Friday, March 02, 2007
Study: 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, report a pair of ecologists from The University of Arizona in Tucson. The pressure to migrate comes from seasonal food scarcity.
Royal Flycatcher (Copyright 2004 Alice Boyle)
"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. "For example, some really long-distance migrants like arctic terns are not fruit-eaters."
Boyle, an adjunct lecturer in UA's department of ecology and evolutionary biology and co-author Courtney J. Conway, a UA assistant professor of natural resources and a research scientist with the U.S. Geological Survey, reported 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 megafamily tree," Boyle said, adding that "supertree" is a technical term among evolutionary biologists.
Yellow-bellied Flycatcher (Copyright 2004 Alice Boyle)
Having the tree let the researchers 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 number one issue that predicted a species' migratory behavior, the team found. Boyle said, "Food availability is the underlying process, not diet and habitat."
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.
 Based on the paper:
Why Migrate? A Test of the Evolutionary Precursor Hypothesis
W. Alice Boyle and Courtney J. Conway
Am. Nat. 2007. Vol. 169, pp. 344-359.
The question of why birds migrate is still poorly understood despite decades of debate. Previous studies have suggested that use of edge habitats and a frugivorous diet are precursors to the evolution of migration in Neotropical birds. However, these studies did not explore other ecological correlates of migration and did not control for phylogeny at the species level. We tested the evolutionary precursor hypothesis by examining the extent to which habitat and diet are associated with migratory behavior, using a species-level comparative analysis of the Tyranni. We used both migratory distance and sedentary versus migratory behavior as response variables. We also examined the influences of foraging group size, membership in mixed-species flocks, elevational range, and body mass on migratory behavior. Raw species analyses corroborated some results from studies that put forth the evolutionary precursor hypothesis, but phylogenetically independent contrast analyses highlighted an important interaction between habitat and diet and their roles as precursors to migration. Foraging group size was consistently associated with migratory behavior in both raw species and independent contrast analyses. Our results lead to a resource variability hypothesis that refines the evolutionary precursor hypothesis and reconciles the results of several studies examining precursors to migration in birds.
A 2006 related paper:
Phylogenetic tests of hypotheses for the evolution of avian migration: a case study using the Motacillidae
The Auk April 2006 123:455-466
The evolution of avian migration continues to be an intriguing research subject, even though relationships between migration and factors such as seasonality clearly exist. The question remains whether these relationships are evident within phylogenies containing both sedentary and migratory taxa. We explore the evolution of migration in the family Motacillidae by evaluating existing hypotheses for the evolution of migration in a comparative, phylogenetic framework at the interspecific level. Many hypotheses to explain the evolution of avian migration - such as the "evolutionary precursor" hypothesis (Levey and Stiles 1992, Chesser and Levey 1998) and the "stepping-stone" hypothesis (Cox 1968, 1985) - are based on New World migratory systems. The central components of these hypotheses should apply across biogeographic realms (i.e. the Old World), given that seasonality and habitat regimes are similar in the New and Old worlds. Using a molecular phylogeny containing most species in the Motacillidae, we investigated the potential interactions of seasonality and ecology with migratory and sedentary behavior. Our results suggest that habitat and migration are not correlated in the manner predicted by the evolutionary precursor hypothesis, but they also suggest the importance of increasing seasonality in explaining the patterns of the evolution of migration, an expected but previously unexamined evolutionary relationship. While understanding the limitations of applying generalizations to a complex evolutionary system such as migration, we have delineated here a broad methodology for testing hypotheses about the evolution of migration within a phylogenetic context.
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