Intense Natural Selection on Body Size and Wing and Tail Asymmetry in Cliff Swallows During Severe Weather

Extreme climatic disturbances provide excellent opportunities to study natural selection in wild populations because they may cause measurable directional shifts in character traits. Insectivorous cliff swallows (Petrochelidon pyrrhonota) in the northern Great Plains must often endure periods of cold weather in late spring that reduce food availability, and if cold spells last four or more days, mortality due to starvation may result. We analyzed morphological shifts associated with viability selection, and how patterns of bilateral symmetry were affected by survival selection, during a four-day period of cold weather in 1992 and a six-day period in 1996 in southwestern Nebraska. Birds that died during the cold were compared to those still alive when the severe weather ended. The event in 1992 killed relatively few birds, but the cold spell in 1996 killed thousands of cliff swallows and reduced their population by about 53%. Climatological records suggest that mortality events comparable to that of 1996 have occurred in only one other year since 1875. Larger birds were favored in the 1996 event. Selection was more intense in 1996 than in 1992 because of more stressful conditions in 1996. Directional selection gradient analysis showed that measures of skeletal body size (tarsus length, culmen width and length) and wing length were targets of selection in 1996. Survivors had lower wing and outer tail asymmetry, and wing and tail asymmetry were targets of selection in both events. Mortality patterns did not differ by sex, but older birds suffered heavier mortality; morphological traits generally did not vary with age. Nonsurvivors were not in poorer apparent condition prior to the weather event than survivors, suggesting that selection acted directly on morphology independent of condition. Selection on body size in cliff swallows was more intense than in studies of body size evolution in other bird species. Larger swallows were probably favored in cold weather due to the thermal advantages of large size and the ability to store more fat. Swallows with low asymmetry were favored probably because low asymmetry in wing and tail made foraging more efficient and less costly, conferring survival advantages during cold weather. This population of cliff swallows may have undergone relatively recent body size evolution.