Density-dependent winter survival of immatures in an irruptive raptor with pulsed breeding.

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.

Author Correction: Arctic avian predators synchronise their spring migration with the northern progression of snowmelt.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Arctic avian predators synchronise their spring migration with the northern progression of snowmelt.

Migratory species display a range of migration patterns between irruptive (facultative) to regular (obligate), as a response to different predictability of resources. In the Arctic, snow directly influences resource availability. The causes and consequences of different migration patterns of migratory species as a response to the snow conditions remains however unexplored. Birds migrating to the Arctic are expected to follow the spring snowmelt to optimise their arrival time and select for snow-free areas to maximise prey encounter en-route. Based on large-scale movement data, we compared the migration patterns of three top predator species of the tundra in relation to the spatio-temporal dynamics of snow cover. The snowy owl, an irruptive migrant, the rough-legged buzzard, with an intermediary migration pattern, and the peregrine falcon as a regular migrant, all followed, as expected, the spring snowmelt during their migrations. However, the owl stayed ahead, the buzzard stayed on, and the falcon stayed behind the spatio-temporal peak in snowmelt. Although none of the species avoided snow-covered areas, they presumably used snow presence as a cue to time their arrival at their breeding grounds. We show the importance of environmental cues for species with different migration patterns.