Smith, Rhonda F. – Investigating raptor migration behavior using orientation cages and wing measurements: A comparison of the Flammulated Owl and Northern Saw-whet Owl in southwestern Idaho. 2009.
Flammulated Owls (Otus flammeolus) are long-distance migrants, while Northern Saw-whet Owls (Aegolius acadicus) exhibit more variable migration tendencies. I investigated the migratory behavior of these species using orientation cages and tested hypotheses concerning (1) the presence of migratory restlessness and orientation, (2) inter-specific differences in migratory restlessness and orientation, and (3) correlates of migratory restlessness and orientation. Only four of 16 Flammulated Owls displayed activity consistent with migratory restlessness although all four owls exhibited significant directionality in their movements. 59 of 97 Northern Saw-whet Owls exhibited activity consistent with migratory restlessness with most of these 59 owls showing preferred directions of orientation. Neither species oriented consistently as a group and, contrary to expectation, Northern Saw-whet Owls exhibited more restlessness in orientation cages. The relative lack of migratory restlessness in Flammulated Owls might be a function of small sample sizes or simply this particular species’ reaction to an unfamiliar situation. In Flammulated Owls, migratory restlessness increased with decreasing natural cloud cover, and was more pronounced in birds with larger flight muscles. Contrary to predictions, Flammulated Owls also showed a marginal increase in restlessness in response to decreasing furcular fat stores which could be a function of small samples sizes. Restlessness in Flammulated Owls did not vary with wind speed, moon disk illumination, nor did restlessness change throughout the migration season. Northern Saw-whet Owls tested under a bright moon (>66% moon disk illuminated) oriented consistently to the northeast, but exhibited variable orientation when the moon was <66% illuminated. These results suggest that, in the presence of a bright moon, owls may chose to seek cover in forested habitat rather than migrate over open areas. Restlessness and orientation in Northern Saw-whet Owls did not relate to age or body condition, wind speed or direction, or simulated or natural cloud cover, nor did orientation vary throughout the migration season. However, under the influence of 100% simulated cloud cover, Northern Saw-whet Owls demonstrated significantly lower migratory restlessness compared with simulated partial cloud cover and clear skies. Furthermore, Northern Saw-whet Owls tested late in the night (2 h before sunrise) showed more restlessness than birds tested earlier in the night and immature saw-whets were more active than adults.