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Nestling development in the alpine : predation risk, parental care, and environmental conditions across the annual cycle de Zwaan, Devin Rhys


Offspring development is a critical life-history stage for altricial songbirds and a prime target for selection, as predation risk is high relative to other life-stages and environmental conditions can induce lasting consequences for life-time fitness. Nestling development rates vary widely among species, populations, and individuals. Rapid development is considered an evolved response to improve nest success given high predation risk at the species or population level. However, it is unclear what drives variation in development rate among individuals and whether offspring or parents have the adaptive capacity to respond to prevailing stressors. I investigated the relative influence of multiple, interacting drivers from across the annual cycle on offspring developmental variation within an alpine breeding population of horned lark (Eremophila alpestris) by integrating ecological observations, behavioural experiments, physiology, and light-level geolocators to track migration. I demonstrated that rapid development was associated with a greater probability of nest success, confirming a selective advantage to fledging quickly. Cold ambient temperatures during the nestling period prolonged development, potentially due to resource constraints or thermoregulatory challenges, but females in better body condition were able to buffer offspring against harsh, early season conditions, enabling rapid development. With elevated predation risk, nestlings left the nest earlier by increasing wing growth. This effect was mediated by predator-specific glucocorticoid responses (stress biomarker) and parental provisioning behaviour. During spring migration, I showed that 59% of adults conducted extended stopovers (mean = 41 days) and subsequently had greater reproductive success during the breeding season. However, periods of extreme cold during stopover were correlated with prolonged offspring development, resulting in a lower probability of nest success. My results demonstrate that: 1) nestlings have the adaptive ability to respond to elevated predation risk, 2) parental care can mediate offspring development in response to suboptimal conditions, and 3) prolonged stopovers may be key components of the annual cycle for alpine larks. By addressing within-population variation, I offer new insights into the eco-evolutionary drivers that shape offspring development across the annual cycle with implications for individual fitness and, ultimately, population-level responses to rapidly changing environments.

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