Modeling phenological reaction norms over an elevational gradient reveals contrasting strategies of Dusky Flycatchers and Mountain Chickadees in response to early season temperatures
Rotenberry, John; Coe, Sharon; Purcell, Kathryn (2021), Modeling phenological reaction norms over an elevational gradient reveals contrasting strategies of Dusky Flycatchers and Mountain Chickadees in response to early season temperatures, Dryad, Dataset, https://doi.org/10.6086/D1S689
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 temperatures. Conversely, high-elevation populations in cooler springs that are sensitive to the risk of having insufficient time to complete a breeding cycle may advance laying onset relative to lower populations (“advance”), 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 three 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 two species.