EthoVision XT: Animal is not tracked in Hidden Zone

Make sure that the Entry Zone is properly linked to the Hidden Zone and not too small

Briefly, the purpose of the Entry Zone is to provide a means for the software to know that if the animal is “seen” in the Entry Zone and then disappears, they must be in the Hidden Zone. For each Hidden Zone you create, you are required to create and link at least one Entry Zone. If the animal being tracked accurately in the Arena and therefore tracked accurately in the Entry Zone, but then goes as “missing” once it enters the Hidden Zone, then there must be a problem with the Entry Zone not being properly “linked” to the Hidden Zone and/or the Entry Zone is too small.  

First thing to check is that the Entry Zone is properly “linked” to its respective Hidden Zone.  To do so, go to your Arena Settings and right-click on the Entry Zone and select “Link Hidden Zones”.  In the Light-Dark box example below, if one right-clicks the Entry Zone “Door” and selects “Link Hidden Zones”, the following box appears:

Thus, it is important to make sure that it is linked to my Hidden Zone called “Dark Side” by making sure the box is checked.  

Please note there’s also an option to “Use the entry zone also as a leaving zone”. By default, the animal is considered to have left the Hidden Zone when it is next detected, that is, when its body point(s) are “not missing”. However, every time an animal pokes its head out, this may be recorded as an exit.  Subsequently, retracting back into the Hidden Zone - without ever really leaving it- would count as entering again. This results in an overestimation of how often the animal enters/exits the Hidden Zone. Therefore, EthoVision XT versions 5.1 and up have the possibility to also use the Entry Zone as leaving zone. The animal is then considered to have left the Hidden Zone when it was first considered to be in a Hidden Zone, and subsequently detected outside of the entry zone. In other words, the exit is counted only when the body point crosses the border between the Entry Zone and the rest of the arena.  It is recommended to use this function if you want to prevent an overestimation of entrances into, and exits from, the Hidden Zone.

If the Entry Zone is properly linked to the Hidden Zone, then the most likely other cause of the issue is that the Entry Zone is too small. Keeping in mind that EthoVision must “see” the animal in the Entry Zone before they disappear (i.e., the enter the Hidden Zone), if the Entry Zone is too small and the animal enters the Entry Zone and leaves it into the Hidden Zone before a sample of data is collected by EthoVision, then they will be determined as “missing” and not in the Hidden Zone. This is more likely to happen if your subject is fast (e.g., a bird flying), or if the Entry Zone is too narrow and the animal can enter the Hidden Zone without its center point ever entering the Entry Zone.  Therefore, please make sure that the Entry Zone is big enough and slightly overlaps the Hidden Zone so that from no matter the angle that the animal enters the Entry Zone, it will be “seen” by EthoVision and determined that it is near the Hidden Zone.  If so, once they disappear (i.e., they enter the Hidden Zone), the software will accurately mark them as being in the Hidden Zone and not “missing”. It's normally ok to make the Entry Zone fairly large (although if "Use this entry zone also as leaving zone" is checked, bear in mind the animal won't be tracked until it leaves the Entry Zone).

Hidden Zones are for situations where the animal is out of view and impossible to track, so the animal is considered to have entered the Hidden Zone if it was in the Entry Zone and then disappeared. If the animal is detected while it is in the Hidden Zone, it's no longer hidden, and therefore it may be tracked as having left the Hidden Zone. If it subsequently disappears without being in the Entry Zone, it will not be counted as going back into the Hidden Zone. You can address this by making the Entry Zone large enough to cover the Hidden Zone.