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1 December 2015 On the Breeding Biology of Northern Saw-whet Owls (Aegolius acadicus)
Jeffrey S. Marks
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We monitored 37 Northern Saw-whet Owl (Aegolius acadicus) nesting attempts at the Boardman Tree Farm (BTF) in north-central Oregon from 2012 to 2015. Nests were initiated from early March to late June and contained an average of 5.7 eggs. Twenty-nine (78%) of the nesting attempts were successful and produced an average of 4.2 fledglings per successful attempt. One male paired with two females and produced 6 fledglings out of 10 eggs. We captured 59 unique nesting adults; 61% of the males (17 of 28) and 58% of the females (18 of 31) were second-year (SY) birds. We captured both adults at 30 nests: both parents were SYs at eight nests, both were after-second-year (ASY) at five nests, the female was SY and the male ASY at eight nests, and the male was SY and the female ASY at nine nests. Considering clutch size and number of young fledged, we found no evidence that pairs composed of SY parents reproduced less successfully than those composed of ASY parents. Sample sizes were small, but neither mean clutch size nor the mean number of young fledged differed significantly among nests of the four combinations of parental ages. One male and two females nested in more than one year; each obtained a new mate the second year. The first female had nested successfully in 2012 and settled in a box 3 km away in 2013; she abandoned her first clutch in 2013 and renested 15 d later in a different box with a male who also had nested successfully in 2012. The other female nested successfully in 2014 and settled in a box 3.7 km away in 2015. Of the 109 banded nestlings that fledged from 2012 to 2014, we encountered only one in a subsequent year through 2015. Our data suggest that breeding-site fidelity and natal philopatry are low in this population, as has been found elsewhere in western North America.

© 2015 The Raptor Research Foundation, Inc.
Jeffrey S. Marks "On the Breeding Biology of Northern Saw-whet Owls (Aegolius acadicus)," Journal of Raptor Research 49(4), 486-497, (1 December 2015).
Received: 15 December 2014; Accepted: 1 June 2015; Published: 1 December 2015
Aegolius acadicus
age-specific productivity
breeding biology
Northern Saw-whet Owl
site fidelity
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