Double-survey estimates of bald eagle populations in Oregon

The literature on abundance of birds of prey is almost devoid of population estimates with statistical rigor. Therefore, we surveyed bald eagle (Halia ectus leucocephalus) populations on the Crooked and lower Columbia rivers o f Oregon and used the double-survey method to estimate populations and sigh ting probabilities for different survey methods (aerial, boat, vehicle) and bald eagle ages (adults vs. subadults). Sighting probabilities were consis tently <1.00, so direct counts were biased estimates of abundance. There wa s a greater probability of observing adult (0.625) versus subadult (0.494) eagles on die Crooked River for different survey methods, but we did not de tect differences in sighting probabilities for adults (0.707) versus subadu lts (0.683) on the Columbia River. The probability of observing bald eagles from aircraft (0.636) was lower than that from heats (0.758) on the Columb ia River. and sighting probabilities from aircraft (0.764) were lower than those from the ground (0.764) on the Crooked River. Precision of population estimates was generally good: most (68%. n = 30) coefficients of variation were <10%. and only 7% were >20%. The results revealed variable and negati ve bias (percent relative bias = -9 -70%) of direct counts and emphasized t he importance of estimating populations where some measure of precision and ability to conduct inference tests are available. We recommend use of the double-survey method to estimate abundance of bald eagle populations and ot her raptors in open habitats.