Passive infrared triggered camera traps: Detecting wildlife, but not as you know it

Camera traps are a useful tool for detecting wildlife, but it is important to have a sound understanding of how they operate.  Dustin Welbourne, author of a recent paper, published in Remote Sensing in Ecology and Conservation, sheds light on how these fantastic tools work and addresses some common misconceptions appearing in the literature.

Camera traps can be used to detect a range of fauna, and often more effectively than traditional detection methods. While there are various types of camera trap, those triggered by passive infrared (PIR) sensor are used most frequently.

If you have been using PIR triggered camera traps you likely have an idea of how they detect wildlife, or at least how you think they detect wildlife. In fact, a quick Google search for descriptions of how PIR sensors detect fauna will return numerous logical and seemingly intuitive explanations.

thermogram-of-a-warm-lizard-moving-across-a-cooler-background

 Thermogram of a warm lizard moving across a cooler background

Unfortunately, many descriptions in the popular press, blogs, and magazines are misleading and at times manifestly incorrect. Worse still, descriptions in the peer-reviewed wildlife research literature have frequently been no better. For instance, an often repeated and yet ambiguous description is that PIR-triggered camera traps detect the difference between an animal and the ambient temperature. This is ambiguous since ‘ambient temperature’ might refer to the environment generally or the air temperature.

The problem with unclear descriptions of how wildlife sampling equipment function arises when we interpret our data or consider how to employ the equipment. When using any device, our interpretation of the collected data is coloured by how we think the device collects the data.

Equally, the manner in which the equipment is used will be constrained by how we think it operates. For instance, if I think PIR-triggered camera traps only detect fauna that are warmer than their environment, as has been suggested, then I would likely infer that because reptiles can often be cooler than their environment that camera traps are not useful for detecting reptiles, as has also been suggested.

using-camera-traps-to-detect-terrestrial-reptiles-is-certainly-possible-and-effective

Using camera traps to detect terrestrial reptiles is certainly possible and effective

Although there are relatively few instances in the primary literature of problematic descriptions directly affecting the interpretation of camera trapping data, wildlife researchers do need to have a clear understanding of how their equipment function.

In short, PIR sensors function by detecting differences in surface temperatures between objects. So, a warm animal walking in front of a somewhat cooler background will be detected, just as a cool animal walking in front of a somewhat warmer background.

Importantly, it is the surface temperatures of objects that the sensor is detecting, not the internal temperature. And, air temperature does not affect the sensor. In essence, the PIR sensor cannot ‘see’ the air, but sees through it.

Our paper aims to raise the understanding of how these valuable tools work.

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