Age-related differential migration strategies in Northern Saw-Whet Owls (Aegolius acadicus)

Abstract

We analyzed differences in wing loading and body condition indices (BCI) between hatch-year (HY) and after-hatch-year (AHY) Northern Saw-whet Owls (Aegolius acadicus) captured during autumn migration in south-central Indiana. From 2002 to 2012, banders captured 1469 owls at two sites, including 826 HY owls and 641 AHY owls. The mean BCI (mass to wing chord ratio) was 0.665 g/mm and did not vary between HY and AHY owls. BCI was lowest in 2005 (0.658 g/mm) and highest in 2008 (0.679 g/mm). During autumn 2007, the banders photographed 267 owls at the two banding sites to enable comparisons of wing loading (mass to wing surface area ratio) and BCI for different age categories. Mean wing loading was 0.242 g/cm2 and also did not vary between HY and AHY owls, confirming the BCI results and suggesting that the age classes were not gaining or losing mass differentially. The median arrival date was 3 d earlier and the nightly median capture time was 10 min later for HY owls. In addition, the mass of HY owls correlated positively with the nightly capture time, whereas this was not true for adults. Together these results suggest that HY Northern Saw-whet Owls in south-central Indiana make up for any limitations in migratory/hunting abilities by migrating earlier each night and foraging more frequently while migrating. Migrating earlier, whether by choice or as a result of density-dependent adult competition, likely affords the younger birds greater access to prey in commonly exploited foraging areas. Available data indicate the potential for regional variation in the mass to wing chord correlation for this species, which needs further research. We also suggest that future studies obtain full wingspan measurements and compare the wingtip shape (amount of point and convexity) between juvenile and adult birds. © 2014 The Raptor Research Foundation, Inc.