Ageing Northern Saw-Whet Owls (Aegolius acadicus) from Remigial Molt Patterns

Abstract

We recorded molt patterns of migrant Northern Saw-whet Owls (Aegolius acadicus) at three locations in the Great Lakes region: Stevens Point, Wisconsin; Whitefish Point, Michigan; and Duluth, Minnesota. We then analyzed the molt patterns of recaptured birds that were banded in a previous season and originally classified as hatch-year birds (i.e., they had no remigial molt). Of these 286 known-age birds included in our analysis, 155 (54%) were second-year (SY), 72 (25%) were third-year (TY), 39 (14%) were fourth-year (4Y), 16 (6%) were fifth-year (5Y), three (1%) were sixth-year (6Y), and one (0.30%) was eighth-year (8Y). Using these known-age birds, we were able to identify definitive molt patterns for the SY and TY age classes. All 155 (100%) SY birds and 52 (72%) of the TY birds matched our ageing criteria. The remaining 28% of TY birds did not match our ageing criteria and were classified as after-second-year (ASY). Of the entire sample (n = 286), only three birds (1%) matched an incorrect age class, including one 4Y that matched the SY age class and two 5Y that matched the TY age class.