= Camera trap
A <camera trap> is a camera arranged to record animals at a location without a person continuously operating it there. The trigger and sampling procedure determine what becomes a recorded encounter. Images can reveal occurrence, activity or recognizable individuals, but a photograph is not automatically an independent animal count.
In a 1995 <tiger> study, cameras along travel routes produced 31 photographic captures of ten identified animals over nine sampling occasions. Researchers analyzed the resulting encounter histories with <mark and recapture> models, including variation in detection probability among individuals.<Estimating tiger Panthera tigris populations from camera-trap data using capture-recapture models by K. Ullas Karanth>[Abstract: photographic identification, sampling occasions and detection heterogeneity]{ref}
An original toy example has three images of animal A, two of B and one of C. There are six photographs, three detected animals and an unknown number of undetected animals. Counting images answers the first question. Recognizing individual markings answers the second. Estimating total abundance additionally requires assumptions about the observation process.
Sampling effort matters too. Two cameras operated for three nights provide six camera-nights of nominal effort, but that equality alone does not make them equivalent to six cameras operated for one night: location and temporal coverage differ. State how encounters are defined and how repeated images are handled before interpreting a photographic detection rate as a biological difference.
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