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Origins of Migration

Why do birds migrate and how did migration over long distances evolve? The answers are hidden within the vast diversity of bird behavior, weather, geography, and the availability of seasonal food sources. At a basic level, migration evolves when birds that move from one area to another produce more offspring than those that do not. When migratory patterns evolve over hundreds or thousands of years the urge to migrate becomes part of the birds genetic make up.

There is still much to be learned regarding the evolution of long-distant migration and the origins of such migratory behavior.

Climatic changes

Over very long periods of time changes such as glacial advances and retreats or even the shifting of continents may have played a role in the development of long-distant migratory behavior.

Gradual dispersement--Neotropical migrants

Neotropical migrants are a group of birds whose migration patterns may have evolved over long periods of time. Their winter homes seem rich in food supplies and nesting locations, so why do they make the arduous and dangerous return trip north in the spring?

One idea is that through many generations the tropical ancestors of these birds dispersed from their tropical breeding sites northward toward the United States and Canadian temperate zones. The seasonal abundance of insect food and greater day length allowed them to raise more young (four to six, on average) than their stay-at-home tropical relatives (two to three young on average). As their breeding zones moved north, the birds continued to return to their ancestral home as cold weather and the related decrease in food supplies made life more difficult. Supporting this theory is the fact that most North American vireos, flycatchers, tanagers, warblers, orioles, and swallows have evolved from Neotropical forms.

In certain species, the evolution of migratory behavior over such long periods of time has resulted in a genetic disposition that favors migration.

Rapid changes in migratory patterns

Under certain environmental conditions migratory behavior can change rapidly, and the origins of the new migratory behavior can be seen.

The new migrants
In the early 1940s House Finches from a nonmigratory population in California were released on Long Island, New York. The harsher New York winters appear to have encouraged the development of migratory behavior. Within twenty years large numbers of House Finches were migrating south to the Gulf Coast states in winter and back north each spring.

This story illustrates how little is really known about the evolution of migratory behavior. How did migratory behavior really evolve in these House Finches? 

Presumably a lack of food in the winter was the initial motivating factor and migration may have evolved according to one of these theories. 

Theory 1. The Dumb Luck Theory.
This theory assumes House Finches, in search of food, fanned out in all directions from the city.

  • Those that went north did not survive the cold.
  • Those that flew east fell into the ocean.
  • Those that flew west apparently did not find enough food to survive.
  • Those that flew south survived. They were able to return to New York in the spring, repeated the behavior in subsequent years, and gradually built a population of finches that migrated south each fall.

Questions still arise. Do all offspring of the migratory finches immediately exhibit migratory behavior or do some take off in the wrong direction and perish? What about the residual population of Houses Finches?  Are a certain percentage of those birds going through the same process, with some birds fanning out in all directions each year, or do they learn from the birds that head south?

Theory 2.  The Inherited Knowledge Theory
Maybe House Finches have an inherited "knowledge" of which direction to fly to find warmer weather and larger food supplies. The decision to act upon this knowledge is influenced by outside elements such as food and temperature.

This theory is supported in a couple of ways.

1. Young birds of many species know to fly south in the fall without support from adult birds. It seems possible that adult House Finches may have the same capability governed by a retained genetic memory that can be triggered by external conditions. This memory points the birds in a southerly direction.

2. The note below on a population of Dark-eyed Juncos is also supportive. Upon arriving on an island with apparently very agreeable conditions, some juncos elected not to return to their migratory behavior and became permanent residents of the island. It may be that they were able to turn off their migratory nature upon arrival at a location that provided adequate resources. If certain species are able to turn off migratory behavior when it is not longer of benefit to them, then perhaps some species can revert to inherent migratory behavior if conditions change to make such behavior beneficial.

Theory 3. The Wireless Internet Theory
Maybe birds have an as yet undiscovered interface to the world around them.  Somehow they are able to sense which directions lead to warmer weather and know that more food will be available in warmer areas.

If this seems far-fetched, consider the Monarch Butterfly. It takes four, five or more generations of Monarch butterflies to travel from their winter homes in Mexico to a short summer in Canada. Yet that final generation lives nine months, migrating back south to the very same valley in Mexico that its great, great, great, great, great grandparents left in early spring. No less a remarkable feat than the idea that birds might have the equivalent of a wireless Internet connection that keeps them in touch with the world around them.

The December 26th, 2005 tsunami caused massive devastation, yet few animals were killed. Elephants have been credited with being able to sense low frequency vibrations in the earth and were able to move to safety in response. With such intense vibrations being so unusual, how did the elephants know that the vibrations were a sign of danger and which way to run? How did flamingos and other animals also know to move to higher ground or other locations? Was their knowledge part of the same special sense that provides first-year shorebirds born in northern Canada with the knowledge that warmer weather and food may be found in the south, and in which direction they should fly to head south?

Tired of the trip
The opposite situation can also occur if conditions are just right. Migrating Dark-eyed Juncos apparently wandered off course and found themselves on Guadeloupe Island, about 155 miles from the coast of Baja California. The environment apparently suited the juncos so well that they abandoned their typical migratory behavior and became permanent residents of the island.

So why did the juncos not migrate north in the spring?

Theory 1. Maybe at least two juncos had weak migratory tendencies and passed on that weak tendency to their offspring. 

This theory assumes that the genetic make up supporting migratory behavior can be lost in a single generation.

If this was the case then why have similar populations not appeared in other parts of the junco's winter range? Dark-eyed Juncos winter throughout the southern United States and into Mexico. If juncos with weak migratory tendencies can find themselves on an isolated island, surely over the years similar juncos will have found themselves in winter locations in Mexico or the southern United States with adequate food supplies and nesting locations. Why did they not elect to abandon their migratory behavior?

Theory 2. Maybe the variances in temperature were so small that there was not a significant enough spring warm-up to trigger migration. Without a sufficiently strong temperature delta, hormonal changes in the juncos did not occur, and the birds did not feel an urge to migrate.

Theory 3. Maybe birds always have the ability to migrate and inherently know which direction they should head in winter months (maybe it is the genes or some 6th sense). The urge to migrate operates like a very sticky switch. It can be turned on or off, but it takes a lot of muscle to do so.

A combination of environmental variables control the flipping of the switch. An overwhelming change in the environment must occur to have the power to flip the biological switch in one direction or the other.

On a point system, it might work like this.

Change in the amount of daylight: 1 to 10 points
Change in low temperature over a 30 day period: 1 to 10 points
Change in high temperature over a 30 day period: 1 to 10 points
Lowest temperature in the past 3 days: 1 to 5 points
Change in food supply of past 30 days: 1 to 10 points
Current food supply: 1 to 10 points
Numer of nesting locations: 1 to 10 points
Quality of nesting locations: 1 to 10 points
Lack of competition for nest locations: 1 to 10 points
Lack of competition for food: 1 to 10 points

The bird assigns a value to each variable and adds it all up.

The urge to migrate is 50 points.

If the total does not exceed 50, the environmental changes do not have enough power to flip the switch and the bird migrates.  If the total exceeds 50 the urge to migrate is overpowered and shuts down, and the bird does not migrate.

For the House Finches, the situation is the reverse. Non-migratory behavior is controlling. When the number of points supporting migratory behavior becomes overwhelming, the migratory behavior switch is clicked back on, and the bird knows to head south.