Predator indices from artificial nests and tracking tunnels: do they tell the same story?

Artificial nests and tracking tunnels are alternative predator encounter devices that can be used to predict predation risk to native species. Tracking tunnels are used ubiquitously in New Zealand, whereas artificial nests are used extensively overseas. To assess whether these devices give similar information about predation risk, we compared tracking tunnel and artificial nest data from 16 native forest fragments in the central North Island over two summers. The fragments were expected to vary in predation risk due to rat control in some fragments, and possibly due to habitat differences caused by stock grazing. We modelled the ‘survival’ of both types of devices, where ‘survival’ is defined as the probability of not being tracked or bitten by a rat over a set period (possum bites were also considered to indicate nest failure, but were rare). We used a Bayesian hierarchical framework that allowed for random effects of individual devices as well as fragments, and random time effects. Data from both devices showed clear effects of rat control, but otherwise gave different results. Tracking tunnel survival varied greatly among fragments in the absence of control, with survival generally higher in grazed than ungrazed fragments, whereas no such pattern occurred in artificial nest survival. Different habitat variables explained variation in survival at both site and fragment level; understorey density was the only useful predictor of tracking tunnel survival, whereas artificial nest survival was correlated with canopy cover, vegetation cover at 1.5 m, and supplejack presence. When both data types were modelled simultaneously, the tracking tunnel data improved capacity to explain the artificial nest data whereas the reverse was not true. Consequently, while it is unknown whether the additional inter-fragment variation detected by tracking tunnels indicates real variation in predation risk, we currently see no reason to prefer the more labour-intensive artificial nests.