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by Tina Watters, Acting Conservation Programs Assistant
With over 3000 hectares of water within the Creston Valley Wildlife Management Area (CVWMA), it's no surprise how many fish-loving bird species like to gather here. The Double-Crested Cormorant (Phalacrocorax auritus) and Great Blue Heron (Ardea herodias) are two of these species that can be observed hunting for fish on CVWMA. Besides being fish lovers both species are also colonial nesters, that is they nest in large groups at rookeries. Rookeries can be established on the ground or in trees. Here in the valley, the herons and cormorants like to nest in a “tree” rookery with their nests overlapping with each other. The rookery is located in cottonwood trees, in Leach Lake, north and east of the Summit Creek area.
One way of spotting the rookery is to look for bare stark-white trees standing out from surrounding cottonwood forest, resulting from acidic cormorant feces that accumulate within nest and on the surrounding branches they perch upon. The buildup of feces over the years results in a change in the chemistry of the soil and kills both the ground and cover vegetation yielding the bare stark-white standing dead trees. Dead trees don’t provide much cover or stability and the cormorants tend to shift to neighbouring live trees in subsequent seasons. The rookery can be moved to a brand new area altogether if trees become unsuitable for nesting. Herons tend to follow the same pattern. Herons build their nests in the center of the colony first, expanding outwards. Cormorants will occasionally utilize unused or abandoned heron nests if they arrive later in the season. Through the years, we have noticed a gradual southwest movement of birds/nests in the cottonwood stand and a gradual increase in nest numbers. Cormorants lay 1-7 eggs per brood while herons lay 2-6 eggs, with both species generally having one brood per season. When nesting attempts are successful, more birds may return in subsequent years. Although having a larger population of nesting cormorants is fantastic, this accelerates the trees' demise.
Surveying cormorant and heron nests up high in trees requires different methods and tools than surveying waterfowl on a pond or a lake. While ducks can be observed from the ground with binoculars and telescopes, colonial nesters such as the cormorants and herons are better seen and more effectively counted from above in the air. CVWMA has been contracting a company specializing in drone imagery to complete aerial surveys of our rookery. Surveys are generally conducted around the same day every year, near the end of June. This is invaluable for consistency and for comparing our results year-to-year. Capturing the images during nesting season is important so that we do not get pictures of unfinished or unoccupied nests. The images usually show nests with young ones inside. This year the photos were taken at 55m, 65m and 75m above the ground, along with some close-ups of various nests. Once the images have been processed, CVWMA staff members review the photos and count/identify nests for both nesting species. Some nests can be difficult to spot, resulting in a very challenging game of “I Spy.”
The past year (2022) was the 9th season of our 10-year monitoring project of the Double-Crested Cormorants for the Pacific Flyway Non-Game Committee. While heron nests used to be more abundant than cormorant nests, the cormorant population in the valley has increased significantly and the number of nests has now surpassed heron nests at the CVWMA rookery. This year we counted 222 (up from 201 in 2021) cormorant nests and only 30 heron nests. As drone imagery produces higher-quality photos every year, counting becomes easier. It will be interesting to see the overall results for the Pacific Northwest at the end of the 10-year monitoring period. It has been exciting to see how the rookery has evolved and changed over time.
Check the rookery out this spring once the Double-crested Cormorants and Great Blue Herons are back from migration!
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