Unraveling the Mysteries of Migration
Fall migration is upon us, that delightful time of year when long-absent birds return, while other familiar feathered friends depart. Unlike spring migration, fall migration is a rather drawn-out affair in California. Some shorebirds begin returning from their northerly breeding grounds as early as mid-summer, while some waterfowl delay their trip south until much later in the fall or even mid-winter. Even so, September is the month that fall migration really begins.
But what, exactly, is migration? Avian migration may be defined as the regular, seasonal, predictable movement of birds, which occurs annually as entire populations travel from their summer breeding habitat to wintering habitat, generally in the pursuit of warmer weather, longer daylight hours, and most importantly, food.
Seventy-five percent of North America's breeding birds exhibit a form of migratory behavior, moving to some extent between summer breeding habitat and overwintering habitat. Many follow the traditional pattern in the Northern hemisphere of flying "south for the winter," and return north again for the summer, chasing suitable temperatures and food supplies.
But migration is much more complex than that.
Migrants may be divided into two categories, known as long-distance and short-distance migrants. Most of the western hemisphere's long-distance migrants travel between North American and Central or South America, splitting their time between two different continents. Short-distance migrants generally migrate within one continent.
An example of a type of short-distance migrant are altitudinal or elevational migrants, species that move "down slope" from mountainous regions when the breeding season ends, seeking the milder climates of lower elevations. In California's Great Central Valley, a number of our overwintering songbirds, like sparrows, kinglets and thrushes, visit us from "up slope," where they breed in the coniferous forests of the Sierra Nevada.
But still, what you learned as a child is more or less accurate: many Northern hemisphere migrants do travel south for the winter.
A few species move in unpredictable patterns, present in large numbers one year and entirely absent the next, as they follow bumper crops of food. These movements, called "irruptions," are not technically "migrations" in the strictest sense because they do not occur annually, nor are they predictable. Irruptions are common among seed-eating northern finches, like Red Crossbills, and some owls, like the Snowy Owl, which are driven south only in years when food is scare in the north.
For others, "spring" and "fall" are arbitrary seasons: some of our western hummingbirds migrate north through the desert Southwest on their "spring" migration in late winter, and return south along mountain chains in late summer; shorebirds that breed far in the north start heading back south on their "fall" migration as early as June. Some form of migration, or movement of birds, takes place all year.
But the peak migration periods, the months that draw out expectant birders in droves, occur during the spring and fall. Exact timing of peak migration depends on location, but generally in North America, migration takes place during April and May, and again from August through October. In California, one of our greatest spots for catching migrants is Point Reyes, on the coast north of San Francisco. Here and elsewhere in Central California, spring migration peaks during April and early May, while fall migration is spread out from August through November.
One quick note: While songbirds on spring migration sport bright and fresh breeding plumage and the males are heard singing their full lyrical songs, those same species look and sound much different during the fall migration. Many species are in drab juvenile or first-year plumage, or they are molting, looking quite disheveled indeed. Songs are more often heard in parts and snatches, or vocalizations are reduced to simple call notes. Because of this, many birders consider identification of songbirds to be much more difficult in the fall than in the spring.
Plenty of birds are rather sedentary, rarely moving more than a few miles from their parents' range where they hatched. Some of these birds include familiar backyard birds, like Mourning Doves, California Scrub-Jays, and California Quail. Some species are even notorious for their homebody tendencies: the Wrentit seldom travels more than 1,300 feet from its birthplace.
At the opposite end of the spectrum, however, are the great migrants of the bird world, extreme long-distance migrants like the Arctic Tern (Sterna paradisaea).
The 4 ounce Arctic Tern claims the title as the world's longest-distance migrant, with some birds traveling around 50,000 miles or more, from pole to pole and back again, every year of their 20 to 30-year life span. (If that number seems high to you, researchers have recently discovered that the actual distance is close to twice that of the previously estimated 20,000-30,000 mile migration. Those that nest in Greenland travel the farthest.) This epic pole-to-pole flight stretches from their Arctic nesting grounds, located as far north as there is land, to the Weddell Sea and Antarctic pack ice in the south, having been reported from 84°N to 78°S. (Remember that the north and south poles are each at 90° North and South, respectively!) In this way, Arctic Terns chase the summer sun, timing their movements to enjoy the longest daylight hours at both ends of the earth.
Though not depicted in the illustration below, some Arctic Terns migrate south along the west coast of North and South America as well, turning up off the coast of Central California between late August and early October.
Highest flyer: Consistently flying at altitudes nearly 23,000 feet above sea level, the Bar-headed Goose (Anser indicus) of Asia migrates over the Himalayas, from breeding grounds in Mongolia and northern China to wintering grounds in India, able to fly with oxygen levels only a tenth of what would be present at sea level.
Fastest flyer: Surprisingly, the migrant that covers the greatest distance in the shortest amount of time, earning it the title of fastest flying bird over long distances, is the Great Snipe (Gallinago media) of Europe and Africa. This plump little bird has been recorded maintaining speeds of 60 miles per hour over the course of its entire 4,000 mile journey, without stopping for a single break.
Longest nonstop flight: But the snipe's long journey is nothing to the bird that wins the title for longest nonstop flight. The Bar-tailed Godwit (Limosa lapponica) flies nonstop from Alaska to New Zealand, a distance of nearly 7,000 miles, in nine days of continuous flight.
Honorable mentions: The avian world is awash with incredible feats of migratory prowess. Some other stand-outs in the field of migration include the Red Knot, which travels 19,000 miles annually, from the Canadian Arctic to Argentina and Chile, then back again, fueled by the eggs of the horseshoe crab; the Sooty Shearwater, which flies 40,000 miles on its figure eight-shaped migratory path, from New Zealand to Antarctica, north to Chile, then on up to feeding sites around the Pacific that include coastal California, Russia and Japan, then back south to New Zealand; and the tiny three inch Rufous Hummingbird, which travels 4,000 miles one way, from Alaska to Mexico, on one of the world's longest migration routes relative to body size.
So, how do birds manage these incredible migratory journeys? This question has baffled naturalists and scientists for generations, and we're just beginning to figure it out.
Aristotle believed that redstarts turned into robins during the winter, which I suppose makes some sense: redstarts migrate to sub-Saharan Africa for the winter, when northerly-breeding robins move south into Greece, so the two birds are never seen together. He also believed that swallows spent winter hibernating in large colonies buried in the soft mud beneath bodies of water, a belief that persisted into the 19th century.
During the Medieval Period, it was common knowledge and touted in medieval encyclopedia-equivalents that Barnacle Geese grew from trees extending over water: young geese that sprouted from the tree dangled by their bills, and those that fell into water survived, while those that fell on land did not.
In the 17th century, the English minister and scientist Charles Morton reasoned that birds migrate to the moon. The birds, after all, were nowhere to be found in the entire world, as far as he could tell, so where else could they go? And birds that landed seemingly out of nowhere descended straight down from above, from outer space it seemed, rather than flying in from a distant horizon as they should do if they had simply been somewhere else on our planet. Convincing, no? But he did get a few things right when he noted that changes in food availability and environmental conditions prompted these great migrations.
These days, we know a little more.
Over the last century or so, information on bird migration has been obtained through bird banding studies, where birds are captured, fitted with coded metal or plastic leg bands or rings, and released. When a banded bird is re-captured somewhere else, it can be identified by the band. Only about 1 in 300 banded birds are ever re-captured, but still, a large body of bird banding data has helped to sketch out the life history of many species.
Radio and satellite transmitters are more modern innovations which have been invaluable in tracking the flight paths of birds around the world. Radar technology is useful in recording migration, as it is accurate enough to record the speed, height, and wing beat frequency of birds flying overhead.
Songbird migration is such a major event, in fact, that flocks of birds in flight appear on weather radar. Today, bird migration can be forecast and tracked in real time through a program called BirdCast. Check it out for yourself!
Physiological adaptations in migratory birds allow them to sense changing day lengths, which triggers birds to begin migration. Even captive birds have been reported exhibiting migratory restlessness during the spring and fall. The need to migrate is designed into their very being.
Birds are able to choose, to an extent, when to begin their migratory journeys, selecting times with favorable wind conditions and even adjusting their flight altitude to maximize tailwinds and minimize headwinds. Additional aid comes from traveling in flocks, as air currents rise near birds' wingtips. Birds flying in V-formations capitalize on this extra bit of lift.
Diurnal migrants, like soaring raptors, also benefit from using the sun's energy in the form of thermal updrafts to help them on their way.
But many songbirds are nocturnal migrants, traveling almost exclusively after sundown. The reasons for this surprising behavior are varied. Perhaps cooler nighttime temperatures help to dissipate the large amount of heat generated by their bodies, or the calmer night air provides less resistance. By traveling in flocks at night, they may avoid diurnal predators, like raptors, and save daylight hours for critical refueling activities, like foraging and resting.
Another physiological adaptation in birds allows them to store large amounts of fat reserves to fuel their long and expensive migratory journeys. Burning fat is far more efficient than burning protein or carbohydrates, and it also produces metabolic water, which helps stave off the very real threat of dehydration during long, nonstop flights. Birds have an amazing ability to pack on fat reserves in preparation for migration, with some long-distant migrant species doubling their weight before their journey. Even tiny little Ruby-throated Hummingbirds pack on 2 grams of fat to their 3-gram bodies before their 500-mile, 20 hour nonstop flight across the Gulf of Mexico.
But how do they find their way???
Some species of birds, like cranes and geese, learn migratory routes from their flock, as young birds migrate for the first time with their parents and family group. Other birds, however, seem to find their way by instinct. In many species, the parents set out for the overwintering grounds before their young are able to make the journey. Miraculously, the young, operating by instinct, are able to find their own way without guidance.
Scientists have been studying bird migration in earnest for the last several decades, and have come to the conclusion that there is not one method by which all birds orient themselves and navigate, but rather a suite of systems.
To orient themselves in the right direction, birds use a sort of solar compass, or the position of the sun. But of course, this isn't useful at night, when most songbirds migrate. Most nocturnal migrants begin their journey each night about half an hour after sunset, just as the first stars are appearing but the glow of the sun is still visible below the horizon. When cloud cover is present, birds are able to see and use the angles of polarized sunlight coming through clouds as a guide. During nocturnal flights, birds orient themselves by the position of the stars and constellations. Earth's magnetic field is an additional compass that many long-distant migrants are apparently able to use.
To navigate along a migratory path, birds may be able to use familiar landmarks to find their way. In the absence of landmarks, such as over oceans or at night, some birds may be able to hear low-frequency sounds from great distances, like the pounding the of the waves along a shore, which would guide them along a coastline even through the darkest, cloudiest night. The idea that some birds may also use their sense of smell has not been highly supported or extensively studied, but is certainly a possibility.
The mechanisms birds use to find their way across thousands of miles are, to my mind, nothing short of miraculous.
Even so, every year, birds turn up thousands of miles off course in unexpected places. These "vagrants" may have been blown off course by a storm, or, perhaps more likely, their internal navigation systems may have just malfunctioned. No one knows for sure, but vagrants always cause quite the sensation in the local birding community!
This fall, keep an eye on the skies (day and night!) as you watch for migrating birds. With high-powered binoculars or a spotting scope, observers can watch birds migrating in front of the full moon. And on quiet nights, the call notes of passing flocks can be heard overhead.
Perhaps the best places for catching fall migrants in California are along the coast, at places like Point Pinos and Elkhorn Slough on the Monterey Bay, and Point Reyes, north of San Francisco. Closer to home (for me) are a number of Central Valley wetlands along the Pacific Flyway, protected as National Wildlife Refuges.
For more migration fun, watch as 118 species migrate across a map of the Western hemisphere.
Also be sure to check out eBird's Status and Trends Maps: type in a species in the search box, then watch as the animated map shows how its distribution changes through the year. This is without a doubt one of my favorite "toys" at the present!
And, one last resource: On eBird's Explore Species page, type in the name of a species, then on that species' page, find the Weekly Bar Chart section and enter your county. A bar chart will appear showing exactly which week of the year you can expect that bird to arrive in your home patch!
As I said before, Sandhill Cranes should be flying overhead very soon!
But what, exactly, is migration? Avian migration may be defined as the regular, seasonal, predictable movement of birds, which occurs annually as entire populations travel from their summer breeding habitat to wintering habitat, generally in the pursuit of warmer weather, longer daylight hours, and most importantly, food.
Seventy-five percent of North America's breeding birds exhibit a form of migratory behavior, moving to some extent between summer breeding habitat and overwintering habitat. Many follow the traditional pattern in the Northern hemisphere of flying "south for the winter," and return north again for the summer, chasing suitable temperatures and food supplies.
But migration is much more complex than that.
Migrants may be divided into two categories, known as long-distance and short-distance migrants. Most of the western hemisphere's long-distance migrants travel between North American and Central or South America, splitting their time between two different continents. Short-distance migrants generally migrate within one continent.
An example of a type of short-distance migrant are altitudinal or elevational migrants, species that move "down slope" from mountainous regions when the breeding season ends, seeking the milder climates of lower elevations. In California's Great Central Valley, a number of our overwintering songbirds, like sparrows, kinglets and thrushes, visit us from "up slope," where they breed in the coniferous forests of the Sierra Nevada.
But still, what you learned as a child is more or less accurate: many Northern hemisphere migrants do travel south for the winter.
Bear in mind, however, that Central California is "south" for many species of Arctic breeders.I remember reading children's books as a kid and being confused: with their East Coast bias, authors wrote of bluebirds returning in the spring and geese leaving in the fall to fly south for the winter. But where I live in the West, bluebirds stick around all year, and geese migrate to California for the winter!
A few species move in unpredictable patterns, present in large numbers one year and entirely absent the next, as they follow bumper crops of food. These movements, called "irruptions," are not technically "migrations" in the strictest sense because they do not occur annually, nor are they predictable. Irruptions are common among seed-eating northern finches, like Red Crossbills, and some owls, like the Snowy Owl, which are driven south only in years when food is scare in the north.
For others, "spring" and "fall" are arbitrary seasons: some of our western hummingbirds migrate north through the desert Southwest on their "spring" migration in late winter, and return south along mountain chains in late summer; shorebirds that breed far in the north start heading back south on their "fall" migration as early as June. Some form of migration, or movement of birds, takes place all year.
But the peak migration periods, the months that draw out expectant birders in droves, occur during the spring and fall. Exact timing of peak migration depends on location, but generally in North America, migration takes place during April and May, and again from August through October. In California, one of our greatest spots for catching migrants is Point Reyes, on the coast north of San Francisco. Here and elsewhere in Central California, spring migration peaks during April and early May, while fall migration is spread out from August through November.
One quick note: While songbirds on spring migration sport bright and fresh breeding plumage and the males are heard singing their full lyrical songs, those same species look and sound much different during the fall migration. Many species are in drab juvenile or first-year plumage, or they are molting, looking quite disheveled indeed. Songs are more often heard in parts and snatches, or vocalizations are reduced to simple call notes. Because of this, many birders consider identification of songbirds to be much more difficult in the fall than in the spring.
 |
| Sandhill Cranes, one of the Central Valley's most notable fall arrivals, will be showing up any day now! |
Plenty of birds are rather sedentary, rarely moving more than a few miles from their parents' range where they hatched. Some of these birds include familiar backyard birds, like Mourning Doves, California Scrub-Jays, and California Quail. Some species are even notorious for their homebody tendencies: the Wrentit seldom travels more than 1,300 feet from its birthplace.
At the opposite end of the spectrum, however, are the great migrants of the bird world, extreme long-distance migrants like the Arctic Tern (Sterna paradisaea).
The 4 ounce Arctic Tern claims the title as the world's longest-distance migrant, with some birds traveling around 50,000 miles or more, from pole to pole and back again, every year of their 20 to 30-year life span. (If that number seems high to you, researchers have recently discovered that the actual distance is close to twice that of the previously estimated 20,000-30,000 mile migration. Those that nest in Greenland travel the farthest.) This epic pole-to-pole flight stretches from their Arctic nesting grounds, located as far north as there is land, to the Weddell Sea and Antarctic pack ice in the south, having been reported from 84°N to 78°S. (Remember that the north and south poles are each at 90° North and South, respectively!) In this way, Arctic Terns chase the summer sun, timing their movements to enjoy the longest daylight hours at both ends of the earth.
Though not depicted in the illustration below, some Arctic Terns migrate south along the west coast of North and South America as well, turning up off the coast of Central California between late August and early October.
 |
| "Simplified figure showing migration patterns of the Arctic Tern, from the breeding sites in Greenland and Iceland to the winter grounds at Antarctica. After initiating the southbound migration (yellow line) the birds paused their migration in the central part of the North Atlantic (small circle) for almost a month before they continue towards the wintering sites at Antarctica (large circle). In spring, the northbound migration (white line) is conducted more than twice as fast in a gigantic "S" shaped pattern through the Atlantic Ocean." Map courtesy of Greenland Institute of Natural Resources Accessed via arctictern.info |
Let's take a look at a few more stand-outs in the world of bird migration.
Fastest flyer: Surprisingly, the migrant that covers the greatest distance in the shortest amount of time, earning it the title of fastest flying bird over long distances, is the Great Snipe (Gallinago media) of Europe and Africa. This plump little bird has been recorded maintaining speeds of 60 miles per hour over the course of its entire 4,000 mile journey, without stopping for a single break.
Longest nonstop flight: But the snipe's long journey is nothing to the bird that wins the title for longest nonstop flight. The Bar-tailed Godwit (Limosa lapponica) flies nonstop from Alaska to New Zealand, a distance of nearly 7,000 miles, in nine days of continuous flight.
 |
| "Flight tracks of nine Bar-tailed Godwits on southward migration equipped with satellite transmitters. One female flew directly from Alaska to the non-breeding grounds in New Zealand on a 11,680 km non-stop journey across the Pacific Ocean, which lasted over 8 days (Gil et al. 2009). © 2010 Nature Education Courtesy of US Geological Survey" Accessed via www.nature.com |
Honorable mentions: The avian world is awash with incredible feats of migratory prowess. Some other stand-outs in the field of migration include the Red Knot, which travels 19,000 miles annually, from the Canadian Arctic to Argentina and Chile, then back again, fueled by the eggs of the horseshoe crab; the Sooty Shearwater, which flies 40,000 miles on its figure eight-shaped migratory path, from New Zealand to Antarctica, north to Chile, then on up to feeding sites around the Pacific that include coastal California, Russia and Japan, then back south to New Zealand; and the tiny three inch Rufous Hummingbird, which travels 4,000 miles one way, from Alaska to Mexico, on one of the world's longest migration routes relative to body size.
 |
"Migratory paths of the Sooty Shearwater (Puffinus griseus).
Source: Shaffer et al. 2006, Proc. Natl. Acad. Sci. 103: 12799-12802."
Accessed via researchgate.net
|
But why do birds migrate?
To be sure, flying 7,000 miles nonstop, or at altitudes over four miles above sea level, are not without their risks. During migration, birds face a number of threats. Severe weather and storms can blow them off course or exhaust them, at best, and drown them in the ocean at worst. They are vulnerable to predation, exhaustion, collision with buildings, and more, and in many cases, don't survive migration.
Why then do birds put themselves at such risk to migrate?
Life exists as a series of trade-offs. To have any one thing, another must be given up. For birds, which expend a considerable amount of energy claiming and defending a territory with suitable nesting sites and food resources, it seems quite the disadvantage to move out of the neighborhood come the end of the breeding season. Surrendering their home turf, these birds risk their lives in a perilous journey fraught with danger - twice - only to return to their former breeding area to start the hard work of establishing a territory all over again.
In this way, birds that stay put and weather the harsh climate and food shortages of winter are at an advantage in that they remain on or near their breeding territory year-round, and may begin nesting earlier in the season. These homebodies save themselves the considerable trouble and expense of annual travel, but of course this advantage is not without its trade-offs. During the winter, food is often scarce and frigid temperatures can be dangerous. The birds who remain on their northern breeding territories through the winter must be prepared to adapt their diets to changing resources, and be equipped to handle cold weather.
Warblers that breed in the north, for example, lose access to insect prey once cold weather arrives. For these small, almost exclusively insectivorous birds, it is worth the risk to migrate to the insect-rich tropics for the winter. In order to avoid costly and dangerous migrations, birds that exist on a diet of insects in the summer must be able to switch to a diet of winter fruits, overwintering insect eggs, and other available food.
So then, why don't our neotropical migrants, like tanagers, warblers and vireos, stay to breed on their tropical wintering grounds? Here, competition probably plays a large role. The number of species of birds (as well as mammals, reptiles, amphibians, and most other life forms) are much higher in the tropics than anywhere else, so while resources are abundant, space is also at a premium and predators are numerous. Birds that risk the journey north also are able to take advantage of longer daylight hours farther from the equator.
 |
| A Yellow Warbler, one of our neotropical migrants |
How do they do it??
So, how do birds manage these incredible migratory journeys? This question has baffled naturalists and scientists for generations, and we're just beginning to figure it out.
Aristotle believed that redstarts turned into robins during the winter, which I suppose makes some sense: redstarts migrate to sub-Saharan Africa for the winter, when northerly-breeding robins move south into Greece, so the two birds are never seen together. He also believed that swallows spent winter hibernating in large colonies buried in the soft mud beneath bodies of water, a belief that persisted into the 19th century.
During the Medieval Period, it was common knowledge and touted in medieval encyclopedia-equivalents that Barnacle Geese grew from trees extending over water: young geese that sprouted from the tree dangled by their bills, and those that fell into water survived, while those that fell on land did not.
In the 17th century, the English minister and scientist Charles Morton reasoned that birds migrate to the moon. The birds, after all, were nowhere to be found in the entire world, as far as he could tell, so where else could they go? And birds that landed seemingly out of nowhere descended straight down from above, from outer space it seemed, rather than flying in from a distant horizon as they should do if they had simply been somewhere else on our planet. Convincing, no? But he did get a few things right when he noted that changes in food availability and environmental conditions prompted these great migrations.
These days, we know a little more.
Over the last century or so, information on bird migration has been obtained through bird banding studies, where birds are captured, fitted with coded metal or plastic leg bands or rings, and released. When a banded bird is re-captured somewhere else, it can be identified by the band. Only about 1 in 300 banded birds are ever re-captured, but still, a large body of bird banding data has helped to sketch out the life history of many species.
Radio and satellite transmitters are more modern innovations which have been invaluable in tracking the flight paths of birds around the world. Radar technology is useful in recording migration, as it is accurate enough to record the speed, height, and wing beat frequency of birds flying overhead.
Songbird migration is such a major event, in fact, that flocks of birds in flight appear on weather radar. Today, bird migration can be forecast and tracked in real time through a program called BirdCast. Check it out for yourself!
 |
| Get your own local migration forecast by following this link: birdcast.info/migration-tools/local-migration-alerts/ |
Physiological adaptations in migratory birds allow them to sense changing day lengths, which triggers birds to begin migration. Even captive birds have been reported exhibiting migratory restlessness during the spring and fall. The need to migrate is designed into their very being.
Birds are able to choose, to an extent, when to begin their migratory journeys, selecting times with favorable wind conditions and even adjusting their flight altitude to maximize tailwinds and minimize headwinds. Additional aid comes from traveling in flocks, as air currents rise near birds' wingtips. Birds flying in V-formations capitalize on this extra bit of lift.
Diurnal migrants, like soaring raptors, also benefit from using the sun's energy in the form of thermal updrafts to help them on their way.
But many songbirds are nocturnal migrants, traveling almost exclusively after sundown. The reasons for this surprising behavior are varied. Perhaps cooler nighttime temperatures help to dissipate the large amount of heat generated by their bodies, or the calmer night air provides less resistance. By traveling in flocks at night, they may avoid diurnal predators, like raptors, and save daylight hours for critical refueling activities, like foraging and resting.
Another physiological adaptation in birds allows them to store large amounts of fat reserves to fuel their long and expensive migratory journeys. Burning fat is far more efficient than burning protein or carbohydrates, and it also produces metabolic water, which helps stave off the very real threat of dehydration during long, nonstop flights. Birds have an amazing ability to pack on fat reserves in preparation for migration, with some long-distant migrant species doubling their weight before their journey. Even tiny little Ruby-throated Hummingbirds pack on 2 grams of fat to their 3-gram bodies before their 500-mile, 20 hour nonstop flight across the Gulf of Mexico.
But how do they find their way???
Some species of birds, like cranes and geese, learn migratory routes from their flock, as young birds migrate for the first time with their parents and family group. Other birds, however, seem to find their way by instinct. In many species, the parents set out for the overwintering grounds before their young are able to make the journey. Miraculously, the young, operating by instinct, are able to find their own way without guidance.
Scientists have been studying bird migration in earnest for the last several decades, and have come to the conclusion that there is not one method by which all birds orient themselves and navigate, but rather a suite of systems.
To orient themselves in the right direction, birds use a sort of solar compass, or the position of the sun. But of course, this isn't useful at night, when most songbirds migrate. Most nocturnal migrants begin their journey each night about half an hour after sunset, just as the first stars are appearing but the glow of the sun is still visible below the horizon. When cloud cover is present, birds are able to see and use the angles of polarized sunlight coming through clouds as a guide. During nocturnal flights, birds orient themselves by the position of the stars and constellations. Earth's magnetic field is an additional compass that many long-distant migrants are apparently able to use.
To navigate along a migratory path, birds may be able to use familiar landmarks to find their way. In the absence of landmarks, such as over oceans or at night, some birds may be able to hear low-frequency sounds from great distances, like the pounding the of the waves along a shore, which would guide them along a coastline even through the darkest, cloudiest night. The idea that some birds may also use their sense of smell has not been highly supported or extensively studied, but is certainly a possibility.
The mechanisms birds use to find their way across thousands of miles are, to my mind, nothing short of miraculous.
Even so, every year, birds turn up thousands of miles off course in unexpected places. These "vagrants" may have been blown off course by a storm, or, perhaps more likely, their internal navigation systems may have just malfunctioned. No one knows for sure, but vagrants always cause quite the sensation in the local birding community!
 |
| Not a vagrant, just a lovely little Hermit Thrush, which I photographed last September shortly after its arrival in the Valley. |
This fall, keep an eye on the skies (day and night!) as you watch for migrating birds. With high-powered binoculars or a spotting scope, observers can watch birds migrating in front of the full moon. And on quiet nights, the call notes of passing flocks can be heard overhead.
Perhaps the best places for catching fall migrants in California are along the coast, at places like Point Pinos and Elkhorn Slough on the Monterey Bay, and Point Reyes, north of San Francisco. Closer to home (for me) are a number of Central Valley wetlands along the Pacific Flyway, protected as National Wildlife Refuges.
For more migration fun, watch as 118 species migrate across a map of the Western hemisphere.
Also be sure to check out eBird's Status and Trends Maps: type in a species in the search box, then watch as the animated map shows how its distribution changes through the year. This is without a doubt one of my favorite "toys" at the present!
And, one last resource: On eBird's Explore Species page, type in the name of a species, then on that species' page, find the Weekly Bar Chart section and enter your county. A bar chart will appear showing exactly which week of the year you can expect that bird to arrive in your home patch!
As I said before, Sandhill Cranes should be flying overhead very soon!
Thank you so much for this article. So informative. I really appreciate it!
ReplyDeleteYou are very welcome! Thanks for reading!
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