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5 December 2019 Social status, forest disturbance, and Barred Owls shape long-term trends in breeding dispersal distance of Northern Spotted Owls
Julianna M. A. Jenkins, Damon B. Lesmeister, Eric D. Forsman, Katie M. Dugger, Steven H. Ackers, L. Steven Andrews, Chris E. McCafferty, M. Shane Pruett, Janice A. Reid, Stan G. Sovern, Rob B. Horn, Scott A. Gremel, J. David Wiens, Zhiqiang Yang
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Abstract

Dispersal among breeding sites in territorial animals (i.e. breeding dispersal) is driven by numerous selection pressures, including competition and spatiotemporal variation in habitat quality. The scale and trend of dispersal movements over time may signal changing conditions within the population or on the landscape. We examined 2,158 breeding dispersal events from 694 male and 608 female individually marked Northern Spotted Owls (Strix occidentalis caurina) monitored over 28 yr on 7 study areas to assess the relative importance of individual (sex, experience), reproductive (annual productivity, mate availability), and environmental (forest alteration, presence of competitor) sources of variation in breeding dispersal distance. Median breeding dispersal distance was 3.17 km, with 99% of all breeding dispersal events <37 km. Mean annual dispersal distances increased by 2.43 km in Oregon and 9.40 km in Washington between 1990 and 2017, which coincided with increases in annual detections of nonnative Barred Owl (S. varia). Frequency of breeding dispersal events, both among and within individuals, also increased over time. Female owls moved farther than males (median of 3.26 and 3.10 km, respectively), and birds with less experience (territory tenure) moved farther than those with more experience. Owls that were single in the year prior to dispersal moved 13–31% farther than those paired prior to dispersal. The greatest environmental change occurring over the course of our study was the expansion of Barred Owl populations. Breeding dispersal distance was positively related to Barred Owls in the study area and disturbance within the originating territory. While it appears that social factors continue to be important drivers of breeding dispersal distance in Spotted Owls, increased competition from Barred Owls and habitat alteration have a contributing effect. Increased breeding dispersal distances should be of concern for conservation efforts and considered in population monitoring because changing dispersal behavior may lead to higher rates of mortality and/or emigration from historical study areas.

Published by Oxford University Press for the American Ornithological Society 2019. This work is written by (a) US Government employee(s) and is in the public domain in the US.
Julianna M. A. Jenkins, Damon B. Lesmeister, Eric D. Forsman, Katie M. Dugger, Steven H. Ackers, L. Steven Andrews, Chris E. McCafferty, M. Shane Pruett, Janice A. Reid, Stan G. Sovern, Rob B. Horn, Scott A. Gremel, J. David Wiens, and Zhiqiang Yang "Social status, forest disturbance, and Barred Owls shape long-term trends in breeding dispersal distance of Northern Spotted Owls," The Condor 121(4), 1-17, (5 December 2019). https://doi.org/10.1093/condor/duz055
Received: 20 June 2019; Accepted: 18 October 2019; Published: 5 December 2019
KEYWORDS
Barred Owl
breeding dispersal
forest disturbance
Northern Spotted Owl
Strix occidentalis caurina
Strix varia
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