Estimating the Number of Bald Eagle Breeding Pairs in Maine Using Dual Frames and Double Observers

• Activity Number: 263
• Type: Topic Contributed
• Date/Time: Tuesday, August 9, 2005 : 10:30 AM to 12:20 PM
• Sponsor: Section on Survey Research Methods
• Author(s): Mark Otto*+ and John R. Sauer and Charlie Todd
• Companies: U.S. Fish and Wildlife Service and U.S. Geological Survey and Maine Department of Fisheries & Wildlife
• Keywords: capture-recapture ; wildlife monitoring ; rare traits ; sampling ; nonresponse

Abstract

When sampling rare populations, a dual frame sample design can take advantage of existing valuable information. Bald eagles are an endangered species in the lower 48 states, and the species is being considered for Endangered Species Act delisting. A monitoring plan is required as part of the delisting process. In a pilot study of active nests, many states maintain lists of nests for managing resources and development. Using a list frame of possible locations is efficient for sampling rare species. We ran an independent area survey by small aircraft of 10km square plots. Double observers were used in the aircraft to estimate detection probabilities. After the observations between the list and area frames were reconciled, the Haines-Pollock dual frame screening estimator was used to estimate the number of active nests in the list frame plus the number of "new" nests found in only the area frame. An "empty nest" rate accounted for the birds missed on single visits in the area frame that were observed on multiple visits to the list sites. List coverage was 86% and the covariance of the area-only estimates were improved from 24% to 10% by incorporating the list frame.