Abstract here from the IMC Congress in Perth, they found the higher-res versions are much better than the less expensive (e.g. iphone) versions when it comes to finding animals. 

Now you see me: How thermal imaging increases detection of cryptic nocturnal mammals in tropical regions

Avril Underwood1 , Mia Dehre-Miller2 1. James Cook University, Smithfield, Queensland, Australia 2.

University of Lancaster, Lancaster, United Kingdom Accurate measures of species abundance and distribution underpin effective wildlife management and conservation strategies. Obtaining these data is very challenging when researching the cryptic nocturnal arboreal mammals of in the dense forest of the Wet Tropics. Our research shows that using a hand-held thermal imager significantly increases the detection rate of all mid-sized arboreal mammals when compared to the most commonly used method of spotlighting. We determine the optimal environmental and mechanical parameters for thermal imaging, and we investigate ‘cost-versus-result’ of hand-held imaging devices currently available on the market. We suggest that hand-held thermal imaging is a great emerging technology for cryptic arboreal mammal detection across tropical forested regions and beyond.

My own experience is with the FLIR iphone attachment, just informal testing, but hasn't been that great at finding animals.  I also think it would work better in the early AM.  In the evening there can still be a lot of rocks and trees still warm from the sun.