Evaluating Techniques for Estimating Post-Breeding-Season Age Ratios for American Woodcock
Kyle O. Daly
Minnesota Cooperative Fish and Wildlife Research Unit, Department of Fisheries, Wildlife, and Conservation Biology
David E. Andersen
U.S. Geological Survey, Minnesota Cooperative Fish and Wildlife Research Unit
Wayne L. Brininger
U.S. Fish and Wildlife Service, Tamarac National Wildlife Refuge
Thomas R. Cooper
U.S. Fish and Wildlife Service, Migratory Bird Program
DOI: https://doi.org/10.24926/AWS.0107
Abstract
Estimating American woodcock (Scolopax minor; hereafter, woodcock) vital rates at a landscape scale requires considerable effort and expense, but provides a means of assessing population response to management. Age ratios derived from capturing woodcock (e.g., age ratios derived from mist-netting or night-lighting) during late summer may be useful proxies of local production, and require much less effort and expense to obtain than estimating local production directly. To assess whether such age ratios were similar to estimates of production derived from estimating vital rates, we estimated post-breeding-season age ratios (juveniles/adult female) at a habitat-management demonstration area in west-central Minnesota using radio telemetry and nest monitoring, and by capturing woodcock using mist nets and night-lighting. In 2011 and 2012 we radio-marked and tracked 41 adult female and 73 juvenile woodcock and monitored 51 broods and 48 nests; we used the collected data to inform population models and derive post-breeding-season age ratios. In July of 2011 and 2012, we captured 204 woodcock using mist nets by exploiting crepuscular movements from diurnal feeding cover to roosting fields and 69 woodcock via night-lighting on nocturnal roosting fields. Estimates of age ratios derived from our population model were 1.07 (95% CI: 0.27 – 2.45) in 2011 and 2.59 (95% CI: 1.04 – 4.95) in 2012. We attribute the higher point estimate of age ratio in 2012 to higher nest and juvenile survival rates during that year. Age ratios from mist-netting were 3.82 (95% CI: 1.99 – 7.13) in 2011 and 2.37 (95% CI: 1.43 – 3.73) in 2012 and from night-lighting were 1.62 (95% CI: 0.69 – 3.28) in 2011 and 0.42 (95% CI: 0.06 – 1.00) in 2012. Age ratio point estimates derived from mist-netting and night-lighting varied considerably between years, with neither method providing a ratio similar to point estimates derived from our population model. The only statistically significant difference (based on 95% CIs) between any of our age ratio estimates was for night-lighting in 2012, with that estimate being lower than the estimates from mist-netting and our population model. Based on these results, age ratios of production of young derived from mist-netting and might-lighting in late summer may not reflect local production, and may be influenced by both local and landscape-scale movements of woodcock following the breeding season. We conclude that without further evaluation of factors that affect post-breeding-season age ratios of local woodcock populations, age ratios derived from mist-netting and night-lighting in summer may not be useful proxies of local recruitment.
