The Western Grebe (Aechmophorus occidentalis) is a piscivorous bird that relies on water for foraging, nesting, and courtship rituals. These birds commonly build their nests in marshy areas characterized by emergent vegetation. These nesting habitats are often subjected to water drawdowns from agricultural use.
Changes in water level can affect nest survival and cause grebe colonies to abandon or change sites in the future. The objective of my investigation was to quantify factors hypothesized to influence grebe colony site selection: mean water depth, and vegetation cover. I conducted my research at Cascade Reservoir in Cascade, Idaho which is home to the largest breeding colony of Western Grebes in Idaho. The water level at Cascade Reservoir can fall ~60cm during the grebe breeding period. I sampled three previous grebe colony sites as well as the current colony site during the 2018 breeding period. I sampled 60 bathymetry points at each of the four sites during peak water level. I also used a UAS (drone) to collect aerial imagery of each site to estimate vegetation cover. I categorized vegetation cover into three categories: open water; sparse vegetation; or full vegetation.
Results/Conclusions
The mean water depth at the current site was 57.84cm while the other sites varied significantly at 36.2cm, 66.9cm, and 87.5cm. The vegetation structure also varied significantly among sites. The current colony site had 90% sparse vegetation and only 0.7% open water, while the other sites were primarily open water with 47%, 58%, and 88%. The grebe colony also had one of the lowest percentages of full vegetation cover at 9.3%. The differences in mean water depth between the sites suggests that water depth may influence colony site selection. Also, there may be a preferred range of water depth that is neither too shallow nor too deep. The low percentage of open water at the colony may inform the colony’s susceptibility to water drawdowns, however, further research is required. These results demonstrate that the Western Grebe requires a specific combination of water depth and vegetation cover. Cascade Reservoir is a highly managed environment and the areas that contain the proper mix of water depth and sparse vegetation may change from year to year. This information can be valuable in informing water use policies as well as predicting future colony sites.