Introduction
Since its development, thermal imaging technology was predominantly used in industrial applications. It was regularly used in construction fields to measure heat flow, identify structural defects (Grinzato, Vavilov et al. 1998, Balaras and Argiriou 2002), detect gas leaks, for powerline maintenance and for the management of forest fires. More recently, applications ranged from determining the structural features of volcanoes (Harris 2013), to medical applications such as detection of breast cancer (Mostovoy 2008, Ng 2009) and monitoring of pain (Agarwal, Spyra et al. 2008), (see (Vollmer and Möllmann 2018) for a full review of the use of thermal imaging across study areas).
Thermal imaging technologies were increasingly used in wildlife surveys from the 1960s (see (Croon, McCullough et al. 1968, Parker and Driscoll 1972)) although their widespread use has been limited due to the cost of the equipment and a lack of exposure of this type of equipment to biologists. The majority of wildlife survey work undertaken with thermal technology was in the detection of large wild animals such as pigs and ungulates using fixed or rotary wing aerial survey techniques (and occasionally comparing these results to visual surveys over the same area) (Parker and Driscoll 1972, Havens and Sharp 1998, Focardi, De Marinis et al. 2001). Limited work has been done on abundance estimates of smaller animals and the detection of fossorial animals and/or their burrows with thermal imagers (Boonstra, Krebs et al. 1994). Burrows of fossorial animals can be difficult to detect with more traditional white-light spotlighting used during ground surveys, particularly where vegetation is present. Additionally, those burrows that can be found often give little and subjective indication as to whether that burrow is occupied.
The detection of occupied burrows is particularly important when the burrowing animal is a pest, such as rabbits (Oryctolagus cuniculus ) in Australia. Rabbits are a significant agricultural and environmental pest and are listed as a key threatening process (Commonwealth of Australia 2016). Rabbits were estimated to cause >AUD$206 million per annum in agricultural losses (Gong, Sinden et al. 2009). Additionally, it was estimated that private and public landholders spend approximately AUD$6 million per annum controlling rabbits. They are listed as a direct threat for 321 species of Australian plants and animals and 75 endangered ecological communities (Commonwealth of Australia 2016). The most effective long-term method of controlling rabbits is the removal of their harbour. Usually this means the destruction of their burrow (hereafter referred to as a warren) through ripping programs, however, the success of ripping programs is greatly influenced by the presence of surrounding active warrens (McPhee and Butler 2010). Reopening of ripped warrens can occur if any nearby warrens remain intact, therefore it is essential that all warrens and warren entrances within the treatment area are located. Where rabbit numbers are high (>5 rabbits/Ha), warrens can be easy to locate due to the lack of vegetative cover. However, where numbers are lower, or non-palatable plants are abundant, warren entrances can be obscured and difficult to find.
Thermal imagers may provide a way to detect these obscured rabbit warrens. Boonstra et al (1994) used thermal imagers to differentiate between occupied and unoccupied arctic ground squirrel (Urocitellus parryii ) burrows (where the location of the burrow was known). They also identified that the presence of dense vegetation was a limiting factor in thermal surveys. Technological development of thermal imagers has progressed rapidly over the last 10 years, and there is a proliferation of thermal imagers available for consumers. Therefore, we felt it time to re-evaluate thermal imagers for the detection of animal burrows. We investigated whether consumer thermal imagers could be used as a tool to assist land managers with identifying rabbit warrens, particularly if obscured by vegetation. Here we investigate the use of thermal imagers to 1) determine whether active and inactive rabbit warrens could be detected with a thermal imager, and (if so) 2) to evaluate the efficacy of consumer imagers to professional imagers and visual inspection.