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.