Climate change reduces the tension of conflicting selection pressures on breeding date in a passerine bird

Unpredictably fluctuating environments create complex selective landscapes that shape the distribution of key life history traits. Identifying the mechanisms behind dynamic patterns of selection is difficult, yet essential for predicting responses to climate change. We combine long-term measures with field manipulation of natural selection on breeding date in a wild bird to investigate whether highly variable spring cold snaps drive fluctuating selection. We show that variation in cold snap intensity leads to fluctuating selection on breeding date-in weak cold snap years, selection was consistently negative; however, in strong cold snap years, its direction reversed. These patterns were mirrored in a field experiment; nests that were food supplemented during cold snaps avoided cold snap mortality leading earlier breeders to have higher fitness. In contrast, in the non-supplemented group earlier breeders had higher cold snap nest mortality and selection was positive. Using nearly a century of climate data, we show that cold snaps are becoming less frequent and paradoxically occurring later which should allow earlier breeders to avoid them, potentially releasing conflicting selection pressures and facilitating a rapid phenological shift. Thus, rather than constraining a species' ability to adapt, climate change can enable a rapid shift to a new phenotypic optimum.

Biological Links between Personality and Plasticity: Testing Some Alternative Hypotheses

AbstractWhen organisms respond behaviorally to a stimulus, they exhibit plasticity, but some individuals respond to the same stimulus consistently differently than others, thereby also exhibiting personality differences. Parent house sparrows express individual differences in how often they feed offspring and how that feeding rate changes with nestling age. Mean feeding rate and its slope with respect to nestling age were positively correlated at median nestling ages but not at hatching, indicating that individuality is primarily in plasticity. Individual differences could arise because of (1) interactions between environmental variables, (2) differences in underlying state or "quality," or (3) differences in the ability to update cues of changing nestling demand. Individual slopes were modestly repeatable across breeding attempts, hinting at the likely action of additional environmental variables, but only brood size was important. I also found few correlates suggesting quality differences. I used short-term brood size manipulations at two nestling ages to test divergent predictions between the three hypotheses. The pattern of correlations between response to the manipulation and individual slope did not fit any single hypothesis. Patterns of sparrow parental care reveal that personality and plasticity are not cleanly separable, and their biology is likely intertwined. New thinking may be needed about the factors parents use in decisions about care and the relevant fitness consequences.