Cultural signatures of punctuated environmental change: measuring the effects of population-wide stress on song distribution in dark-eyed juncos and song sparrows

Severe weather events can dramatically alter a species evolutionary trajectory. Previous landmark studies on populations in transiently hostile environments have measured traits that are under direct selection during those events. However, phenotype shifts that are not inherently adaptive to the fluctuating environment may also occur. Stress, especially during development, can cause important phenotypic changes in individuals, including impaired learning. Thus, learned behaviors, such as birdsong, may exhibit unique evolutionary dynamics as a result of widespread environmental stress. We hypothesize that ecosystem-level stressors may cause population-level changes to birdsong as entire cohorts experience developmental stress, learn songs imperfectly, and become the song tutors for future generations. In 2016, an unprecedented drought affected western New York State, a hotspot for community-science-generated birdsong recordings, and presented a unique opportunity to test our hypothesis through a natural experiment. We analyzed publicly available community-science birdsong recordings of two species, Junco hyemalis and Melospiza melodia, recorded between 2006-2020 in the drought-affected region and two control regions. We found that population-level song features had changed in the species with more complex songs (song sparrow) in the drought area after 2016, but not in the control area or in the species with a simple song (dark-eyed junco), implying that stress-induced deficits may disproportionately affect song traits that are more difficult to learn. These results demonstrate that environmental events can drive population-level trait evolution due to disruption in learning, with potentially important implications for speciation.