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Abstract
The climatic stability of tropical climates is a fundamental driver of the latitudinal gradient in earth’s biodiversity, due in part to the promotion of narrow niches in tropical organisms. However, the extent to which this pattern reflects narrow fundamental or realized niches remains largely unknown, particularly for complex traits and behaviors involved in resource acquisition. This thesis examines the plasticity of resource-based behavior in tropical birds through two distinct lenses. In the second and third chapters, I investigated how reduced variability in tropical wind regimes shapes the aeroecology of nocturnal bird migration through Colombia relative to the conditions that the same individual birds face at temperate latitudes, applying and developing remote sensing methods with weather radar and bioacoustics. I found that stable tropical winds have distinct effects on migration timing depending on geographic context. Within the Andes mountains, supportive winds explain the altitudes at which birds fly, but not the volume of birds aloft on a given night, resulting in lower nightly variation in migration intensity. However, lapses in prevailing headwinds east of the Andes drove higher variation in migration intensity similar to temperate latitudes. With a reduced relationship between forecastable weather and the magnitude of nightly migration, predicting high volume migration events to target for conservation initiatives may be more challenging in tropical climates. I used bioacoustics to show that many of the species captured by this radar dataset are different from those detected by traditional, ground-based monitoring. In the third chapter, I evaluated the plasticity of foraging behavior and diet among resident forest birds living along the slopes of the Ecuadorian Andes to elucidate mechanisms of community assembly and vulnerability to climate and land use change. I found evidence that niche packing of morphologically similar species may be offset by greater behavioral plasticity in foraging behavior at species-rich lower elevations where competition is likely more intense and invertebrate prey more diverse. We also tested the extent to which the breadth and similarity of birds’ foraging and dietary niches are shaped by the environmental and competitive gradient across elevation versus species identity and phylogenetic similarity. The specific behaviors and substrates that birds used were far more strongly associated with species identity than elevation, particularly for behaviors requiring specialized morphology that is phylogenetically conserved. In contrast, species identity had little effect on prey selection, which was more strongly associated with elevation. Our findings suggest that elevational range dynamics and niche packing of tropical montane birds are more strongly shaped by phylogenetic constraints on foraging behavior than by specializing on specific prey taxa, highlighting the importance of maintaining structural integrity in tropical forests for preserving functional diversity. Studying each of these systems addresses how the resources of wind and food structure biodiversity in the tropics, fills critical gaps in natural history knowledge, and highlights the urgency of immediate action to conserve critical habitat.