In many situations, colonial waterbird colony size is best evaluated from aerial photographs taken during the breeding season. This is typically the case where colonies are not accessible or when it is not feasible to count all birds or nests from ground locations or by boat. This dataset presents high resolution aerial imagery and corresponding nest counts of Caspian terns (Hydroprogne caspia), double-crested cormorants (Nannopterum auritum), Brandt’s cormorants (Urile penicillatus), California gulls (Larus californicus), ring-billed gulls (Larus delawarensis), American white pelicans (Pelecanus erythrorhynchos), great blue herons (Ardea herodias), great egrets (Ardea alba) nesting in the Columbia River Basin, during the 2024 breeding season. Photos were taken from a fixed-wing aircraft during peak nesting, and colony size was estimated by digitizing photos and enumerating visible birds using ArcGIS. Colony size was reported as the number of birds on colony, and, in the case of terns and cormorants, the number of active breeding pairs. These data are part of a wider project to evaluate the efficacy of management actions to reduce the impacts of predation by piscivorous colonial waterbirds on Endangered Species Act (ESA)- listed juvenile salmonids (smolts; Oncorhynchus spp.) in the Columbia River Basin.

Three fixed-winged aerial surveys were conducted in 2024, on the 17th – 18th May, 31st May – 1st June, and 15th–19th June, to help identify all active nesting colonies of piscivorous waterbirds in the region. To estimate the size of waterbird colonies, aerial photos were taken at oblique angles using a Canon EOS 7D digital SLR camera with one of two image stabilizing lenses, Canon EF 70 - 200mm or Canon EF 100 - 400mm. The lens and camera settings used are included within the image metadata. Photos were taken through the open window of the plane during low-altitude flyovers, ~800 – 1,200 feet above the colonies. Aerial images were reviewed, and select images were imported to ArcGIS (ESRI ArcGIS Pro 3.30 and ArcMap 10.8.1, Redlands, CA) to be used for manual colony photo counts. Using ArcGIS, point shape files were created for each image or series of images, depending on colony size. Points were placed manually on either attended nests or individual birds and then summed to give the total. To estimate the number of breeding pairs, direct counts of attended nests were used. However, in some situation,s it was not possible to count breeding pairs or nests; in these cases, the number of individuals was used as a surrogate for breeding colony size.

Each photo was counted by at least two counters. To generate an estimate of error, all counters conducted two complete passes of the same photo(s). A pass was defined as examining all areas thoroughly enough that any bird captured in the photo may be counted. To improve the accuracy and consistency of the counts, the total count from each counter was always within 10% of the other. If the count data resulted in differences larger than 10%, counters conducted a third pass or recounted until the average of both counts was within 10%. Once all images were counted, the peak count was determined for each colony and species by comparing total counts from each aerial survey. 

Each species had a specific counting method.

American white pelicans 

All individual American white pelicans that were visible on or near the breeding site were counted. Birds flying over the colony were not counted. Once chicks were large and mobile, no counts were made, and the photo(s) were recorded as “mobile chicks- not counted”.

California gulls and Ring-billed gulls 

All individual gulls that were visible on the colony were counted. Birds flying over the colony were not counted. The two gull species are not consistently distinguishable from aerial photos, therefore, California gulls and ring-billed gulls were grouped and counted together as the total numbers of breeding "gulls" at a colony. If chicks were large and mobile, no counts were made and the photo(s) were recorded as “mobile chicks- not counted”.

Caspian terns 

Caspian terns were counted as either individuals or as attended nests, depending on the site. When counting individuals, all Caspian terns visible on the colony were counted. When counting attended nests, all Caspian terns sitting in nest scrapes were counted. If a pair was present at the nest scrape, only one of the birds was counted in an attended nest count. If chicks were large and mobile, no counts were made, and the photo(s) were recorded as “mobile chicks- not counted”.

Double-crested cormorants and Brandt’s cormorants

Early in the breeding season adults were sitting tight on nest structures making nest sites easier to identify, thus attended nests were counted. However, as the breeding season progresses and the number and size of chicks increases, the nest identity can be difficult to determine or be lost. If this was the case, then no counts were made and the photo(s) were recorded as “mobile chicks- not counted”.

Great blue herons and great egrets

Some double-crested cormorant breeding sites were within mixed species colonies; in these cases, great blue herons (Ardea herodias) and great egrets (Ardea alba) that attended nests were also counted.