Feral cats (Felis catus), cats that live in the wild and can survive without human resources, are a key threatening process to native species around the world (Medina et al., 2011; Risbey et al., 2000; Sims et al., 2008; Smith and Quin, 1996), and cause billions of dollars damage to the natural and agricultural environment (Pimentel, 2007; Stelzer et al., 2019). Predation by feral cats has been identified as one of the major obstacles to the successful reintroduction of extirpated native fauna (Hardman et al., 2016; Moseby et al., 2011; Priddel and Wheeler, 2004; Short, 2016). Therefore, the suppression of feral cat populations is a critical component to the successful conservation of small to medium-sized native fauna (Fischer and Lindenmayer, 2000; McKenzie et al., 2007).
While implementing methods of feral cat control is difficult; measuring the outcomes of feral cat control, the pre- and post-management abundance of cats, is harder. Detecting feral cats is difficult because they are a cryptic species that avoids interactions with humans (Gosling et al., 2013) and in some environments cannot be readily detected via remote sensing technologies, such as camera-traps, despite the use of lures (Moseby et al., 2015; Stokeld et al., 2016). Estimating the abundance of an animal species typically requires capturing or identifying individual animals on multiple occasions (Lancia et al., 2005). Capturing feral cats on multiple occasions is extremely difficult, requiring the use of multiple labour-intensive techniques (McGregor et al., 2016) because feral cats frequently lack the unique markings required to identify individuals remotely (Rees et al., 2019). Failure to detect a feral cat when it is present (false zeros) is detrimental to parameter estimation in abundance estimation models (MacKenzie and Royle, 2005; Moilanen, 2002). Therefore, even models that do not require information on individuals, such as occupancy models, require detection probabilities to be greater than 0.3 (MacKenzie et al., 2002).
Despite the difficulties, our need to monitor the efficacy of feral cat management continues to drive research with the aim of estimating the abundance (Bengsen et al., 2011; Elizondo and Loss, 2016) or occupancy (Comer et al., 2018; Hohnen et al., 2016; Jean-Pierre et al., 2022; Vanek et al., 2021) of feral cats on the global landscape. And unreliable estimates derived from detection probabilities below 0.3 are being published (Krauze-Gryz et al., 2012; Taggart et al., 2019; Wysong et al., 2020b). In 2021, we had a similar intent to estimate the abundance of feral cats via camera-traps, but on 130 camera-traps we only detected cats on five occasions (C. A. Lohr et al., 2021). All the referenced exemplar studies used camera-traps to detect feral cats.
Therefore, there is a need to improve our ability to detect feral cats using camera-traps and minimise the risk of false zeros. Prior studies have tested several lures for feral cats including artificial olfactory lures, food-based lures, and visual lures with insignificant results (Read et al., 2015; Stokeld et al., 2016). Spraying wooden stakes with 4 ml whole cat urine on a single occasion increased the detection rate for cats sixfold but still only generated detection rates of 6 to 22% (Hanke and Dickman, 2013). Increasing survey duration for cameras with a food lure to 30 days can generate appropriate detection probabilities (Moore et al., 2020).
Building on the results generated by Hanke and Dickman (2013), we compared camera-traps with the olfactory lure commonly used for feral cat research in Western Australia, Catastrophic (Outfoxed Pest Control, Victoria, Australia) against passive cameras (no lure), and Magnum-Scrape Drippers® (Wildlife Research Center, Minnesota, USA) filled with whole cat urine. We hypothesised that the slow automated application of cat urine over time would produce a greater magnitude of difference between passive cameras and lured cameras and might generate detection probabilities above 0.3.