Abstract
Highly mobile predators can show strong numerical responses to pulsed resources, sometimes resulting in irruptions where large numbers of young invade landscapes at a continental scale. High production of young in irruption years may have a strong influence on the population dynamics unless immature survival is reduced compared to non-irruption years. This could occur if subordinate individuals (mainly immatures) are forced into suboptimal habitats due to density-dependent effects in irruption years. To test whether irruptive individuals had lower survival than non-irruptive ones, we combined necropsy results (N = 365) with telemetry (N = 185) from more than 20 years to record timing and causes of mortality in snowy owls (Bubo scandiacus), which irrupt into eastern North America during winter following high breeding output caused by lemming peaks in the Arctic. Mortality was more than four times higher in irruption years than non-irruption years, but only for immatures, and occurred disproportionately in early winter for immatures, but not adults. Mortality was also higher in eastern North America, where owl abundance fluctuates considerably between years, compared to core winter regions of the Arctic and Prairies where populations are more stable. Most mortality was not due to starvation, but rather associated with human activity, especially vehicle collisions. We conclude that immature snowy owls that irrupt into eastern North America are limited by density-dependent factors, such as increased competition forcing individuals to occupy risky human-altered habitats. For highly mobile, irruptive animals, resource pulses may have a limited impact on population dynamics due to low subsequent survival of breeding output during the nonbreeding season.
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All datasets used in this study are available upon request from the authors.
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Acknowledgements
We thank all collaborators, researchers and volunteers who helped trap and tag snowy owls over the years, including SNOWstorm supporters and collaborators. This study was funded by Natural Sciences and Engineering Research Council of Canada (NSERC) CREATE-Environmental Innovation scholarship # 241061 to RAM, NSERC discovery grant # 20317 to KLW, Hawk Mountain James A. Kushlan award to RAM, and by generous donors of Project SNOWstorm. Research in the Arctic was funded by NSERC grant to GG, the network of center of excellence ArcticNet, the Polar Continental Shelf Program of Natural Resources Canada, the Fonds de Recherche du Québec Nature et Technologies, International Polar Year program of the Government of Canada, Weston Family Foundation, Glencore Xstrata and First Air. Research conducted at veterinary facilities was funded by Centre Quebecois sur la Santé de la Faune (CQSAS) and the Canadian Wildlife Health Cooperative (CWHC). We thank all reviewers and editors for valuable comments on the manuscript. This is Project SNOWstorm contribution number 009.
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Communicated by Robert L Thomson.
By combining 20 years of telemetry and necropsy data from snowy owls in North America, the authors demonstrate that survival during irruption years is influenced by density-dependent factors resulting from the high abundance of first-year owls on the wintering landscape. This research sheds light on our understanding of pulsed resources and cautions against overestimating the role of ‘boom’ years on long-term population growth for highly mobile, irruptive species.
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McCabe, R.A., Therrien, JF., Wiebe, K. et al. Density-dependent winter survival of immatures in an irruptive raptor with pulsed breeding. Oecologia 198, 295–306 (2022). https://doi.org/10.1007/s00442-021-05057-9
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DOI: https://doi.org/10.1007/s00442-021-05057-9