Acoustic recorders reveal the impact of invasive rats on nocturnal burrow-nesting seabirds in Haida Gwaii.

Haida Gwaii supports 1.5 million nesting seabirds, including 50% of the global Ancient Murrelet population and significant nesting populations of several other seabird species. Invasive rats pose a significant threat to the conservation of these populations. In 2010, Gwaii Haanas National Park Reserve and Haida Heritage Site initiated the Night Birds Returning project with the goal of eradicating invasive rats on a number of islands. I used automated acoustic recording units (ARUs) to explore seabird presence and relative abundance of Ancient Murrelets Synthliboramphus antiquus, Cassin’s Auklets Ptychoramphus aleuticus, Fork-tailed Storm-Petrels Oceanodroma furcata and Leach’s Storm-Petrels Oceanodroma leucorhoa on rat-infested and rat-free islands in Gwaii Haanas, during the main breeding season. I assessed the effect of rat presence using two measures derived from acoustic data, relative abundance and attendance period. Relative abundance was higher on rat-free islands, compared to rat-infested islands, but was still less than half that of rat-free islands. Attendance periods were also longer and vocal activity more regular on rat-free islands. My results showed a statistically significant negative effect of rat presence on detections of Ancient Murrelets (p = 0.01) and Fork-tailed Storm-Petrels (p = 0.03), but the effect was not significant for Cassin’s Auklets (p = 0.31) and Leach’s Storm-Petrels (p = 0.47). That rats had not extirpated all seabirds from these islands came as a surprise. Indeed, data suggest that breeding may in fact be occurring, though whether this results in successful fledging still remains unknown. On a positive note, my results suggest that seabird recovery may occur more rapidly following rat eradication on these islands, compared to islands where the species of interest are extirpated.