USING VIDEO MONITORING TO ASSESS THE ACCURACY OF NEST FATE AND NEST PRODUCTIVITY ESTIMATES BY FIELD OBSERVATION

Nest fate and nest productivity are key demographic parameters for understanding songbird population dynamics, yet little consideration has been paid to assessing and improving the accuracy of these estimates in the field. We considered the magnitude and sources of error in field estimates of nest fate and productivity for 13 species of boreal forest songbirds, the implications of this error when estimating rates of nest survival and population growth, and the utility of common field cues used to assess fate. Using video from 127 nests, we found that observers correctly identified 85% of nest fates but overestimated nest productivity by up to 35%. This resulted in population growth rates being overestimated by 6%. Field estimates were less accurate when nestling age approached the estimated fledge date and when the nest was depredated. Accuracy of field estimates can be improved by focusing on nest condition and the presence of fecal droppings outside the nest. Spending additional time searching for family groups would be prudent when nests are deemed successful on the basis of nestling age alone. Nest predators force fledged one or more nestlings from 14% of nests. The fate of force-fledged young is unknown. Our measures of error declined as increasingly younger force-fledged individuals were considered successful. Resolving this uncertainty would further improve the accuracy of field-based estimates. We encourage the use of video to quantify and improve the accuracy of field estimates and to evaluate the potential for differential bias in error within variables of interest. Received 6 October 2011, accepted 13 April 2012.