Cooperative Fish and Wildlife Research Units Program: Massachusetts
Education, Research and Technical Assistance for Managing Our Natural Resources

Wilson, T.L., J.H. Schmidt, W.L. Thompson, L.M. Phillips. 2014. Using double‐observer aerial surveys to monitor nesting bald eagles in Alaska: Are all nests available for detection? The Journal of Wildlife Management 78: 1096–1103.


The abundance of nesting eagles is often identified as the parameter of primary interest for monitoring their populations. We compared the standard dual‐frame estimator, which is recommended in the bald eagle post‐delisting monitoring plan, with a Bayesian multistate capture‐recapture approach to estimate the total number and number of active nests (nests with incubating adults) along the remote Kenai Fjords National Park coastline from 2009 to 2012. Two independent observers conducted aerial surveys of random transects during peak nest initiation in May. Both methods produced similar estimates of nest abundance, but the Bayesian multistate model allowed more flexibility to accommodate shifting management priorities. Estimates of the total number of nests and the number of active nests increased by approximately 49% between 2009 and 2012. This increase was much greater than expected based on feasible rates of nest loss and creation for our study area, indicating apparent estimator bias. Survey‐specific conditions (e.g., aircraft height) that made some nests unavailable to both observers were the most likely cause of the bias. We recommend that bald eagle nest monitoring include 2 surveys during the early breeding season to reduce bias of annual capture‐recapture estimators. Our results demonstrate that incomplete availability may be an important source of bias for many double‐observer aerial wildlife surveys. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.