Two publications appeared dealing with natural selection in cliff swallows during severe climatic events. One, in Ecosphere, documented that during the 1996 mass mortality event birds living in the smallest and largest colonies survived better over the 6-day period of bad weather than did swallows residing in intermediate-sized colonies. The results suggest disruptive selection on colony size, favoring the two extreme group sizes. We have rarely seen this form of selection in other contexts. Unusual weather events thus may select for group sizes in novel ways, and such selection can maintain the range of colony sizes we see in nature. The other paper, in the Wilson Journal of Ornithology, looked at two weather-related mortality events in 2004 and 2017, and examined whether natural selection on morphology was the same as in the 1996 catastrophe. In 2004, selection on wing, tail, and skeletal measures was generally the same as in 1996, but the weather event in 2017 showed reverse patterns of selection, with skeletally smaller birds favored. Both papers suggest that selection can fluctuate enough during rare climatic events to influence stasis in both behavior and morphology.
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