The process of migration stems from an adaptation of climatic seasonality and animals have developed various strategies to complete the journey between a wintering and breeding ground. Understanding the migratory behavior and determining when and where mortality occurs during the annual cycle is fundamental to understand population dynamics and implement appropriate conservation measures. Based on a big data set and advanced statistical methods, we inspected the phenology of migration of a polygynous land bird, the Macqueen s bustard, Chlamydotis macqueenii. We explored its migration strategies between sex, age, season and geographical origin. We show that departure for migration depended on age in the fall with juveniles being the first to leave and on age and sex in the spring with juveniles departing later and males induced to arrive early in spring to secure high-quality territories. Birds breeding at higher latitudes were the first to leave in the fall and more likely to perform longer stopovers. Bustards exhibited different strategies for spring and fall migrations: spring migration was significantly longer than fall migration with more but shorter stopovers. Survival was lower for juveniles experiencing their first migration and for all birds during fall migration and on their wintering ground. Experience linked to social hierarchical pressures and environmental conditions might be the key drivers of migration strategies and survival in long-distance polygynous migrants.
In conservation translocation, released animals should have comparable fitness to their wild counterparts to effectively contribute to the species demography. Captive-bred animals frequently exhibit lower fitness performances, which can often be attributed to an inadequate release strategy. Untimely release of migrant animals may interfere with key events such as their migration and breeding. In Kazakhstan and Uzbekistan, declining wild populations of Asian houbara (Chlamydotis macqueenii) are reinforced in their breeding grounds with captive-bred individuals. Using data from 6 years of monitoring, we compare eight breeding traits and the productivity of wild and captive-bred females released in two distinct seasons (autumn vs. spring) considering the effects of age and time in the season. Females released in the spring nest prior to their first migration, and females released in the autumn nest following their first migration. Our results highlight that captive-bred and wild females have similar breeding traits and productivity. Breeding probability, laying date, and egg volume varied, depending on the release season and female age. One-year-olds released in autumn have a significantly lower breeding probability compared to wild and spring-released females. However, 1-year-old females released in the spring nest later and lay smaller eggs than wild and autumn-released birds; effects which appear to be carried over with age. Age has a positive effect on breeding probability, egg volume, re-clutching probability and advancement of nesting date. These findings suggest a complex interplay of release timing with migration and breeding, resulting in short- and long-term effects on population demography, emphasizing its importance in conservation translocation.
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