Abstract
Western grebe (Aechmophorus occidentalis) populations in North America declined by 90% between 1970 and 2017. Western Grebes nest in breeding colonies and traditional nest monitoring methods cause substantial disturbance to the breeding colony. We explored a non-invasive technique to estimate nest survival and to identify the factors that influence nest survival. We used a small unmanned aerial system (drone) to map and monitor the largest grebe colony in Idaho at Cascade Reservoir. We conducted six flights between 20 June – 11 July 2018 and used the photographs from each flight to create an orthomosaic image that we then digitized and georeferenced. We used the orthomosaic images to assess fine-scale habitat selection for 940 nests as well as recreate nest histories and estimated nest fate for 709 grebe nests. We created a resource selection function using use/non-use data to assess nesting habitat preference. Similarly, we used program MARK to model nest survival. We found that Western Grebes preferred to nest in water between 40-80cm and probability of use increased as distance to open water increased. Our nest survival analysis indicated that when extrapolated to a 30-day nesting cycle, our model estimated that nest success was 51.8%. Probability of nest survival was positively correlated with water depth at the nest and the aggregation index, and negatively correlated with distance between the nest and the colony center and distance to deep water habitat. The results of this study can be used to inform conservation efforts by identifying which habitat characteristics Western Grebes are selecting for as well as identifying areas of the colony that are most.