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

Abstract

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.

Albinism and Phenotype of Barn Swallows (Hirundo Rustica) From Chernobyl

“…The same person measured all birds in a standardized way, thus the differences cannot be attributed to changes in measurement methods or definitions of characters. These changes are very large compared to responses to selection recorded in other birds, such as 0.50-0.75 standard deviations in Darwin's finches (Grant and Grant 1995) and a similar magnitude or even larger in cliff swallows (Petrochelidon pyrrhonota; Brown and Brown 1998;Price et al 2000). The initial responses to selection in the studies by Grant and Grant (1995) and Brown and Brown (1998) were subsequently followed by evolutionary responses in the opposite direction.…”

Intraclutch Egg-Size Variation in Magellanic Penguins

“…Predation, climatic events, nesting density, fights, and foraging distance (see Stokes and Boersma 1998, 2000, Boersma and Rebstock 2009 can also affect reproductive success in Magellanic Penguins. Similar environmental variables influence selection of traits in other birds, including weather (Brown and Brown 1998), food size (Boag and Grant 1981), and human interference (Brown and Brown 2013). Additionally, there is some intrasexual competition in male Magellanic Penguins (Renison et al 2002) that may impose selection on morphology, similar to results in Snow Petrels (Pagodroma nivea; Barbraud 2000).…”

Geographic variation in the G matrices of wild populations of the barn swallow

“…Selection on morphological components has been demonstrated to occur in a wide range of organisms (Endler, 1986;Fairbairn and Reeve, 2001;Kingsolver et al, 2001) and in various species of birds (Holland and Yalden, 1995;Brown and Brown, 1998;Larsson et al, 1998;Merila et al, 1999;Balmford et al, 2000;Barbraud, 2000;Nowakowski, 2000;Przybylo et al, 2000), including barn swallows (M ller, 1993;M ller and Tegelstrom, 1997;M ller et al, 1998;Brown and Brown, 1999). In Nebraska, a period of severe weather resulted in size-related mortality of barn swallows (Brown and Brown, 1999) indicating that, as with other bird species natural selection acts upon at least some morphological components.…”