The contents of this dissertation integrate a diverse set of disciplines, spanning biological and physical sciences, intact and human modified environments, and across time scales. Each chapter covers seemingly disparate topics, from atmospheric science and environmental history to behavior, physiology, and their roots in evolutionary biology. Each chapter, however, is firmly embedded in natural history, pushing disciplinary boundaries with the shared goal of understanding the consequences of environmental gradients, through time or space, on the organisms around us. In Chapter 1, we analyzed a time-series of black carbon deposited on >1,300 bird specimens to reconstruct historical levels of atmospheric black carbon in the US Manufacturing Belt. Among the insights revealed through our analysis, we found that prevailing emission inventories underestimate black carbon emissions through the first decades of the 20th century, suggesting that the contribution of black carbon to past climate forcing may also be underestimated. In Chapter 2, we leveraged field experiments to empirically test the capacity in birds to use human disturbance within their environment. We discovered a capacity in insectivorous birds to exploit human disturbance in primary subalpine forest to gain novel foraging opportunities. In Chapter 3, we explored physiology, behavior, and life history to ask how diverse functional demands on the avian flight muscle are reconciled in birds that experience different environmental and ecological pressures. In this chapter we leveraged an intersectional framework in which we asserted that flight muscle phenotypes are a manifestation of the combined and balanced selective pressures that individuals experience as a result of multi-class identity (i.e. species, age, and sex). We found that age- and sex-structured variation in flight muscle phenotypes is associated with the distinct functional demands and selective pressures imposed on different demographics. Finally, in Chapter 4, we experimentally tested how birds mitigate the physiological stress of shifting environments in seasonal mountains. Through common garden experiments, in which we manipulated temperature and partial pressure of oxygen, we found that birds with higher degrees of seasonal movements exhibit increased physiological flexibility in traits that influence whole-organism metabolism, suggesting a mechanism to mitigate the physiological stress of shifting environments. This chapter provides empirical evidence for the role of metabolic flexibility in facilitating seasonal movements in and out of high elevation environments. Together, these projects leverage natural history to ask new questions about the ecology, evolution, and environmental history of birds in natural and human impacted environments.