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Why and How do Birds Migrate?
Migration in birds is predominantly a genetic predisposition. Migration appears to be programed into all bird’s genetic make-up. It is part of their survival instincts. They are not unique to this. Most all animals have migratory predispositions. They move from abundance to abundance for food. Primitive people did the same thing.
Heck, some modern people do the same thing. I met a large number of (snow birds), people from Canada and the Great Lakes regions, that spend their winter months in South Texas while I attended the Port Aransas Whooping Crane Festival. Beats shoveling snow . . .
The migration of birds, however, is more spectacular and is used around the world as a harbinger of seasonal change. These migrations have been considered and studied for a very long time.
Historic Thoughts on Migration
Bird migrations have been commented upon for thousands of years. They are mentioned in the Bible and in ancient Greek writing. Below are some early ideas about why birds disappeared each winter and where they went.
- In the middle ages, people thought high flying birds were heading for the moon for winter.
- The changes in plumage from summer to winter colors was noticed and lead some to believe birds were changing into something different. Lngersoll, author of the below book about bird myths, states that Sophocles, a Greek playwright, thought that the Hoopoe and the hawk were the same bird.
- Lengersoll also says that Egyptians thought that storks and cranes carried smaller birds across the Mediterranean on their backs.
- Swallows and swifts were believed to hibernate in caves and hollow trees.
Read About It!
Birds and Food
Reasons for Migration
Generally, birds migrate to move from areas of low or decreasing resources to areas of high or increasing resources and the requirements of raising the next generation. The differences between the way different species accomplish these things has more to do with environmental triggers, a combination of changes in day length, lower temperatures and changes in food supplies.
As a rule, birds choose to nest in the Northern Hemisphere and spend their non-breeding season in the Southern Hemisphere. Why isn’t it the other way around? It’s sunny and temperate in southern latitudes most of the time, right?
- One reason for this may be that Southern Hemisphere territories are hotter in Spring than northern territories, making it a harsher climate for nesting and raising chicks.
- Another reason may be due to the higher abundance of predators in territories with year-round warm temperatures.
- There is also the fact that the days are longer in high latitudes so there is more time for foraging activity. Also, birds that migrate north have larger clutches than birds of the same family that don’t. They have the longer day hours to keep the extra chicks fed.
- There is more land mass to be had in the north and more fresh water lakes, which lead to less competition for nesting sites. The farther south one gets the more area is taken up by oceans.
If migration is a matter of being in the best place to insure the survival of chicks, it stands to reason that northern territories are chosen because the climate, landscape, food availability and ability to protect chicks from harm is greater.
Next to nesting needs is the availability of food. Food sources, such as flower nectar and insects, wax and wane seasonally. Flowers start to die off in late summer, so around August, Hummingbirds will begin their southward movements. Insects become harder to find as the first frosts happen, so flycatchers and other insect eating birds head south for warmer climates in the Fall. Fish and game eating birds will also migrate for similar reasons.
Migration moves bird populations from one spring and summer cycle in the Northern Hemisphere to another in the south. This seasonal cycle occurs because the planet rotates at an angle giving us the effect of opposite seasons between the north and southern hemispheres. Assistant Professor Ken Rubin, University of Hawaii, Department of Geology and Geophysics explains that the northern hemisphere winter is in Dec, Jan and Feb, whereas the southern hemisphere winter is in June, July, and August.
“Each of our summers is 6 months away from our winters in our respective hemispheres. The difference is that in the North people might say "I am dreading a cold January" whereas in the South you might say "I am looking forward to a warm January."
The below video is long, but the first five minutes gives a great overview of bird migration. The rest is very interesting stuff too. Take out some time to see it all.
Bird Migration Overview
Bird Migration Differences
Not all birds migrate as far as others. Distance varies depending on species and their geographic placement.
Short Distance Migrants – Some birds like tropical hummingbirds migrate from high altitudes during the breeding season to low altitudes during non–breeding seasons. Most of this is due to the need for nectar and the loss of flowering plants at high elevations during colder seasons. Northern Bobwhites living in the Rocky Mountains are also short migrators. Lower altitudes offer warmth and more food, but higher altitudes offer fewer predators; so they go high for nesting and low for winter forage.
Medium Distance Migrants – Some birds travel, but do so on a periodic, almost at will schedule. They live in temperate climates where spring and winter climates may not be severely different, so get through the winter with various coping skills and by varying their diets.
For example, Eastern Blue Birds in the more northern ends of their range will migrate, but sometimes only to the closest points where food is still available. In this way, a Blue bird that summers in Canada may head to Tennessee in the fall. If food becomes scare as the winter progresses, they may move down to Alabama. Blue birds in southern range areas may not migrate at all. Other examples of this are Blue Jays and Killdeer.
Long Distance Migrants – Cornell Lab of Ornithology describes these as Neotropical migrants. “Approximately 350 species fall into the Neotropical migrant category. That is nearly half of all bird species in North America. These birds breed in the United States and Canada. They winter in the Caribbean, Mexico, Central America and South America. Neotropical migrants include raptors, vultures, waterfowl, shorebirds, and passerine species such as hummingbirds, thrushes, warblers, orioles, and tanagers.”
Here are some of the long distance record holders and their round trip distances.
- Northern Wheatear – Alaska and Canada to Sub-Saharan Africa, 9,000 miles
- Barred-tail Godwit – A satellite tagged bird traveled from Alaska to New Zealand, 7,145 miles. That is the record for non-stop flights by birds. A round-trip would equal 11,899 miles.
- Pectorial Sandpipers – From Alaska into South America, 18,000 miles
- Short-tailed Shearwater – Loops the Pacific Ocean annually, 27,000 miles
- Arctic Tern – Greenland to Weddell Sea in Antarctica, 44,000 miles
These long distance travelers are believed to be choosing these wide distances in order to (leap-frog) or bypass latitudes where habitats are already occupied by short and mid-distance migrators. Getting past these high population areas gives long distance migrators wintering territories with less competition. These birds develop strong ties to their destinations returning to the same winter and spring sites year after year.
Arctic Tern Travels
The southward migration of birds, as mentioned before, seems to be triggered by environmental conditions, a combination of changes in day length, lower temperatures.
Short and Mid-distance migrators are reacting to temperature changes primarily. Their migration choices only involve staying where food is available. Even if the weather is severe, some birds that follow these shorter migrating patterns will remain if they have sufficient winter forage. Long distance migrators seem to be triggered more by day length. They travel far distances between summer and winter territories, so will move with the earliest seasonal changes.
We see geese fly in their V formations as well as some other birds, but I’ve yet to see large groups of Hummingbirds or Eagles flying by. There is a reason for this.
- Some birds travel by day, referred to as Diurnal migrators. These include most of the larger birds such as raptors and storks. These birds need the thermal columns or rising hot air to gain altitude. They have the hardest times flying over large bodies of water where thermals don’t exist. To combat this, they usually hopscotch between islands when traveling over water is required. We don’t see them much because they fly at very high altitudes and over wide areas. Bald eagles, for example, tend to migrate in groups spread out a half mile apart and twenty to thirty miles in length.
- Nocturnal migrators, on the other hand, travel at night. Most hummingbirds, warblers and flycatchers follow this timing. These birds will travel through the night and then land, spending several days refueling for the next night flight. Night travel helps these birds avoid overheating and becoming in-flight meals for traveling raptors.
It is believed that migration northward starts as birds near breeding readiness. The hormonal changes cause heavy feeding to build fat reserves and changes in plumage. I’ve noticed that Gold Finches wintering in Texas stay until their spring colors begin to return. As the males regain their black caps, they start to disappear. In many cases, males start migrating first to establish territories. Females and juveniles will then follow later.
Both north and south migrations can also be timed by the weather. Wind currents change with the seasons. As winter arrives, strong winds from the north push cold air southward.
Birds generally take to the skies on the trailing edge of cold fronts. This allows them to avoid most of the turbulence of warm air and cold air collisions on the leading edge. These southward air currents are the reason for the four North American Flyways. Cold air currents funnel around the Rockies separating the western and central flyways. Air currents also flow down through the Mississippi Valley creating that flyway. The Eastern flyway is created by south and easterly moving currents along the seaboard. These tail-winds push the birds along allowing them to save energy and fly faster. This information was gathered mainly through the study of water birds, geese and ducks, by banding and analyzing hunter reports.
Song Bird Migration Flyways
What is good for a duck isn’t quite the same for a song bird. A Cornell Lab Study taken between 2004 and 2011 used crowd sourced data turned in by birders to the E-bird data collection site to study the movements of passerines. The data was then mapped out using computer models. It showed where songbird species were moving over multi-year time spans. The study discovered that these birds move in three flyways.
- A western group consisting of 31 species
- A central group of 17 species
- 45 species in an eastern group.
These flyways are wider spread than those of ducks and geese and the central and eastern flyways overlap considerably.
When fronts are slow moving or become stalled, a condition called Fall-out occurs, where birds seem to literally fall out of the skies as rain, low overcast and foggy conditions develop.
The day right after one of these cold fronts pass is what birders watch for. Those that take note of weather patterns will gravitate to these fall out areas taking advantage of the sudden abundance of grounded birds. As air conditions become cool and dry and when tail-winds return, the journey resumes. Waiting grounded birds then rise up in huge flocks to continue their journeys.
1. Most songbirds migrate at 500 to 2,000 meters of altitude, but some fly as high as 6,800 meters; swans have been recorded at 8,000 meters and Bar-headed Geese at 9,000 meters.
2. Shore birds fly high over water but low over land; while land birds fly high over land and low over water.
3. Land bird migration paths follow a clockwise flight pattern. They fly more to the east heading south and more westerly going north.
4. Geese and ducks follow a fairly narrow flyway going both north
Types of Navigation
How do Birds Navigate?
Bird navigation is a complex subject, but I will offer some highlights about previous and on-going research into the subject. These different methods are believed to be used in conjunction and possibly in different orders of importance by different bird species.
Many birds, such as Whooping Cranes learn the route. They migrate with their parents from their breeding grounds to their southern ranges. It takes a Whooping Crane five years to reach full maturity, so they follow the family flock for quite a while before becoming independent.
Birds have an amazing ability to home-in on their desired destination.
“In an experiment, “Manx Shearwaters were flown by plane from their nesting island off the coast of Great Britain to two different locations. One group was released near Boston, MA, and another near Venice, Italy. Shearwaters do not fly over land so both groups must have taken an over water route, which would be especially convoluted from Venice. Both groups of birds returned to their nesting burrows within 14 days, covering approximately 250 miles per day.”
Birds actually understood navigation by the skies long before we did. They use the position of the sun and stars to guide their migrations.
- Sun Compass – “Birds have a time compensation ability to make allowances for changes in the sun's position over the course of the day. This theory is supported by an experiment in which pigeons were placed in a closed room with an altered cycle of light and dark. Over a period of a few days their 24-hour cycle was reset. The birds were then released on a sunny day. Because their "internal clock" had been reset, they misinterpreted the position of the sun and made a predictable error in their homing direction. Pigeons actually ignore the position of the sun relative to its position in the sky, relying on its azimuth direction, i. e. the compass direction at which a vertical line from the sun intersects the horizon.”
- Star Compass – “In 1967 Cornell scientist Stephen Emlen used Indigo Buntings to prove that birds orient themselves to stars for migration. His research indicated that young birds do not learn star patterns themselves, but learn a north-south orientation from a rotational star pattern.”
Some birds use the Earth's magnetic field to navigate. This excerpt from Live Science explains it.
“Researchers discovered molecules called cryptochromes, which change their chemistry in the presence of a magnetic field, in the retinas of migratory birds' eyes. “When light hits these molecules, their chemistry changes and magnetism can influence them," Mouritsen said. The molecules might then affect light-sensing cells in the retina to create images, which would help the brain navigate during flight, he added.
A direct connection between the specialized cells and the region of the bird's brain active during magnetic orientation, was discovered by Mouritsen and his team recently found such connections between the cryptochrome-holding retinal cells and the "cluster N" region of migratory birds' brains, located in part of the brain responsible for vision.
"Cluster N is highly active during magnetic field orientation at night, when migratory birds fly," he said, explaining that non-migratory birds don't seem to use it during night flight. "We can't see what birds see, obviously, but they may pick up some sort of shading in their vision at night to act as a compass."
Birds could also navigate using landscape features, such as coastlines, mountain ranges and waterways. This is the least understood of the types of navigation birds may use. Another theory is that they use their sense of smell. Again, this is hard to prove and is controversial.
Birds are complex animals to study, but in recent years a great deal of new information has been discovered to understand how they live and migrate across the planet. We have come a long way from when Aristotle suggested that birds hibernate, or from the days tiny birds were considered too fragile to fly great distances under their own power. Birds are tough and much stronger with more endurance than we ever thought possible. So next time you see migrating birds, consider how amazing the accomplishment is.
© 2015 Sherry Thornburg