Evaluation of a Survey Method for Estimating and Monitoring the Number of Active Bald Eagle Nests in Kenai Fjords National Park

Thompson, William; Hall, Shelley; Lindsay, Charles

The Southwest Alaska Network (SWAN) is monitoring the condition of key natural resources or "vital signs" in five national park units in southwest Alaska, including Kenai Fjords National Park (KEFJ). Bald eagles were selected as a vital sign because of their important ecological role in freshwater and marine coastal systems. The proposed monitoring objectives are to estimate long-term trends in nest occupancy and productivity of bald eagles in SWAN parks. In KEFJ, bald eagle nest surveys have not been conducted since 2002 and previous surveys, which were primarily boat- and ground-based, did not incorporate a sightability correction. The U. S. Fish and Wildlife Service (USFWS) recently proposed a double-observer method for surveying bald eagle nests that provides a sightability correction for nests that are missed. Another component of the USFWS design requires an up-to-date list of previously located eagle nests within the area of interest. An evaluation of the feasibility for adopting this design to monitor nesting bald eagles in KEFJ would require an updated map of nest locations. Therefore, the primary objectives of this study were to produce an updated map of active and empty nests along the coastline and in nearby inland areas of KEFJ, field-test the double-observer portion of USFWS protocol, generate a sightability adjusted estimate of the number of active nests along the coastline and in nearby inland areas of the park, and estimate how much of the park's coastline and nearby inland areas could be feasibly surveyed for nests via helicopter under existing cost and logistical constraints. Observers detected 44 active nests with incubating adults or eggs and 36 nests that were emptyduring the seven-day survey that covered approximately 500 mi (800 km) of park coastline. Thirty-seven of the active nests were in Sitka spruce, four were in mountain hemlock, two were on the ground and one was in a balsam poplar. Thirty-three of 44 (75%) active nests were within 65 ft (20 m) of the shoreline (maximum distance = 1680 ft [512 m]). The clearly best supported double-observer model included a time-of-day covariate and estimated the number of active nests to be 65 (95% Bayesian credible interval: 50, 101). The time-of-day covariate likely helped account for heterogeneity in sighting probabilities induced by observer fatigue and lighting/shadow conditions that occurred both during early morning and late afternoon. As the next steps in field testing and evaluation, we recommend 1) using simulations to obtain apreliminary estimate of the minimum size and number of park coastline segments that should be randomly sampled to meet monitoring objectives and 2) performing nest surveys within randomly chosen coastline segments to evaluate survey precision, costs and logistical constraints that will help inform the design for monitoring nest occupancy of bald eagles along the coastline and in nearby inland areas of KEFJ.

Evaluation of a Survey Method for Estimating and Monitoring the Number of Active Bald Eagle Nests in Kenai Fjords National Park

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