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Have you ever wondered why some birds fly south for the winter? Bird migration is a fascinating natural phenomenon that has intrigued humans for centuries. Each year, millions of birds undertake perilous journeys often across vast distances, facing numerous dangers along the way.
But what exactly triggers their migration? How do they find their way across continents and oceans? And what dangers do they encounter from human activity and climate change? This article takes a closer look at the fascinating intricacies of bird migration, uncovering the driving forces behind their migrations, the sophisticated timing and navigation methods they employ, the different types of migratory birds, and the impact of environmental and human factors on their chances of survival.
Physical & Biological Reasons for Bird Migration
Some birds travel thousands of miles during their migration, and it makes you wonder why they put in all that effort. Well, it’s down to several reasons, but the main one is food!
Birds migrate when food sources become scarce. During the winter, food isn’t as readily available in colder climates, so birds will head to areas where meals are more abundant. However, they don’t stay here year round because then the food sources in these tropical locations would dwindle owing to the presence of too many birds. So, when summer comes around, they head back to their original homes.
Another common reason for bird migration is for breeding purposes. Not all locations are suitable for raising young, so some species will head to places that are better for their brood. They’ll go where there is more food, more shelter, and where there are breeding colonies that can offer them protection; as they say, there’s strength in numbers!
In some cases, birds will head to warmer climates merely to escape the cold. Imagine having to spend the winter in below-freezing temperatures. It would be incredibly difficult for birds to survive these extremes so moving somewhere warmer literally helps them to survive.
Understanding How Birds Time their Migration
Birds are amazingly intelligent creatures so it’s not that much of a stretch of the imagination to think that they’d just know when it’s time to go. Of course, there are factors that affect this.
Knowing when to migrate has a lot to do with environmental factors, such as changes in the temperature. Birds sense this, and it signals that it’s almost time to leave. In research performed by NASA, it was noted that much of the North American bird migration was triggered by atmospheric changes known as Rossby waves.
The weather also has a lot to do with where the birds choose to stop over in both the winter and the spring.
In addition to this, changes to the length of the day and the local food resources can all be triggers for migration.
However, migration and breeding often go hand in hand so it’s many of the same factors that stimulate both. Changes to the metabolic rate, influenced by the bird’s thyroid gland are the biologically known effects that cause them to prepare for migration. This includes increasing their fat stores by eating more.
Exploration of Bird’s Migratory Types
When most of us think about bird migration, we imagine that all species ‘fly south for the winter.’ While there are species that do this, it certainly isn’t the only type of migration.
Latitudinal – when birds migrate latitudinally, they are simply moving from north to south. Many of us are familiar with this migration pattern which occurs mainly in northern species that are looking for warmer climates during winter.
Longitudinal – many European birds undertake longitudinal migration, which means they are moving from east to west and back again.
Altitudinal – birds that live at higher altitudes, such as mountain species like the gray wagtail, may migrate to lower regions if there is a lot of snow or particularly cold weather.
Seasonal – this type of migration takes place in line with the birds breeding habits and is probably the most well-known type, with birds arriving in spring and leaving again in fall.
Vagrant – vagrant birds will often appear well outside their normal range and this is often a result of them flying off course due to factors like extreme weather.
Irruptive – this is one of the more spontaneous types of migration where birds will suddenly move to a new region due to factors like food scarcity. The waxwing is one example of a bird that will behave like this, but they typically only do it every six or seven years.
Nomadic – birds such as the waxwing and black swan may be seen to do nomadic migration. This occurs when there is a lack of resources in one part of a region so the birds will move to another part of the same region. For this reason, it is possible that in some ranges, there may be a complete lack of a particular species at any one time. However, they will return when resources become plentiful again.
Drift – drift migration is not intentional but is caused by freak weather such as high winds or storms, which can cause birds to drift off course and end up in places where they wouldn’t usually be seen.
Molt – when birds molt their feathers at the beginning of the year, this can make them more vulnerable to predation. Therefore, they will migrate to somewhere safer for a short period.
Reverse – birds won’t often be seen reverse migrating in large numbers as this is usually accidental. It typically occurs in fall when young birds lose their way and begin going in the wrong direction along a migratory route.
Pre-Migration Rituals of Birds
Migration is no mean feat, and it’s not something birds can just do as and when they please. There’s a lot of preparation involved for these amazing animals to travel long distances.
Double their Body Weight (Hyperphagia)
Birds have a very hard journey ahead of them and this requires a lot of energy. By gaining body fat, this serves as fuel for their migration and so birds will go into a feeding frenzy which is known as hyperphagia.
Hummingbirds are an example of birds that behave in this way and they’ll lap up as much nectar as possible in the lead-up to their migration. In fact, during this time, it’s thought that this species will consume the equivalent of its own body weight in nectar every day; and that’s on top of nearly 2000 insects!
For hummingbirds, this accumulation of body fat gives them the energy to fly for almost 600 miles (966 km) without stopping!
The species of bird will depend on how much weight the individual puts on. However, many species, particularly water birds that migrate over very long distances, can double their body weight during this time of hyperphagia.
Shrink their Internal Organs
With all that additional body fat, birds need to do something to make space; they have the incredible ability to shrink their digestive organs just before they depart.
And after all, they won’t be using their digestive organs, as many species will fly long distances without stopping. But how do they do this? Well, it’s all down to their amazing ability to absorb body tissues. Some species, such as the bar-tailed godwit, can absorb up to 25% of the tissues from organs like the liver and kidneys.
What’s more, once the bird reaches its destination, it will gorge on food in order that their organs will grow back to their original size!
Bird Navigation Uncovered: How Do They Find Their Way?
Unlike humans, birds don’t have GPS, Google Maps, and other navigation tools. They have to rely on what nature provides for them, and it’s pretty amazing to think that they’re able to find their way across thousands of miles using these tactics.
It’s only been during the last century that humans have been able to better understand how birds navigate. What we have discovered is truly fascinating.
The earth gives off a huge magnetic field, right from its very core, and it is this that is one of the birds’ primary methods of navigation. But what’s really interesting is that they don’t just sense the gravitational pull of the magnetic field, it goes down to sensing at a molecular level.
Quantum effects in tiny molecular fragments are actually formed within the eyes of the birds which allows them to ‘see’ the earth’s magnetic field.
For many hundreds of years, human sailors would use the stars as a way of navigating and it was a pretty reliable method. So, it’s little wonder that birds use this cue from nature to find their way from A to B.
We can be almost certain that birds use the stars to navigate direction thanks to studies performed in the 40s. When stars within a planetarium were flipped, the direction of the birds mimicked this. But it isn’t only the stars that show them the way; during hours of daylight, the birds rely on the position of the sun.
As I have just touched upon, birds will use the sun as a way of helping them navigate. In the 1950s, experiments were performed to determine whether birds did actually use the sun as a navigation tool, and the results were conclusive. In a controlled environment, mirrors were used to alter the position of the sun, and in turn, the birds which were European starlings, also altered their patterns and direction.
The world is full of significant landmarks, and humans have long used these to determine where they are on the planet. Birds are no different and will use different landmarks to guide them on their journey.
This is most common in their home territories where birds will recognize a particular building, for example. From there, they will see a second point and know to fly in that direction, and so on until they reach their destination.
Awe-Inspiring Migrations: Birds that Travel Incredible Distances
Some birds only migrate very short distances but there are some species that travel thousands of miles. These long journeys can take days or even weeks to complete and in many cases, the bird will do it all in one fell swoop!
1. Bar-Tailed Godwit (Limosa lapponica)
I could not write a list of the most incredible bird migrations without including the bar-tailed godwit, the bird that holds the record for the longest non-stop flight. These birds will travel on a transequatorial flight from Alaska to New Zealand which covers more than 7,500 miles (12,070 km)! However, the longest individual recorded flight was more than 8,400 miles (13,518 km)!
The bar-tailed godwit spends the entire time in the air, not stopping for the duration of the journey. Some scientists suggest that it can take around a week, but when tracking individuals, the journey lasts around 11 days.
Of course, spending this long flying requires some serious energy. So, before they leave the northern hemisphere in late August, they will gorge on food to boost their fat reserves. They’ll also shrink their internal organs, and it helps that their bodies are designed to be aerodynamic.
2. Ruby-Throated Hummingbird (Archilochus colubris)
Hummingbirds are among some of the smallest avian creatures on the planet, but that doesn’t mean that they can’t go the distance. In fact, the ruby-throated hummingbird can fly as far as 1,200 miles (1,931 km) without a break!
These birds make their journey from the east coast of America down to warmer Central America between the end of August and the beginning of September. They will stay here until around May.
What’s amazing is that these birds, unlike many other species, migrate alone. Where you may see flocks of other species migrating together, the hummingbird takes this mammoth journey in solitude, beating its wings up to 50 times a second.
In order to give themselves the energy to fly, ruby-throated hummingbirds will up their food intake for up to four days prior to flying. What’s more, they’ll enter a state known as torpor where they lower their metabolism and body temperature to conserve energy.
3. Arctic Tern (Sterna paradisaea)
The Arctic tern holds the record for the longest animal migration on the planet. The birds travel an enormous 59,000 miles (94,951 km) every winter. They’re found on the Farne Islands, off the coast of Northumberland in the United Kingdom, and travel all the way to the other end of the earth, ending up in Antarctica.
What is fascinating is that these birds have a very different tactic to others to survive the flight. As opposed to putting on weight and not eating for the duration of the flight, they’ll continuously swoop down, taking fish out of the ocean; that brings a new definition to meals on the go!
The individual tern that holds the record for the longest migration, left the UK in July and traveled for months. It took a stop in South Africa and then spent time around the Indian Ocean until October, when it continued its flight to Antarctica, where it remained until the following spring.
4. Common Swift (Apus apus)
During the breeding season, the common swift can be found all over the northern hemisphere. However, when winter comes around, these birds will head south to places like South Africa, Mozambique, and Zimbabwe.
The swift’s migration is triggered by a lack of food when insect populations in places like the UK and Europe become sparse. They’ll usually only remain in these areas for a few months, beginning their annual migration around July. However, this may vary by region as swifts will usually leave once there are fewer than 17 hours of daylight. In more northerly areas, like Scandinavia, this happens much sooner.
The swift is a very incredible bird and is known to hold the record for staying in the air for the longest time. One individual was recorded to remain in flight for 10 months and on average, swifts will make a round trip of more than 60,000 miles (96,560 km), traveling up to 497 miles (800 km) in a single day!
5. Red Knot (Calidris canutus)
In the Arctic tundra, you will find thousands of red knots during the breeding season. However, their winter migration takes them further south, and they’ll travel up to 20,000 miles (32,186 km) from their breeding grounds. Unlike many other species, they will make this long journey twice a year, putting them at the top spot for the longest total migration.
Before making its journey, the red knot will fill up on food which could include spiders, crabs, bivalves, and larvae, among other things. It then uses these fat reserves for the flight which could take up to three days of continual flight.
6. Short-Tailed Shearwater (Puffinus tenuirostris)
The short-tailed shearwater is one of 37 species of shearwater, and this particular species spends the winter, between October and January, in New Zealand. During spring in the northern hemisphere, these birds are usually found along the east coast of North America, but some vagrants have been found as far north as Ireland.
Some of these short-tailed shearwaters will travel incredible distances from Alaska in the north down to Australia and New Zealand in the south. This covers a one-way distance of around 9,320 miles (15,000 km) which they complete in just four weeks!
7. American Golden-Plover (Pluvialis dominica)
If you thought that traveling long distances was something to be impressed by, then consider this; the American golden plover does this at lightning speed. These birds can travel up to 60 miles per hour (97 kph), allowing them to cover their migratory distance of 20,000 miles (32,186 km) much more quickly. There have even been reported cases of individuals flying up to 80 miles per hour (129 kph).
This mammoth journey takes the American golden plover from the Arctic tundra in the north of Canada down to South America in early summer. These birds travel in very large and noisy flocks, so they’re hard to miss.
While these birds can fly up to 3000 miles (4,828 km) without stopping, they do take regular breaks to refuel and rest.
8. Western Sandpiper (Calidris mauri)
Out of all of the shorebird species in North America, the western sandpiper is one of the most abundant. These birds are known for migrating in some of the largest flocks, and it’s thought that the entire breeding population travels through the Copper River Delta within a space of four weeks during the spring.
Western sandpipers will travel from North America, to the coasts of South America as well as parts of the Caribbean where they’ll spend their time on mudflats and beaches. What’s more, the first time these birds migrate, they’ll generally remain here for the summer, only returning to their breeding grounds in their second year of life.
9. Northern Wheatear (Oenanthe oenanthe)
The northern wheatear makes its annual migration from its breeding grounds in Alaska down to South Africa in late summer, typically around August. They can travel a whopping 9,072 miles (14,600 km), which is surprising considering they are such small birds.
They will remain in South Africa between September and February when they make their return journey, covering up to 180 miles (290 km) in a single day. The birds use magnetic cues to navigate their way on this epic journey, and they’re thought to share these abilities with the European robin.
While there may be birds that make longer migrations, the Northern wheatear has the longest migration of all songbirds.
10. Great Snipe (Gallinago media)
The great snipe, an endangered species of wading bird, breeds in the marshlands of the north eastern parts of Europe, particularly in Russia. However, they overwinter in Africa, covering distances of up to miles 4,349 miles (7000 km). Amazingly, these birds will take a non-stop flight, and they can complete the entire migration in anywhere between 60 and 90 hours!
The likely reason that the birds are able to cover such huge distances in such a short time is their speed. There is currently no other known animal that can move as quickly over as many miles as the great snipe. Even without tailwinds, these birds can travel at up to 60 miles per hour (97 kph).
They’ll plump up before their flight, ensuring that they have good fat reserves for energy. In a typical migration flight, they may burn up to half of their current body weight.
11. Sandhill Crane (Grus canadensis)
You’ll find sandhill cranes all over the world, as they are the most common species of crane. These birds head down to the southern United States and Mexico for winter, where they will spend their days around shallow waterways.
The sandhill crane does not make one continuous flight when migrating. Instead, it will take a break and in March of each year, you can see a spectacular gathering of up to half a million of these birds along the Platte river basin in Nebraska. This is up to 80% of the entire population in North America. They’ll rest here for between 4 and 6 weeks, feeding on agricultural crops before moving on with the rest of their journey.
12. Blackpoll Warbler (Dendroica striata)
The blackpoll warbler holds the record for the longest migration of any warbler species in North America. It makes non-stop transatlantic crossings that put many other species to shame, flapping their wings up to 20 times per second!
These birds make a latitudinal migration, spending the spring in their breeding grounds, until it travels as far as 1,800 miles (2,896 km) without stopping to its winter grounds in Central and South America. Despite being just 3 ounces (85 grams), these birds have serious stamina, flying for up to three days without stopping.
Sadly, there are a lot of risks to the blackpoll warbler during migration. With flights often taking place at night, collisions with lighthouses are common. While the birds are still abundant, it’s thought that, in recent years, their numbers may have dropped by as much as 88%.
13. Swainson’s Thrush (Catharus ustulatus)
Swainson’s thrush is common across the United States and Canada, but in winter, it will journey towards South America, such as Argentina and Venezuela.
During migration, these birds can be very noisy and use a series of peeps for communication which are also common at breeding time. Moreover, since they are very territorial, it’s not unusual to hear ‘sing offs’ between Swainson’s thrushes as they establish territory.
The birds will forage for wild berries and other foods to provide them with energy for their long journey. Moreover, they’ll molt their feathers early in the year to make way for fresh new ones that are much better for flight.
A Bird’s Eye View of Climate Change: Understanding Its Effects on Migration
Climate change has an impact on so many things on planet Earth, and bird migration is sadly no exception. It’s thought that, as a result of climate change, there have been physical changes to migratory birds, including longer wings and smaller bodies. During studies, it was demonstrated that directly after periods of warming, there was an almost immediate decrease in the size of migratory birds’ bodies.
On top of this, rising temperatures have altered the behavior of birds including at what time they start their migration. Over the last few decades, it’s thought that migration timings could have been altered by as much as 17 days. Generally speaking, migration is taking place much earlier than it did just two decades ago. Arriving at their destination too early could mean that the birds don’t have access to essential resources and may face more competition with native species.
If that wasn’t concerning enough, we have to consider the impact of melting sea ice. In many cases, this creates a barrier for birds during migration, but when this melts, studies have shown that the path migratory birds take could be affected. Where they may usually take a flight across the North Atlantic, these same species may now make trans arctic flights heading towards the Pacific.
How Human Actions Affect Migratory Birds
While there are concerns about the environmental impacts on bird migration, humans are to blame for making this natural process more challenging than it should be.
Humans have illuminated the planet to an astonishing level. Did you know that, during the Indian festival of Diwali, the country is so lit up that it can be seen as a brightly glowing patch from space?
This may be wonderful for humans, but for migrating birds, light pollution is a major problem. You see a lot of birds migrate during the hours of darkness, and any unnatural light can disorientate them and throw them off course.
When they lose themselves like this, it becomes more likely that they’ll collide with buildings and other structures. In fact, it’s thought that as many as 988 million birds become fatalities to this every year.
Many birds migrate to Latin America during winter, but because of deforestation in this part of the world, habitat loss is becoming a serious issue. But the problem can also be seen in the breeding grounds of migratory species which is leading to rapid declines in their numbers. And this isn’t just happening in North America.
Over in the UK, it has been reported that as many as 29 migratory species, that migrate to Africa, are in decline, while short-distance migratory birds seem to be thriving and even increasing in numbers.
Some species rely on a stopover partway through their migration but the habitat in these locations is now declining thanks to things like agriculture, property development, and mineral or gas mining, among other things.
Humans are constantly looking for ways to be more eco-friendly when sourcing energy, and there’s no better way to achieve this than through the use of wind turbines. While this method of harvesting energy does have its advantages, it could be having a very serious negative impact on bird migration.
Much of this is to do with habitat loss as a result of onshore wind farms. Each wind turbine requires up to 0.4 miles (700 m) around it, and this is all flight space that’s being paid for by birds.
What’s more, there are plenty of reports of birds colliding with wind turbines and while the numbers might not be as high as rumors would suggest, they’re worrying enough that something needs to be done about it. That’s exactly what scientists are doing. They are looking at the potential decrease in collisions as a result of painting wind turbines black since studies in Norway showed a significant decrease in collisions when just one blade was painted.
And it’s not just onshore wind farms that are causing problems. In the UK alone, there are more than 160 offshore wind farms, which are disrupting habitat and causing collisions for seabirds. This is largely because many of these species migrate under the cover of darkness and cannot see the wind turbines before it’s too late.
Did you know that as many as 1 billion birds die every year as a result of colliding with windows? And that’s just in the United States! Buildings that are illuminated at night pose even more of a risk for migratory birds, and it’s thought that a single building could be responsible for as many as ten birds every year. During migration, there are much higher numbers of birds in the skies and so the chances of a collision also rise. What’s more, younger birds, on their first migration are far less experienced and therefore much more likely to collide with glass.
The global bird trade is big business and as a result, thousands of migratory birds are targeted by hunters every year. In the United States, the passing of the Migratory Bird Treaty Act made it illegal to hunt or kill migratory birds. However, there are no laws like this in every country of the world. In fact, in some countries, like Malta, bird hunting is considered to be part of the culture.
According to surveys, as many as a third of all migratory shorebirds have been lost since the 1970s. Between Asia and Australia, as many as 32 species that use this migratory route are now considered to be globally threatened. However, on the Asian end of this route, many of these birds are hunted and sold for food.