Drivers and Consequences of Partial Migration in an Alpine Bird Species

1. Partial migration, where a portion of the population migrates between winter and summer (breeding) areas and the rest remain year-round resident, is a common phenomenon across several taxonomic groups. Yet, although several hypotheses have been put forward to explain why some individuals migrate while others stay resident – as well as the fitness consequences of the different strategies – the drivers and consequences of the decision to migrate or not are poorly understood. 2. We used data from radio-tagged female (n=73) willow ptarmigan Lagopus lagopus in an alpine study area in Central Norway to test if i) the decision to migrate was dependent on individual state variables (age and body size), ii) individuals repeated migratory behaviour between seasons, and iii) the choice of migratory strategy was related to nesting performance.3. Partially supporting our prediction that migratory strategy depends on individual state, we found that juvenile birds with small body sizes were more likely to migrate whereas large juveniles stayed resident. For adult females, we found no relationship between migratory strategy and body weight. We found strong evidence for high individual repeatability of migratory strategy between seasons. Migratory strategy did not explain variation in nesting performance among individuals, suggesting no direct influence of the chosen strategy on nesting success. 4. Our results indicate that partial migration in willow ptarmigan is determined by juvenile body weight, and that migratory behaviour becomes a part of the individual life history as a fixed strategy. Nesting success was not affected by migratory strategy in our study population, but future studies should assess other traits to further test potential fitness consequences.

Southerly movements of Willow Ptarmigan during the winter in and near the prairie provinces

Southerly movements of Willow Ptarmigan during the winter in and near the prairie provinces

Survival and cause-specific mortality of harvested willow ptarmigan (Lagopus lagopus) in central Norway

Survival is a key demographic component that often vary as a result of human activities such as recreational harvest. Detailed understanding of seasonal variation in mortality patterns and the role of various risk factors is thus crucial for understanding the link between environmental variation and wildlife population dynamics, and to design sustainable harvest management systems. Here, we report from a detailed seasonal and cause-specific decomposition of mortality risks in willow ptarmigan (Lagopus lagopus) in central Norway. The analyses are based on radio collared (n=188) birds that were monitored across all seasons, and we used time-to-event models for competing risks to estimate mortality patterns. Overall, annual survival was estimated at 0.43 (SE: 0.04), with no distinct difference among years (2015/16 to 2018/19) or between sexes. Analysis of mortality risk factors revealed that on the annual basis, the risk of harvest mortality was lower than the risk of dying from natural causes. However, during the autumn harvest season (Sept. – Nov.), survival was low and the dominating cause of mortality was harvest. During winter (Dec. – Mar.) and spring seasons (Apr. - May), survival was in general high and did not vary between males and females. However, during the spring season juveniles (i.e. birds born last year) of both sexes had lower survival than adults, potentially because they are more prone to predation. During the summer season (June – Aug.) females experienced a higher hazard than males, underlining the greater parental investment of females during egg production, incubation and chick rearing compared to males. Our analyses provide unique insight into demographic and seasonal patterns in willow ptarmigan mortality risks in a harvested population, and revealed a complex interplay across seasons, risk factors and demographic classes. Such insight is valuable when designing sustainable management plans in a world undergoing massive environmental perturbations.