Flying south for the winter?
In American slang, millions of “snowbirds” bail out of the North American winter and fly to a warmer climate. Now we learn that human snowbirds are simply emulating a pattern set by a large group of Western-Hemisphere birds.
Hundreds of North American birds migrate south for the tropical winters, and fly north to eat vegetation and insects that prosper during long summer days. That helps them raise more young.
Where did this pattern originate? Until now, the standard answer was this: The birds evolved into new species in the tropics, which began to migrate northward for the summer.
Now, in a study in Proceedings of the National Academy of Sciences, scientists compared the genetic relationships among more than 750 North American birds to their migratory habits, and found a curious similarity between birds and snowbirds.
Both, it would seem, were once resident in the northern, temperate zone, and then began flying winters to the tropics.
The great untangling
Untangling the web of bird migration is not so easy, says Benjamin Winger, a graduate student in evolutionary biology at the University of Chicago. “Migratory birds visit different parts of the globe that often are far apart, and different species go to different places … and to figure out how it all began has been pretty challenging.”
The study looked at emberizoid passerines – the largest lineage of New World migratory birds, which includes warblers, sparrows, orioles, blackbirds and cardinals. Because fossils do not document past migrations, the researchers crossed a database on genetic relationships against data on birds’ current winter and summer ranges.
Essentially, the scientists assumed that if two migratory species descended from the same ancestor, it was also migratory. “We were looking back to see if we could trace changes in migratory behavior through time,” says Winger.
The model they created begins with “a combination of where they exist today, and how they are related to one another, to give a probabilistic view of the past… to tell us the most likely way they did evolve,” Winger says.
The presence of hundreds of non-migratory passerines in the American tropics has been considered evidence that migration originated in the tropics. The logic is similar to what a botanist would use: To track the origin of, say, potatoes, they might focus on an area with intense potato diversity, like the Andean highlands.
We asked Winger about whether he’d over-stepped a boundary, since genes are traditionally thought to affect structures, not behaviors. “Generally speaking, migration is assumed to be genetically programmed, and thus inherited, in many birds, including the species in this study,” he told us.
Migration is “a complex group of characters,” he added. “For example: the ability to navigate and orient; the practice of eating extensively to put on fat before long migrations; the ability to detect certain weather patterns — all of these are aspects of bird biology that are part of migration. It is reasonable to assume that the migratory patterns in this group of birds are heritable enough to be studied in this phylogenetic context.”
Winger, with colleagues Frederick Barker and Richard Ree, also concluded that some non-migratory tropical birds once migrated. (Kinda reminds us of the U.S. and Canadian ex-patriates who have fled the icy northern winters to settle long-term in Central and South America.)
A glacial process?
We Why Filers, occupying land in Wisconsin that was covered by vast glaciers 12,000 years ago, had to wonder how that would affect bird migration, but Winger told us the ice ages are just a blip in the roughly 20-million year history described by the phylogeny. “Many birds are thought not to have existed over much of North America at the recent glacial maximum, but were still found somewhere in North America, so the glacial cycles are ignored.”
“This is a really thought-provoking article,” says Anna Pidgeon, associate professor of forest and wildlife ecology at the University of Wisconsin-Madison. “It would be really interesting to see if their best ranked model, showing that migratory populations evolved from a temperate source with the winter range shifting toward the equator … holds up in the Europe-to-Africa migrants. Similar results from modeling Old World migrants would lend huge support to the ideas asserted here, that the colonization of the tropics came from temperate zone species.”
A statistical peek into the past is not conclusive, but it may be the best way to explore the origin of bird migration, Winger says. “We did not find exclusive evidence for the out-of-North-America pattern, but do find more evidence for it” than for the reverse. “There are fewer instances of [migrating] birds coming from the tropics than you might expect, given the lopsided diversity numbers in the tropics.”
– David J. Tenenbaum
Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer
- Temperate origins of long-distance seasonal migration in New World birds, Benjamin M. Winger et al, PNAS Aug. 4, 2014. ↩
- Why timing of bird migration is changing. ↩
- Toronto’s FLAP provides useful tips for homeowners to cut down on window collisions. ↩
- Planned stadium and other windows threaten birds. ↩