A study on the causes of bird migration
- From: "Howard Lester" <hlester@xxxxxxxx>
- Date: Wed, 7 Mar 2007 10:28:18 -0700
This was presented today in the weekly on-line "Faculty and Staff news" at
the University of Arizona. Note that Clorox had nothing to do with THIS
study! ;-)
5. Why Do Birds Migrate?
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 either eating fruit or living in
non-forested environments were the precursors needed to evolve migratory
behavior.
Not so, reports a pair of ecologists from The University of Arizona. The
pressure to migrate comes from seasonal food scarcity.
"It's not just whether you eat insects, fruit, nectar or candy bars or where
you eat them ? it matters how reliable that food source is from day to day,"
said W. Alice Boyle, an adjunct lecturer in the department of ecology and
evolutionary biology. "For example, some really long-distance migrants, like
arctic terns, are not fruit-eaters."
Boyle and Courtney J. Conway, a UA assistant professor of natural resources
and a research scientist with the U.S. Geological Survey, report their
findings in the March 2007 issue of American Naturalist.
To figure out the underlying pressures that drive some birds to leave home
for the season, the team wanted to examine a related set of species and
compare their size, food type, habitat, migratory behavior and whether they
fed in flocks. Boyle and Conway focused on 379 species of New World
flycatchers from the suborder Tyranni. One of the largest groups of New
World birds, the Tyranni includes kingbirds, flycatchers, phoebes and such
southern Arizona birdwatchers' delights as vermillion flycatchers and
rose-throated becards. Tropical members include manakins and cotingas.
First the scientists had to construct the first "supertree" for New World
flycatchers.
"No one has ever compiled all those birds together into one mega-family
tree," Boyle said, adding that "supertree" is a technical term among
evolutionary biologists.
Having the tree allowed the researchers to compare a variety of traits
across the many species of Tyranni by using a computer analysis called
phylogenetic independent contrasts.
The technique allowed the scientists to sort out whether a bird was
migratory because that's what species on their side of the family tree
always did or whether the bird's travel habits had some ecological
correlates.
Food scarcity was the No. 1 issue that predicted a species' migratory
behavior, the team found. "Food availability is the underlying process, not
diet and habitat," Boyle said.
One strategy for dealing seasonal changes in food availability is migration.
The team also found that species that forage in flocks are less likely to
migrate.
"If you are faced with food scarcity, you have two options," Boyle said.
"You can either forage with other birds or you can migrate."
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, she said. "Flocking
can be an alternative way to deal with food shortages."
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.
The National Science Foundation and the Natural Sciences and Engineering
Research Council of Canada funded the work.
.
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