Why do some birds fly thousands of miles back and forth between breeding and non-breeding areas every year whereas others never travel at all? One textbook explanation suggests that eating fruit or living in nonforested environments were the precursors needed to evolve migratory behavior. Not so, report ecologists W. Alice Boyle and Courtney J. Conway of the University of Arizona, Tucson, in the March issue of the American Naturalist.
Conway is also a research scientist with the U.S. Geological Survey. The two showed the pressure to migrate comes from seasonal food scarcity. It's the first time the technique called phylogenetic independent contrasts has been used to identify the causes of bird migration. "It's not just whether you eat insects, fruit, or candy bars, or where you eat them -- it matters how reliable that food source is from day-to-day," Boyle said. "For example, some really long-distance migrants like Arctic Terns are not fruit-eaters."
The new research indicates that one strategy for dealing with seasonal changes in food availability is migration. The team also found that birds that forage with others of the same species are less likely to migrate.
"Flocking can be an alternative way of dealing with food shortages," Boyle said. When birds band together to search for food, the group is more likely to find a new patch of food than is one lone individual.
To figure out the underlying pressures that drive some birds to leave home for the season, Boyle and Conway focused on 379 species of New World flycatchers from the suborder Tyranni. For all those species the scientists compared the species' size, food type, habitat, migratory behavior, and whether the birds fed in flocks.
A universal assumption about bird migration has been that short-distance migration is an evolutionary stepping stone to long-distance migration. The team's work contradicts that idea by showing that short-distance migrants are inherently different from their globe-trotting cousins.
Citation: W. Alice Boyle and Courtney J. Conway, "Why migrate? A test of the evolutionary precursor hypothesis" American Naturalist 169:344-359 (2007)
Founded in 1867, The American Naturalist is one of the world's most renowned, peer-reviewed publications in ecology, evolution, and population and integrative biology research. AN emphasizes sophisticated methodologies and innovative theoretical syntheses--all in an effort to advance the knowledge of organic evolution and other broad biological principles.
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