DOBSERV

Although point counts are frequently used in ornithological studies, basic assumptions about consistency in
detection probabilities often remain untested. We apply a double-observer approach developed to estimate
detection probabilities for aerial surveys (Cook and Jacobson 1979) to avian point counts. At each point count,
one observer is designated "primary", and indicates to the other ("secondary") observer all birds detected. The
secondary observer records all detections of the primary observer, as well as any birds not detected by the
primary observer. Observers alternate primary and secondary roles during the course of a survey. The approach
permits estimation of observer-specific detection probabilities and bird abundance. We developed a set of
models incorporating different assumptions about sources of variation (e.g., observer, bird species) in detection
probability. Single observer point counts generally miss varying proportions of the birds actually present, as
observer and bird species were both found to be relevant sources of variation in detection probabilities.
However, using the double observer approach, overall detection probabilities (probability of being detected by at
least 1 of the 2 observers) were very high (>0.95) yielding precise estimates of avian abundance. We believe that
most questions meriting the effort required to carry out point counts also merit serious attempts to estimate
detection probabilities associated with the counts. The double-observer approach is a method which can be used
for this purpose.