Data from: Winter wren populations show adaptation to local climate

Most studies of evolutionary responses to climate change have focused on
phenological responses to warming, and provide only weak evidence for
evolutionary adaptation. This could be because phenological changes are
more weakly linked to fitness than more direct mechanisms of climate
change impacts, such as selective mortality during extreme weather events
which have immediate fitness consequences for the individuals involved.
Studies examining these other mechanisms may be more likely to show
evidence for evolutionary adaptation. To test this, we quantify regional
population responses of a small resident passerine (winter wren
Troglodytes troglodytes) to a measure of winter severity (number of frost
days). Annual population growth rate was consistently negatively
correlated with this measure, but the point at which different populations
achieved stability (λ = 1) varied across regions and was closely
correlated with the historic average number of frost days, providing
strong evidence for local adaptation. Despite this, regional variation in
abundance remained negatively related to the regional mean number of
winter frost days, potentially as a result of a time-lag in the rate of
evolutionary response to climate change. As expected from Bergmann's
rule, individual wrens were heavier in colder regions, suggesting that
local adaptation may be mediated through body size. However, there was no
evidence for selective mortality of small individuals in cold years, with
annual variation in mean body size uncorrelated with the number of winter
frost days, so the extent to which local adaptation occurs through changes
in body size, or another mechanism remains uncertain.