The challenges of recognising individuals with few distinguishing features: Identifying red foxes Vulpes vulpes from camera-trap photos

Abstract

Over the last two decades, camera traps have revolutionised the ability of biologists to undertake faunal surveys and estimate population densities, although identifying individuals of species with subtle markings remains challenging. We conducted a two-year camera-trapping study as part of a long-term study of urban foxes: our objectives were to determine whether red foxes could be identified individually from camera-trap photos, and highlight camera-trapping protocols and techniques to facilitate photo identification of species with few or subtle natural markings. We collected circa 800,000 camera-trap photos over 4945 camera days in suburban gardens in the city of Bristol, UK: 152,134 (19%) included foxes, of which 13,888 (9%) contained more than one fox. These provided 174,063 timestamped capture records of individual foxes; 170,923 were of foxes ≥ 3 months old. Younger foxes were excluded because they have few distinguishing features. We identified the individual (192 different foxes: 110 males, 49 females, 33 of unknown sex) in 168,417 (99%) of these capture records; the remainder could not be identified due to poor image quality or because key identifying feature(s) were not visible. We show that carefully designed survey techniques facilitate individual identification of subtly-marked species. Accuracy is enhanced by camera-trapping techniques that yield large numbers of high resolution, colour images from multiple angles taken under varying environmental conditions. While identifying foxes manually was labour-intensive, currently available automated identification systems are unlikely to achieve the same levels of accuracy, especially since different features were used to identify each fox, the features were often inconspicuous, and their appearance varied with environmental conditions. We discuss how studies based on low numbers of photos, or which fail to identify the individual in a significant proportion of photos, risk losing important biological information, and may come to erroneous conclusions.

Individual and seasonal variation in contact rate, connectivity and centrality in red fox (Vulpes vulpes) social groups

“…Cameras were active continuously for 40 days for four consecutive seasons: spring (March-May; birth, early cub rearing), summer (June-August; late cub rearing/early independence), autumn (September-November; onset of dispersal), winter (December-February; peak dispersal/mating). The first survey on each territory varied; consecutive surveys were ≥39 days apart 35,54 . Since foxes are crepuscular/nocturnal 55 , ‘days’ started/ended at noon to ensure independent sampling.…”

“…Where possible, sex was determined from visible features. Foxes were identified in 99% of photographs 54 . A fox was only considered resident on one territory/season: residents were photographed on ≥20 days/season and shared ≥2 associations with another resident 7,33 .…”

Understanding the intricacy of canid social systems: Structure and temporal stability of red fox (Vulpes vulpes) groups

“…Between July 2013 and June 2015 we positioned camera traps in 4 to 6 residential back gardens in each of seven fox territories; cameras were positioned to record visits to locations (food patches) where the householders provisioned the foxes regularly [61,65]. All residents at our field sites gave us permission to conduct the study in their gardens.…”

“…Not all territories were surveyed concurrently due to logistical constraints. Full details of the timing of the surveys in each territory, camera trapping techniques, data collection and handling, are given in [61,65]. We only included foxes > 5 months old in the analyses and identified the individual fox in 99% of capture records; full details on the techniques used to identify each fox, and levels of accuracy, are given in [65].…”

“…[52,55,57]. Some foxes visited several territories so appeared in more than one interconnected network of animals behaving socially [50,61,65]. To avoid preconceived assumptions about social structure, there was no criterion for inclusion or exclusion of an individual from a particular network, and so all foxes were retained and territories analysed independently.…”