Photogrammetric monitoring of double-crested and Pelagic Cormorants nesting sites on urban bridges and remote seacliffs in the Salish sea

Double-crested (Phalacrocorax auritus) and Pelagic Cormorants (P. pelagicus) nest colonially on cliffs and offshore islets. In the Salish Sea, a shift from natural sites to bridges has occurred. Significant cormorant colonies are nesting on Vancouver city bridges, conflicting with provincial bridge management efforts. In recent years, exclusionary measures have been implemented to keep bridges cormorant-free and more are proposed. This study investigated their nest-site selection on urban bridges and at a natural seacliff in the Salish Sea by using time-series data captured by GigaPan®, Sony, and GoPro systems from May 2020 to March 2021. Results show breeding occurred between April and August with nest-site selection dependent on central bridge structures, and on heterogeneous cliff ledges. Bridge nest site selection demonstrates a central colony preference. Nests begin to disappear (fall off) from the bridge in August, as the young fledge from the nests. These results suggest further monitoring is needed to demonstrate nest-site selection and inform cormorant management on Vancouver's bridges.