I agree with Peter that a handheld thermal camera would fit your needs best.
It works during the night and by daylight, helps to find unobstrusive or well camouflaged species. The only thing is: useful devices are still quite expensive, they start at < 1300,- EUR with a resolution of mostly 384x288 pixel with different lens (19mm, 23mm, 25mm, 30mm) depending on manufacturer (Liemke, Guide, Pulsar, FLIR and others) and model. Higher resolutions are available, 640x480 start at about 3000,- EUR, depending on features.
The attached picture is an extracted image of a footage made from a distance of about 70 meters. It shows a thawny owl while attracting a female by singing at a cavity. With my bare eye I couldn't see anything. Some minutes later the owl started flying through the sparse forest and could be followed by the thermal imager for about 200m.
Last edited by Vollmeise : Monday 11th December 2017 at 19:28.