Factors affecting bird attacks on small mammal traps

Live trapping is a common survey technique used to sample small terrestrial mammal populations, but too often traps are disturbed by non-target species, especially by birds such as corvids, resulting in a considerable waste of survey time and effort. This study aimed to test whether visual cues associated with trapping increased the disturbance rate of traps by birds, which are renowned for their attraction to visual stimuli. We manipulated trap covering (plastic bag, hessian and uncovered traps) in conjunction with the presence and absence of flagging tape, and randomly assigned treatments to traps in 180 locations for checking in either the morning or the evening. The plastic bag cover was the primary cause of disturbance rates with approximately 50% of all traps with plastic bags disturbed compared to uncovered traps (2%) or traps covered in hessian (9%). Flagging and time of day had no effect on the likelihood of disturbance. It is probable that trap-raiding birds may have associated plastic with scavenged foods from nearby picnic areas and sources of rubbish. We suggest that hessian provides a reasonable alternative to plastic bags, giving shelter for trapped animals from rain and exposure while reducing the likelihood of trap attack by birds.