Using GPS location data to assess red fox responses to predator exclosure setup and hatching at shorebird nests

Wire cages (i.e., predator exclosures) are frequently used to protect shorebird nests from predation. While exclosed nests often have higher survival than unexclosed nests, concerns exist over whether the conspicuous structures might draw attention of predators, potentially increasing risks to adults and newly-hatched chicks. We present a new approach to investigating predator responses to exclosures and other stimuli at shorebird nests. We used location data from GPS-collared red foxes (Vulpes vulpes) to examine short-term spatial responses of foxes to exclosure setup and hatching at piping plover (Charadrius melodus) nests on Fire Island, New York. We compared mean distances between fox locations and piping plover nests, and the proportions of fox locations within a 100-m buffer of nests, before and after exclosure setup and before, during, and after hatching. Mean distances from fox locations to nests were similar before versus after exclosure setup (x = 641 m before, 675 m after; permutation test for matched pairs, [PTMP], T = 0.28, P = 0.56, n = 9 foxes), before versus during hatching (x = 417 m before, 340 m during; PTMP, T = -0.93, P = 0.18, n = 5 foxes) and during versus after hatching (x = 340 m during, 330 m after; PTMP, T = 1.00, P = 0.84, n = 5 foxes). The proportions of fox locations within 100-m buffers surrounding nests were similar before versus after exclosure setup (x = 0.01 before, 0.03 after; PTMP, T = 0.26, P = 0.53, n = 9 foxes), before versus during hatching (x = 0.04 before, 0.15 during; PTMP, T = -1.44, P = 0.09, n = 5 foxes), and during versus after hatching (x = 0.15 during, 0.14 after; PTMP, T = 0.84, P = 0.80, n = 5 foxes). In 10 of 19 of our individual-level comparisons, there were no significant differences in the distances from fox locations to nests between paired event periods. In the other 9 individual-level comparisons, we found evidence of shifts in fox space use between paired event periods, but no clear directional trend across individuals. We conclude that the foxes in our study did not appear to consistently key in on exclosure setup or hatching at piping plover nests at the spatial scales considered in our comparisons. We recommend that managers continually re-evaluate the effects of exclosures at their management sites, as changes in nest predation, abandonment rates, and adult plover survival may all influence the relative benefit of exclosures for population growth. Our study illustrates the potential for using spatial data to investigate predator responses to various management actions taken to protect nesting shorebirds and other prey species of conservation interest.