Drought to deluge: Differential impacts of snow on mountain chickadee reproduction across the Sierra Nevada mountains

Abstract

Changing climates are reshaping animal populations, but our understanding of how demographic trends are shaped by individual responses to local environmental conditions has historically been limited to long-term studies with restricted spatial scales. Participatory science (i.e. citizen science) programs have improved our ability to identify geographic variation in population-level trends, but these programs often lack insight into the individual-level processes that underly such variation. Here we present a new research framework that uses long-term data to validate participatory science observations as measurements of reproductive success, then explores reproductive responses to environmental variation across large geographic scales. We use this approach to investigate how reproduction in a montane songbird, the mountain chickadee (Poecile gambeli), varies in response to extreme scarcity and extreme accumulation of snow throughout the Sierra Nevada Mountains in North America. Reproductive responses to snowfall extremes varied considerably across elevations and latitudes. In the northern Sierra Nevada, mountain chickadee reproductive performance was negatively affected by snow deluge conditions at high elevations, whereas snow drought conditions reduced reproductive output at low elevations. In the central Sierras, drought conditions negatively impacted both elevations, but deluge conditions improved reproductive performance at both low and high elevations. Reproduction in the southern Sierra Nevada was nearly unaffected by spring snow levels. These variable responses to extreme snow accumulation across environments corroborate increasing evidence that biological responses to environmental variation are often nonstationary across spatial scales. Our results show that long-term studies can be used to inform and interpret participatory science data to provide a framework that strengthens our understanding of how individual behaviors underly differential responses to environmental variation across geographic scales.