Oct. 26, 1998 To Charles Darwin, evolution through natural selection occurred over countless generations. To Charles Brown, a University of Tulsa biology professor, natural selection took place literally overnight in 1996 as he witnessed the starvation death of half the cliff swallows he has studied in Nebraska for 17 years.
"The population in our study area, which is a region of about 100 miles along the Platte River, fell by 53 percent during a six-day cold spell in May," explains Brown. "Mortality was probably in the vicinity of 30,000 birds, but about 28,000 survived."
Basically, Brown found that the survivors were larger and more symmetrical than those that died, and those who lived through the bad weather have apparently passed on the genes for larger size to their offspring.
Brown's findings are presented in the October issue of Evolution in the article "Intense Natural Selection on Body Size and Wing and Tail Asymmetry in Cliff Swallows During Severe Weather."
"This event is an example of evolution in contemporary time," says Brown. "When we talk about evolution, we're talking about gradual changes that have occurred over millions of years, so it's rare to be able to go out and demonstrate natural selection in the time frame of this study. In this case, essentially overnight."
Brown, his wife, Mary, a research associate in TU's biological sciences department, and student assistants spend the summers along the Platte River observing, trapping, and banding swallows to determine life span, migration patterns, sex and health. They have banded more than 100,000 birds. The research represents the longest running study of coloniality in vertebrates. The goal is to uncover the advantages and disadvantages of group living.
On May 24, 1996, began six days of cold weather, with highs ranging from 40 to 50 degrees, and some rain daily. As a result, flying insects were not active, which sharply curtailed the food supply of the insect-eating swallows, who normally catch their prey in flight at about 200 feet above the ground.
"They can go four days without food, and they can eke out a living if it is in the 50s, but dry. But when you put rain on top of it -- that's what was so bad," Brown says. By the fifth and sixth day, when lack of food took it's toll, the researchers' instincts took over, asking: What traits do the survivors have that are absent in the ones who succumbed?
They collected more than 1,800 dead cliff swallows and measured their wings, legs and beaks. They measured the same features on about 1,000 survivors. Many of the birds sampled -- dead or alive -- had been banded. "They were ones that we knew something about; we had their history. We had 10-year-old birds that were dying."
The survivors were larger skeletally, including bigger beaks and bigger legs. "They probably were able to hold more fat going into the bad weather, which would help cope with food scarcity. Surviving swallows had shorter wing and tail feathers, but Brown has no clear explanation for that difference.
Among the dead birds, the right and left sides did not match in size as closely as those who lived. The difference in lengths was about 5 percent.
Brown says some researchers believe that symmetry is a reliable indicator of an individual's "quality" -- meaning overall vigor, including disease resistance. "In addition, there is a great deal of interest now in asymmetry as a cue for mate choice," he says. "It is believed that many wild animals, particularly females, will pick symmetrical males, although the reason is not clear. But this case provides a good example of why a female would want a symmetrical male."
Presumably evenly formed individuals are survivors who will pass those symmetry qualities to their young. "Our research actually provides rationale for this widely held view that symmetry is important," says Brown. "This is the first example of natural selection for high symmetry in the wild."
"We've looked at the offspring of the 1996 survivors, and they are also big and symmetrical," Brown points out. "So these traits appear to be genetically based."
The same patterns were seen among samples of 25 barn swallows that survived and 21 that perished in the study area.
They also found that the survivors tended to be younger birds. "We suspect that the older birds had arrived earlier and were probably further into nesting at the time the event occurred, and therefore, they probably had fewer fat reserves; they had used up more energy in laying eggs and incubating eggs. Many birds are real fat when they first arrive on the breeding grounds, and that's believed to be so that they can endure some bad weather. The younger birds probably arrived more recently, and they still had some of these reserves."
Temperature and rainfall records since 1875 for the study area show only two similar events, in 1967 and 1996, "and that really indicates to me that we witnessed a very rare event." A similar weather spell occurred in 1992, but relatively few birds were involved.
Brown, who has served as a curator of ornithology at Yale's Peabody Museum of Natural History, wrote "Swallow Summer," a journal-type book published this year about his passion for birds and the personal challenges of scientific research. He and his wife wrote the book "Coloniality in the Cliff Swallow: The Effect of Group Size on Social Behavior."
The field work is funded by the National Science Foundation and the National Geographic Society. The Browns met when he was in graduate school in Princeton University. She was his first assistant, and the project became his dissertation project.
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