Modeling phenological reaction norms over an elevational gradient reveals contrasting strategies of Dusky Flycatchers and Mountain Chickadees in response to early season temperatures

We developed an approach to distinguish among three alternative strategies birds may employ relating the timing of egg-laying across elevations to annual variation in spring temperature (phenological reaction norms), which we applied to analyze breeding phenology of two species over an elevational gradient in the Sierra Nevada, California. In a “simple offset” strategy, birds at different elevations initiate breeding relative to environmental temperature in a consistent manner, in that breeding onset is triggered by a critical temperature regardless of when it occurs in the spring. Elevation-specific reaction norms based on multiple years are offset (high-elevation birds start breeding later) but parallel. In a “delay” strategy, in cooler springs populations at higher elevations that are sensitive to early-season weather-related risks may delay laying onset relative to those at lower elevations, yielding a high-elevation reaction norm that diverges from a low-elevation one at cooler tem...