Considerations for a threatened seabird: The impact of shoreline avian predators on at-sea marbled murrelets

Abstract

We tested the influence of on-shore avian predators on the at-sea distribution and abundance of marbled murrelets (Brachyramphus marmoratus) during their breeding season in Haida Gwaii, British Columbia, Canada. We conducted a field experiment using deterrent predator decoy kites that mimicked flying bald eagles (Haliaeetus leucocephalus). In the summers of 2018 and 2019, we conducted at-sea surveys of murrelet distributions along inshore and offshore transects with and without kites flying, and tallied real eagles along the shoreline and fish schools encountered. Kites negatively influenced overall murrelet counts (estimate = −1.19, 95% CI = −1.88 to −0.51), but we did not detect inshore to offshore movements within the study site. Our results indicate murrelet movement out of the study area in response to the kites. Overall murrelet counts were also lower when real eagle counts were higher (estimate = −0.22, 95% CI = −0.35 to −0.09). When kites were flying, a higher proportion of the murrelets remaining along inshore transects were found between rather than adjacent to kite locations (estimate = −1.61, 95% CI = −2.67 to −0.54), indicating avoidance of kites. Since avian predator populations have steadily increased in the past 30 years throughout the murrelet's breeding range, these avoidance effects could create a negative bias in long-term shoreline count trends. Our findings highlight the importance of considering non-lethal predator effects on murrelets when conducting at-sea censuses and constructing conservation plans.