PS 41-129 - Rapid Natural Selection of Eastern Bluebirds during Severe Winter Weather

Abstract: Climate change has forced organisms to respond phenologically, physiologically, morphologically, etc. However, extreme events can revert the direction of these responses, be they plastic or adaptive. In February 2021, two severe snowstorms swept the United States, setting new records of snowfall and below-freezing temperature, both in terms of magnitude and duration. This event caused massive die-offs, particularly in birds and bats, which may have been associated with rapid natural selection. Focusing on Eastern Bluebirds, our objective was to determine if the snowstorm caused natural selection in their morphology and plumage coloration. We hypothesized that conditions during such a severe winter storm could lead to a micro-evolutionary change. Specifically, we predicted that these storms would favor larger individuals with smaller extremities following the thermal tolerance and starvation endurance hypotheses. Additionally, high-quality individuals (e.g., with lower asymmetry) should also be more likely to survive, but we expected individuals with more ornamented plumage would have been disadvantaged because this trait, so important in sexual selection, is costly to maintain.

We collected all fatalities and captured all survivors encountered in nest boxes at three sites in Arkansas, Mississippi, and Oklahoma. We measured dead and non-survivors for 10 morphological variables and six derived variables (e.g., asymmetry score). We extracted feathers from four body regions and obtained eight coloration variables using a reflectance spectrophotometer. Applying an information-theoretic approach on generalized linear mixed models, we selected the morphological and coloration variables that best explained the probability of a bluebird surviving the snowstorm.

Survival probability was not correlated with body size but was associated with a longer tarsus, and a wider, longer, and deeper beak. These results were counterintuitive because beak and tarsus are the primary areas through which birds lose heat. We speculate that these longer extremities may have helped with foraging in the snow. Additionally, as expected, both male and female bluebirds with duller breast and tail were more likely to have survived. However, asymmetry was not associated with a lower survival. Longer extremities were favored in extreme winter conditions and should also present a thermoregulatory advantage in the increasingly warmer summer conditions, so these traits may persist. However, if natural and sexual selection act against each other, plumage ornamentation traits may stabilize.