| I used an independent double-observer method to examine detection probabilities in ground-based waterfowl pair surveys and a mark-recapture approach to examine sightability bias in ground-based waterfowl brood surveys. Sightability in pair and brood counts varied considerably among observers and species. A more experienced observer had higher pair sightability (91%) than a novice observer (79%). Brood sightability was highest for diving ducks (57%) and lowest for mallards (32%) and blue-winged teal (31%). Pair sightability was also influenced by wetland size, duck density, and precipitation, whereas brood sightability was also influenced by time of day, duckling density, and date. Based on these results, I derived brood-to-pair ratios adjusted for sightability-bias. I found brood-to-pair ratios explained 10--60% of the variation in nesting success among 8 study sites. My results show that traditional duck surveys that fail to account for detection probabilities result in dramatic underestimates of population size, especially for broods. |
