An appraisal of the use of hydrogen-isotope methods to delineate origins of migratory saw-whet owls in north America

Abstract

Northern Saw-whet Owls (Aegolius acadicus) breed throughout the boreal forest of North America, but little is known about their population trends or distribution within this region. Analysis of stable hydrogen isotopes (σ2H) in feathers can delineate origins of a variety of avian migrants, but raptors are reported to have high intrafeather isotopic variance and mean σ2H values higher than predicted from σ2H isoscapes specific to raptor feathers, making assignment of geographic origin sometimes difficult. We examined the applicability of σ2H analysis of sawwhet owl feathers to delineating origins of migrants and to assessing differences in the migratory behavior of adult and young owls by using multiple generations of feathers from owls captured during fall migration at the Delta Marsh Bird Observatory, Manitoba, 2006-2007. Values of σ2H in saw-whet owl feathers were higher than predicted from a σ2H isoscape specific to raptor feathers and from patterns of movements inferred from analysis of band recoveries. This effect was pronounced in adults, while values of σ2H in feathers of hatching-year owls fell primarily within the range predicted for the boreal forest northwest of Delta Marsh. Significant differences in σ2H values among feather generations suggest that physiological or behavioral differences between adults and young give rise to greater 2H enrichment in adult feathers. These results indicate that current σ2H isoscapes for feathers cannot be used to track adult saw-whet owls reliably and further research into the mechanisms of 2H enrichment in owl feathers is required. © The Cooper Ornithological Society 2013.