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In some areas of the world, having flocks of birds darkening the skies for a few days each year is a common occurrence. It is often a spectacle to see especially considering that the average bird’s brain is only the size of a nut.
It is even more amazing to know that not only are these avians flying at the same time, but they are flying towards the same location, at pretty much scheduled moments each year, like clockwork.
So why do birds leave their home and fly thousands of miles? How do they know where to go throughout the years? Why do some birds migrate while others opt to stay as residents the entire time?
This article aims to discuss in detail the phenomena known as migration, with the goal of unlocking its mysteries and understanding the logic of this majestic yearly occurrence.
What is Migration?
Migration, simply put, is the movement done by different species of birds from one location to another based on certain conditions such as the occurrence of winter in the animals’ natural habitat. This is done via a fixed location and direction, as well as during similar times of the year.
Different species of birds migrate to different locations and at varying distances. This can range from simply altitudinal migration to distances that can reach up to tens of thousands of meters, even across the world’s major oceans.
It has been reported that almost half of the world’s bird species practice migration. Studies have shown that the major reason for this activity is due to dwindling resources, especially during winter. To survive, these birds migrate to locations where food is more abundant and temperatures are better for breeding purposes.
While migration has been going on for thousands of years, there are also some inherent dangers for the birds during their long flights. Predation and accidents caused especially by man-made structures can occur. As a result, a majority of the migrating birds do not survive.
Historical Records of Migration
Bird migration is an event that has been witnessed since pretty much the documented history of man. In fact, native Micronesians and Polynesians have known about migration thousands of years ago. Migration has also been ingrained in Samoan tradition.
The Ancient Greeks are also known to be aware of bird migration, as proven in the works of writers such as Aristotle and Homer. Pliny the Elder, another renowned Greek writer, has observed the movement of birds, specifically cranes, from Scythia to the Nile.
In fact, migration has also been referenced in the Bible, primarily in the Books of Job and Jeremiah. Passages relating to bird migration have included species such as turtle doves, swifts, cranes, and hawks have been mentioned.
As for more modern times, bird migration has been observed and discussed in numerous publications such as Thomas Bewick’s A History of British Birds, published in 1797. It was also during this period that certain misconceptions about where birds go when they suddenly become scarce in a locale were pushed forward.
Why Birds Migrate to Other Places
Birds migrate because of food scarcity, as well as to increase the chances of producing larger clutches during the breeding season. This dramatically increases the chances of survival in a species, especially for those birds that live in locations where climate changes drastically throughout the year.
During winter, food sources such as fruits and insects drop. Competition between birds amidst dwindling food supplies will cause a drop in the population. Similarly, other seasonal changes such as fall or rainy season can also trigger migration to occur.
Research has shown that birds also want to take advantage of longer days to extend their breeding periods. This also allows birds to forage for more food that they can feed their young. In the absence of these conditions and as days become shorter in their natural habitat, bird species will then migrate to other locations where breeding and nurturing conditions are more desirable.
In some locations, endemic bird species do not migrate. These are considered as residents as these species can be found in the same location at any given point throughout the year. The reason for this is that their current location provides them with adequate resources and conditions for proper feeding and breeding.
Despite some decrease in resources such as food, it is not enough to trigger migration for residents. This holds true even if other bird species migrate and live in the same area during the same period. This shows further resilience and adaptability of residents compared to other bird species despite changing conditions on their home turf.
While it is said that over 40% of the world’s birds migrate, and 75% of birds in North America do migrate the rest are considered as residents. These include:
- Northern Cardinals
- Black-Capped Chickadees
- Turkey Vultures
- European Starlings
- Great Horned Owls
- Anna’s Hummingbirds
- Pileated Woodpeckers
- Tufted Titmice
It is also worth noting that it is not only birds that can fly practice migration. Flightless birds like the Adelie Penguin walk over 8,000 miles as they follow the sun across the frigid Antarctic continent. Some birds have even been recorded to walk over 10,000 miles, making them some of the longest migration paths on record.
Ostriches, on the other hand, are still debated by scientists on whether they migrate or not. While ostriches have been observed to move from one location to another, some experts state that the distance traveled between locations is too short to be deemed as “true” migration.
However, an interesting thing related to this is that the ostrich’s close relative, the emu, has been shown to travel thousands of miles and exhibit migratory behavior.
It is also important to state that birds in captivity like chickens and turkeys do not have migratory instincts. Whether or not their earlier ancestors practiced migration before domestication is unclear, though it has been observed that even wild turkeys do not migrate as well.
In summary, it is clear that birds migrate due to a number of reasons, including but not limited to finding a location where:
- Temperatures are warmer
- Days are longer
- Food and water are more abundant
- Breeding conditions are more optimal
Usually, birds will migrate if these conditions are not met in their current location at the current time, and they will only return once conditions are close to ideal once again.
Why Don’t Birds Hibernate?
Hibernation pertains to the process by which select animal species survive winter or other prolonged periods by undergoing a dormant or semi-dormant state, usually by going through long periods of sleep and minimization of resources required by the body to survive.
This practice is often done by mammals such as rodents as well as some species of bears and marsupials. During hibernation, animals often bury deep in the ground or go into caves to sleep for weeks to months at a time. Hibernation also entails lowering the body temperature and base metabolic rates during the period.
Many species of reptiles, considered to be the closest relatives to birds, are known to hibernate as well. Even amphibians like certain frog species, such as the wood frog, can survive by being frozen in a body of water during winter only to wake up and reanimate once they have thawed.
Early in human history, people believed that birds, primarily swallows, also hibernate. Many have postulated about the disappearance of the swallows during winter, only to return the following spring. These include sleeping underground, hibernating in or around bodies of water, or even change forms.
It was only until the very late 1700s that these myths were dispelled, and that migration was universally accepted as the reason for these yearly disappearances.
Interestingly, there is a species of bird that hibernates. This is the common poorwill, a species of North American nightjar, which can be found in parts of the United States, Canada, and Mexico.
This species, rather than hibernating, can lower their body temperature and metabolic rate throughout the winter to minimize energy consumption. Common poorwills also hide under piles of rocks and can hibernate for weeks or months at a time.
Hummingbirds and doves are also known to be less active during those times, although they are not dormant but merely lethargic or enter a state of torpor, unlike real hibernation.
The most accepted reason as to why birds migrate and do not hibernate is that birds are more equipped to travel longer distances due to their wings and relatively lighter frames. Obviously, bears and even some rodents are much larger than birds and would have limited range. Thus, hibernation would be a better option for them.
Another reason is that birds migrate not just because of scarcity of food, but also because they prefer warmer weather above all else. As for other animals that hibernate, they expect decreased sources of food during the winter months.
Millennia’s worth of evolution has provided certain animals with the ability to enter a lethargic state, reduce energy consumption, and be able to live off any fats that they have stored in their bodies during prolonged periods. Birds, on the other hand, have adapted through other means, namely migration.
Preparation for Migration
Once migration season comes in, bird species experience a number of changes, both inside and out. These are necessary for their preparation for the long trek towards their new home for the coming months.
The first thing that occurs is that birds will often molt and grow new feathers. Much like vehicle owners having their cars maintained and changing oil and other components in preparation for a cross-country trip, birds also ensure that their feathers are in tip-top shape for their long trek ahead.
Another major change that occurs is that their appetite will ramp up. This will allow birds to store more fat as a lot of energy will be utilized during migration. Some birds might even increase their weight to up to two times their usual size, which can pose an increased risk of predation as well as cause more energy expenditure due to more power needed to support their weight in flight.
Some bird species change their diet as well. From eating foods rich in protein, like insects, birds like warblers will feast on fruits and nectar instead. The reason for this is that these foods can more easily be converted into fat, which is then stored in the body for future energy consumption. Protein, on the other hand, like carbohydrates is related more to short-term energy use.
The hormones that are produced within birds’ bodies during this period can also cause them to become restless, especially at night. Caged birds that have been known to migrate may bang onto a certain portion of their cell, the location of which pertains to the general direction of where its flock will go.
Why Birds Migrate at Night
Most birds migrate at night. There are various reasons for this, but the main factor, scientists agree, is temperature. Nocturnal flight causes less overheating, which would help consume less energy and cause less strain on the birds.
Another reason as to why birds migrate during the night is that most bird species navigate using the stars. For diurnal migratory birds, they typically utilize landmarks like coastlines and mountain ranges in order to know where to go. Another theory is that there are fewer predators at large during the day, as well as overall less activity that would hinder their movements.
Nocturnal migration also provides the added advantage of having the birds able to reach stopovers in the morning, which would allow them to forage and refuel more efficiently. Oftentimes, they would stay for several days before moving on.
When flying at night, birds are threatened with colliding not only with man-made structures but with each other. Birds are able to circumvent this with a form of short calls to each other. These are different from the usual “songs” done while in their habitat and are mainly used for positioning and to allow strays to come back to the flock even in pitch-black conditions.
How Do Birds Navigate During Migration?
Birds navigate through a combination of different methods, some of which we are only beginning to understand with others some scientists are even unsure of. What we are sure of, however, is that birds’ navigation skills are some of the best in the animal kingdom and are even in some way or form genetically imprinted on the species themselves.
Birds have highly attuned senses of sight and smell that help them navigate. They can navigate using these senses, and they have a memory of where to go based on these visual and olfactory cues. The former is present in birds that migrate during the days as they are able to recognize mountains, rivers, and other landmarks.
Migration is also guided by celestial bodies such as the stars and the sun. This is especially impressive as this is the same way that sailors have been able to navigate the seas since ancient times. This practice might have been developed by observing how birds use stars for navigation purposes.
It has also been shown that birds utilize a form of magnetoreception, that is, being able to perceive the Earth’s magnetic field in order to guide them toward their intended destination.
There has also been some proof that navigation is somewhat imprinted genetically into the bird’s young as, even though their parents are not present, they are able to migrate and follow the rest of the flock. However, this might be a case of following the group rather than some innate desire to migrate.
Also, research has shown that the drive to migrate and plot out where to go is passed down from generation to generation. Those who have made previous trips can remember their flight path and it is passed down to the younger birds, allowing the practice to continue throughout hundreds to thousands of years.
Flyways are paths that birds follow during migration. These flyways can span thousands of miles and can go across different continents and the world’s largest oceans. These paths have been developed over time and are desired due to the abundance of food, stopovers, as well as desirable weather and wind patterns.
Flyways are not necessarily the shortest path from Point A to Point B. In some cases, some flyways also merge with one another. Experts have distinguished that there are eight major flyways, although some minor ones do exist.
These eight flyways are:
- The Pacific Americas Flyway
- Central Americas Flyway
- Atlantic Americas Flyway
- East Atlantic Flyway
- Mediterranean/Black Sea Flyway
- East Asia/East Africa Flyway
- Central Asia Flyway
- East Asia/Australasia Flyway
The Central Asia Flyway is the flyway with the smallest area, while the Pacific Flyway is considered as the longest. The most used flyway, on the other hand, is the East Asia/Australasia Flyway. With the Central Asia Flyway spanning over 30 countries, it can be seen that, even by our standards, these migratory paths require a lot of endurance and will require several weeks to complete.
Contrary to popular belief, not all migratory birds go to different locations that can reach thousands of miles away (also known as latitudinal migration). In fact, there are birds that move within the same spot, but only at different altitudes. This is known as altitudinal migration.
Altitudinal migration is common not only to birds, but some animals such as different species of goats and deer as well as Monarch butterflies also practice moving from lower to higher heights and vice versa.
When it comes to birds, there is one major reason as to why altitudinal migration is better than long-distance migration. This is because all the conditions that would ensure good living conditions are still available in the area, albeit at a different height.
A major reason as to why some birds go to higher altitudes is to lower the chances of predation, especially during the breeding season. Whether it be for the eggs that they hatch and nurse or for the bird parents themselves, lower risks of predation occur at higher altitudes especially if fewer potential predators can reach their areas.
Other triggers for altitudinal migration include more succulent fruits, nectar, or prey at higher areas, trees, or branches, triggers related to breeding, and slight changes in temperature, among others. It is not rare for altitudinal migration to coincide with when fruits are in season or when it is breeding season.
During altitudinal migration, birds can go to higher cliffs, or even simply to higher branches of larger trees. This can range from just a few feet to a couple of thousands of feet. Birds that practice altitudinal migration are typically found in areas with temperate or tropical climates.
Examples of birds that practice migration altitudinally include:
- Yellow-Eyed Juncos
- American Dippers
- American Robins
- White-Ruffed Manakins
- Resplendent Quetzals
- Hawaiian Geese
- Prairie Falcons
- Many species of birds of prey
- Many species of hummingbirds
Research has shown that those who migrate altitudinally have a far lower mortality rate as compared to those who travel latitudinally. This is due to the shorter distances traveled, fewer risks during the journey, and overall similar conditions to the ones that they are currently used to.
Conversely, migratory birds that will need to travel further distances will have to face numerous threats, and often a high number of fatalities, many of which will be discussed in the next section.
What are the Threats to Migration?
Migratory birds experience a lot of perils when migrating. Chief among them is climate change, whereas their destination as well as any stopovers during their flight has drastically changed and can no longer sustain the population. In some cases, 30 percent or higher of the total population are wiped out at the end of the trek.
Another threat to migration is bad weather, especially above open waters. Thunderstorms and heavy rains can disorient birds, causing them to be exhausted and eventually fall into the water and drown.
Other factors include death by poisoning from insecticides, especially during layovers, as well as collision with man-made structures. Colliding with buildings, trains, wind turbines, and oil platforms is pretty common. Contrary to popular belief, collision with airplanes is rare as planes and birds fly at different altitudes.
Hunting is another threat that migratory birds face. While there are agreements between countries in place that disallow hunting of migratory birds, not all species are protected and regulations against illegal hunting are sometimes not enforced as strongly.
Light pollution, in some cases, can also cause deaths to migrating birds. Excess light from buildings and other man-made structures can disrupt birds’ navigation capabilities. This holds especially true for birds that navigate by the stars.
Perhaps the biggest threat to migration is deforestation and urbanization. This results in habitats and stopovers slowly but surely being destroyed. This results in fewer living areas and a scarcity of resources for the birds to thrive. This in turn results in a higher mortality rate and lower clutches coming back with each coming year.
Changes in Migration Patterns
Migration patterns have changed drastically and have evolved drastically ever since the Ice Age. Climate change, specifically global warming, and other environmental changes have affected migratory patterns in many different ways.
For the past twenty-five years, experts have observed an almost two-week change as to when some birds started to migrate. As stopover locations and overwintering quarters have been destroyed to create space for urbanization, routes have also been altered.
Pollution is also a major contributing factor to the drop in the bird population and survivorship. Plastics can cause fish-eating birds to drown, and it can also cause digestive problems which would hinder their ability to feed.
These effects have resulted in ecological shifts as well as higher mortality rates for migratory birds. It has also resulted in irruptions where birds move beyond their normal range in order to find more adequate sources of food.
In some cases, vagrancy also occurs. Birds belonging to a flock may get lost and even join other bird species and flyways by mistake. This can cause the appearance of a bird species not endemic to the region.
The Human Impact of Bird Migration
While relatively minor in most cases, bird migration can impact humans as well. For one, tourism in the form of avid bird-watchers would increase in areas where birds would stop by or end their migration journey. These locations can serve as prime spots for ornithologists to be able to study and observe rare bird species.
Bird migration also affects agriculture. Crops can get destroyed during the feeding frenzy of birds, which is one of the reasons why farmers often use pesticides to avoid this from happening. On the other hand, birds also carry with them seeds that can help introduce existing and new species of plant life in an area.
Perhaps the most negative aspect of bird migration to the human race is its possible risk of spreading disease. Viruses such as avian flu, West Nile virus, and Marek’s disease can be spread to remote locations by these birds, affecting not only other local or fellow migratory birds but humans as well.
Birds’ migration behavior varies from species to species, and there are even individual birds that excel in performance far beyond their flock’s capabilities. This can result in some impressive and interesting feats done by these creatures:
Here are some interesting records related to bird migration:
- A bar-tailed godwit was recorded to have flown 8,435 miles non-stop. This covered the distance from Alaska to Australia without food or rest. This is a Guinness World Record for the longest non-stop flight for any bird.
- Arctic terns travel over 22,000 miles each year from their breeding grounds to their overwintering locations and back. This is the longest recorded migration path for birds.
- Swifts have the ability to fly non-stop for up to 10 months, research has found. During migration, they have been shown to fly up to 500 miles each day, making them one of the fastest migratory birds on record.
- The shortest migration known is by the North American Blue Grouse. These birds are altitudinal migrants and will only move about 1000 feet or 300 meters during migration.
- Red Knots usually fly 1500 miles at a time before stopping. These are timed to coincide with their feeding patterns where in they eat up to double their body weight before continuing their journey.
- The Bar-headed Goose migrates to one of the highest points on Earth. Crossing over to the Himalayas, they can reach up to 7,000 meters, or 23,000 feet, above sea level.
- Adelie penguins hold the record for the longest land-based migration. These penguins walk over 8,000 miles to complete their migration patterns.
Migration is a natural process within the animal kingdom, and birds primarily do this in order to ensure the survival of their species. Failure to adapt through migration may cause a drastic drop in their population and may lead to extinction.
Migration is also a wonderful sight to behold, especially for bird watchers. In some cultures, it is a sign of the changing seasons and of rebirth. For many, seeing huge flocks of birds in the sky is not only majestic but close to divine.
While experts have extensively studied the mechanics of bird migration, there is still a lot that needs to be learned. There have also been efforts to assist endangered species during their migration, as well as efforts to help them search for better grounds due to the destruction of their original overwintering grounds.
Like evolution, migration patterns have changed over the years, and our actions as human beings have also affected this fragile process. Man-made structures, deforestation, and the decrease of their natural habitat remain the greatest threats to migratory birds.
As such, we must also do our part to ensure that these creatures still have a place to go to as well as a home to return to, ensuring that the balance of the ecosystem is maintained and ensuring that the species will thrive well until the foreseeable future.