Resumen en inglés

This dissertation investigates how climatic asynchrony shapes reproductive timing and population dynamics in Neotropical birds, integrating macroecological analysis, fine-scale field studies, and bioacoustic monitoring to test the Asynchrony of Seasons Hypothesis (ASH). Chapter 1 presents the first large-scale, quantitative analysis of breeding seasonality in Colombian resident birds, using over 80,000 records from banding programs and GBIF. Generalized additive models and circular statistics revealed that breeding activity is markedly seasonal across Colombia, with peaks in the first half of the year and variation driven by latitude, elevation, and trophic guild. These findings challenge the traditional view of aseasonal reproduction in tropical birds and establish a national-level baseline for phenological research. Chapter 2 shifts to a local scale, examining bird assemblages across a 30 km elevational gradient in the Upper Magdalena Valley. Year-round field sampling and hormone analysis showed clear breeding asynchrony between lowland and foothill populations, with distinct seasonal peaks despite geographic proximity. Breeding activity correlated with gross primary productivity and solar radiation, while hormone levels peaked synchronously across sites, suggesting that local environmental conditions modulate reproductive output even under shared endocrine cues. These results offer empirical support for ASH within a climatically heterogeneous tropical landscape. Chapter 3 uses passive acoustic monitoring to assess whether vocal activity—a behavioral proxy for reproduction—also reflects asynchronous breeding dynamics. Over 120,000 recordings revealed displaced acoustic peaks along the elevational gradient, with lowland sites peaking earlier in the year. Vocal activity was weakly correlated with rainfall but aligned closely with reproductive activity measured in the field. At the species level, focal taxa exhibited asynchronous vocal phenologies across sites, suggesting that bioacoustic patterns mirror reproductive asynchrony and may contribute to ecological divergence. Overall, this work demonstrates that reproductive phenology in Neotropical birds is not only seasonal but also spatially structured across geographic and environmental gradients. Climatic asynchrony—especially variation in productivity and radiation—emerges as a key driver of reproductive divergence, with implications for population differentiation even in the absence of physical barriers. By combining large-scale datasets, field-based endocrine and phenological assessments, and machine learning-based bioacoustics, this dissertation offers a robust framework to explore temporal divergence in tropical birds. The findings advance our understanding of avian life-history evolution and highlight the need to incorporate temporal environmental heterogeneity into models of population dynamics, especially under climate change scenarios.