limiting the rate that the tags arrive at the antenna. Another solution is to use multiple antennas to increase detection opportunities.
Tag collision has a direct effect on detection efficiency, the measurement accuracy of a PIT tag monitoring station.