Cohort-specific differential vulnerability to capture of mallards and wood ducks in baited swim-in traps

Abstract

Attaching leg bands to birds directly before fall hunting seasons is a primary component of monitoring waterfowl in North America. While capture and recovery data are primarily used for estimating survival and harvest distribution, these data may be used to estimate age ratios and other demographic rates, especially if recapture data are available from subsequent trapping and release events. We estimated recapture rates of mallards Anas platyrhynchos and wood ducks Aix sponsa and used those rates to estimate differential capture vulnerability, which is vital to estimate true age ratios from banding data. Posterior estimates of mallard age and sex cohort and location-specific recapture rates varied among capture locations from a mean of 0.004 (0.002- 0.008 95% credible interval) to 0.547 (0.486-0.609). Ratios of recapture rates among cohorts also varied, meaning no single differential vulnerability estimate would be useable across the study area. Our estimates of differential capture vulnerability for mallards, using the ratio of recapture probabilities, averaged 2.64 for adult female to adult male and 5.42 for juvenile female to adult female, with significant variation. Wood duck cohort-specific recapture rates were similar across locations. Similar wood duck recapture rates resulted in similar estimates of differential vulnerability, 1.24 for the adult female to adult male ratio and 1.30 for juvenile female to adult female. The wide range of recapture rate estimates we found for mallards suggests location- specific characteristics may have a strong effect on capture probability. Differences in recapture rates and apparent survival likely resulting from emigration suggest if recapture data are to be used in population modeling, location-specific information is needed. The ability to monitor multiple demographic parameters using a single scheme improves continued assessment of population status. We recommend increased collection of in-season recapture data by biologists during active banding operations. Banders should be aware of the potential value of live, in- season, encounters in monitoring populations and modeling demographic rates.