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 3 alternative strategies that 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 the breeding phenology of 2 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 delay laying onset relative to those at lower elevations, yielding a high-elevation reaction norm that diverges from a low-elevation one at cooler temperatures. Conversely, high-elevation populations in cooler springs that are sensitive to the risk of having insufficient time to complete a breeding cycle advance laying onset relative to lower populations (“advance” strategy), yielding a high-elevation reaction norm than converges with a low-elevation one. Both delay and advance strategies imply an elevation-dependent interaction between temperature and date (photoperiod) in influencing laying onset. Examined across 3 elevation groupings, phenological reaction norms of Mountain Chickadees (Poecile gambeli) were essentially parallel, consistent with simple offset, whereas Dusky Flycatchers (Empidonax oberholseri) relationships were more complex. In cooler springs, mid-elevation flycatchers bred comparatively late relative to lowest-elevation birds (delay), implying greater sensitivity to early-season risks, but still with sufficient time to complete a breeding cycle. However, high-elevation flycatchers bred comparatively early relative to mid-elevation populations (advance); delaying at these highest elevations may not be an option. Our approach revealed differences in risk sensitivity that were consistent with other ecological differences between the 2 species. LAY SUMMARY Birds at higher elevations usually begin laying later than those at lower elevations mainly because temperatures that cue breeding occur later at higher elevations. And spring storms that bring rain to lower elevations may produce snow at higher ones, which might favor delaying breeding even longer. However, with shorter breeding seasons at higher elevations, delaying breeding risks running out of time to complete the breeding cycle, favoring relatively earlier laying. Dusky Flycatchers in the southern Sierra Nevada demonstrate both responses, depending on elevation: compared with lowest elevations, mid-elevation birds delayed breeding in cooler years, whereas highest-elevation birds advanced lay dates in cooler years compared with mid-elevations; delaying may not be an option. No such risk sensitivity was observed in Mountain Chickadees, where differences across elevations in first egg dates were constant. Cavity-nesting and other ecological attributes of chickadees likely buffer them from risks to which open-cup-nesting flycatchers are more susceptible.