2024 Oregon and Washington double-crested cormorant (Nannopterum auritum) colony surveys – aerial photos and count data

Aerial surveys were undertaken to monitor selected double-crested cormorant (DCCO; Nannopterum auritum) colonies in Oregon to fulfill monitoring requirements associated with the United States Fish and Wildlife Service (USFWS) final environmental impact statement: Management of Conflicts Associated with Double-crested Cormorants (USFWS 2020). Survey methods follow guidelines established by the Pacific Flyway Council monitoring strategy: A Monitoring Strategy for the Western Population of Double-crested Cormorants within the Pacific Flyway (PFC 2013). This dataset presents high resolution aerial imagery and corresponding nest counts from areial surveys of double-crested cormorant (DCCO; Nannopterum auritum) colonies in Oregon and Washington. Survey results are intended for use by USFWS and Pacific Flyway Council members, including ODFW and other state and federal agencies, to determine population size and evaluate long-term trends in abundance of DCCO within the Pacific Flyway. Additionally, colonies in coastal Washington were surveyed opportunistically and aerial photos were shared with WDFW. Monitoring DCCO numbers is essential for evaluating effects on population size of permitted management actions currently underway across the western United States.

Methods
We conducted aerial surveys at focal double-crested cormorant (DCCO; Nannopterum auritum) colony sites in Oregon and Washington during May and June of 2024. Colony sites were photographed if cormorants were present, or if there were any signs of nesting activity (i.e., nest structures). We also opportunistically surveyed and photographed four supplemental cormorant colony sites along our survey route. Flight timing was based on a best estimate of peak nesting activity from monitoring conducted in previous years and from available published literature. We surveyed and photographed each colony from a fixed-wing aircraft (Cessna 180 Skywagon) at altitudes between 700 and 1,500 feet above ground level. The pilot circled above each target colony, and we used a mirrorless digital camera (Canon R5) with an image-stabilized lens (Canon 100 – 500mm; f/4.5-7.1) to photograph the colonies. Photographs were taken from the front right window of the aircraft, which was opened at survey speeds to reduce potential distortion.

For Oregon colonies, we followed protocols and procedures identified in the Cormorant Management Plan to measure colony size as the number of apparent active nests at each colony at the time that the oblique aerial photography was taken. Colony counts were performed by two independent reviewers using Dot Dot Goose software (Version 1.7.0) to analyze aerial imagery taken during survey flights. We estimated the number of breeding pairs at each DCCO colony as the average nest count of the two photography reviewers. When counting colony photography, reviewers identified active nests by the presence of adults on nest structures in incubating or brooding posture. Nests were considered active if they were unattended but contained eggs and/or chicks. All counts were obtained from aerial photos and no ground visits were conducted as part of this project. Washington colony photos were not counted or analyzed by Oregon State University for this project.