2024 Pacific flyway region Caspian tern (Hydroprogne caspia) colony surveys – aerial photos and colony count data

The 2024 Caspian tern (Hydroprogne caspia) minimum breeding population in the Pacific Flyway region was estimated by census of known active and historical colony sites. We collected survey data for 155 colony sites (c.a., 98%, 88%, and 93% of known active, historical, and potential colony sites, respectively), including 37 colonies that were confirmed to be active in 2024. This is the fourth population survey conducted under a monitoring strategy developed by the U.S. Fish and Wildlife Service in 2015 and undertaken every third year. As part of this effort, Oregon State University flew aerial surveys from fixed-wing aircraft to locate and photograph Caspian tern colonies in Washington, Oregon, California, Alaska, Nevada, and Utah. Oblique aerial photos were analyzed to estimate the number of nests at active colonies. Here we provide aerial photographs and count summaries for active colonies surveyed by Oregon State University in 2024. Additional colonies in the Columbia River Basin are archived in a separate dataset, “Aerial photos and colony counts of nesting colonial waterbirds in the Columbia River Basin, 2024”, DOI: 10.5061/dryad.k98sf7mht.

Methods
In 2024, we attempted to collect colony size estimates from all known active, historical, and potential colony locations in the Pacific Flyway region, and for any new colony sites identified during our census effort. Surveys of all colony sites were targeted to capture the peak colony attendance, based on regional and site-specific nesting chronologies from previous years. Numbers of attended nests were quantified using different methods depending on colony location, accessibility, collaborator availability, and independent ongoing research or monitoring efforts. These methods included fixed-wing aerial surveys, UAV surveys, boat surveys, and ground visits. Aerial photography of active colonies was obtained whenever possible, which allowed researchers to efficiently observe colonies across large geographic regions during a short window during peak nesting. Aerial surveys provided high resolution oblique photography (Canon DSLR and mirrorless cameras with 100-600 mm equivalent zoom lenses) taken while flying over colonies at low altitude (ca. 500 – 1200 feet AGL). For larger colonies, multiple images were stitched together into high-resolution files using Adobe Photoshop (Version 25.1; San Jose, CA). When possible, these stacked images were captured in rapid succession using 12 fps bursts to avoid artifacts from perspective shifts (parallax) due to photographing from a moving aircraft platform and to eliminate double counting birds in flight. Ground based photos were used when provided by collaborators, and if aerial photography was not possible. Photography of each active colony was analyzed by two independent observers using either ArcGIS Pro (Version 3.3; ESRI, Redlands, CA) or DotDotGoose (Version 1.7.0; Ersts 2024) to enumerate nests and loafing adult terns. Counts were finalized when count totals were within 10% (calculated by dividing the difference between the two counts by the mean of the two counts). This imagery was funded by the Bureau of Reclamation (Reclamation), US. Department of Interior. Funding was provided by Reclamation as part of its mission to manage, develop, and protect water and related resources in an environmentally and economically sound manner in the interest of the American public. Funding was provided through Grant R23AP00195. The views in this report are the author's and do not necessarily represent the views of Reclamation.