Aerial photos and colony counts of nesting colonial waterbirds in the Columbia River Basin, 2023

Description

Notes

Methods

Three fixed-winged aerial surveys were conducted in 2023, on the 13th–15th May, 3rd–5th June, and 15th–16th 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 is 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 2020. ArcGIS Desktop: Release 10.8.1. Redlands, CA) or DotDotGoose (Ersts, P.J. Version 1.7.0. American Museum of Natural History, Center for Biodiversity and Conservation) 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. In both programs, 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 situations 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 clearly 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".