Tropical animals respond to rainfall in population-specific ways. In extremely wet regions, endotherms experience heavy rains as stressors with consequences for behavior and demography. Ultimately, such stressors can affect the relative strength of abiotic selection, reducing the scope for sexual selection and other sources of biotic selection. We studied population-level differences in the response to biogeographic gradients of rainfall in White-ruffed Manakins (Corapipo altera) on the Caribbean slope of Costa Rica, a species having a lek mating system subject to strong sexual selection. Previously, we documented behavior in a population inhabiting an extremely wet location where estimates of apparent survival were low and the turnover of display sites and dominant, displaying males was high and positively associated with local rainfall. Males also frequently engaged in coordinated display, and sub-adult males practiced in the presence of adults. Over three breeding seasons (2017–2019), we studied the same species and metrics in a population located only 110 km away, but in a location receiving roughly half as much rain. We tested behavioral predictions of four alternative mechanisms linking rainfall to reproductive behavior. Using data derived from 1350 hr of observations at 40 display logs, we found fewer sub-adult males practicing at display sites and coordinated adult male displays were less frequent. Multiple mechanisms potentially underlie links between rain and the population-specific differences we documented. Our results are consistent with rain constituting an important source of abiotic selection for tropical endotherms and modulating the scope for sexual selection near the extremes of a species’ hygric niche.