Climatic niche variation in genetically distinct populations throughout the annual cycle for a migratory parulid bird, Cardellina pusilla

At the intraspecific level, environmental heterogeneity can create opportunities for local adaptation and population differentiation. However, studying the effects of environmental heterogeneity in migratory animals is challenging due to the complexity of their natural history, which is conditioned by their long journeys each year. Cardellina pusilla is a widespread Neotropical migratory bird that comprises six genetically distinct populations. We conducted a detailed study of the potential for differentiation among those populations in their climatic niche space and preferences across the annual cycle. Previous studies had focused mainly on the breeding part of their range, leaving the non-breeding range under-sampled. We filled in those sampling gaps with genetic samples in key regions of the non-breeding grounds to analyze variation in the realized climatic niche among the six genetically distinct populations. We found that niche breadth was wider during the non-breeding season than the breeding season. There were also clear differences in breeding niches among populations, with that of the Eastern Boreal population being the most differentiated from the other populations. The non-breeding niches were more similar among populations, but the Eastern Boreal population still showed low overlap with the other populations. Comparisons from seasonal climatic niche overlap and potential migratory destinations revealed that several populations (e.g., Coastal California, Pacific Northwest) would experience a better climatic match if they occupied non-breeding grounds used by other populations, yet they do not do so. This pattern is consistent with recent theoretical frameworks suggesting that philopatry, site fidelity and energy efficiency are stronger drivers of migratory connectivity than climatic niche. Our findings demonstrate substantial intraspecific niche variation across the annual cycle, that populations follow distinct migratory strategies, and that non-breeding ground fidelity may limit migratory animals' ability to adjust to climate change. Conservation efforts should prioritize the protection of historically used non-breeding areas and recognize that population-specific vulnerabilities are shaped by migratory fidelity, not by potential climatic optima.