Migration is described as a yearly seasonal movement of a population of animals, in this case birds, generally north to south, to and from breeding and wintering grounds.
Many are familiar with an overhead passing form of canada geese noisily flying over in their typical v formation. Seeing these large flocks form twice a year, coming back to the area from their wintering grounds then again in the fall and they gather together once again to head further south to avoid their food sources getting covered by ice and snow.
This natural phenomenon in birds has been recorded as early as 3,000 years ago with storks and swallows by ancient greek authors.
But before it was confirmed that birds actually migrated to a different location, there were some interesting beliefs about where birds actually disappeared too. Some believed they traveled to the moon and back. Another theory was some species like swallows hibernated by burying themselves in mud and clay at the bottom of rivers. Or maybe that redstarts just simply transform themselves into robins for the winter.
Thankfully due to curious minds, banding birds and other technological devices, we are learning more and more each year about the science behind this event.
Scientists have used several different methods to help unlock this mysterious avian journey. From banding individual birds, to attaching little lightweight devices like backpacks onto birds called geolocators.
About 1800 of the approximately 10000 species in the world are migratory, with North America about half of the breeding species are migratory.
It may seem like a lot of work for these species but it serves a vital purpose and is important for many reasons.
Two of the main reasons are for food and nesting locations.
The longer daylight in the north allows for seasonal increase in insects, plants and seeds and a longer time for successfully raising young.
But as it gets colder there are less insects, less fruit and other vegetation available, so to keep a steady supply of resources, many species need to move to an area that is sustainable for their population
While the cold is a motivational factor, in the end it really comes down to lack of food to sustain them more than freezing temperatures, for most species at least.
Migration can be broken down into a few different categories, short, medium and long scale.
Short scale migration can be as simple as higher or lower elevation along a single mountain. An example of this can be seen in the dusky grouse, whose short migrations consist of moving from its wintering area in the mountains down to the deciduous forests for summer, a change of altitude of about 1,000 feet.
A Medium scale can be moving a few states north or south depending on time or year. And long distance can move to completely different continents or hemispheres.
But the winner for farthest migration is the arctic tern. This small pale gray and white seabird with long wings and a forked tail, sports a black cap in its breeding plumage with bright red beak and legs. This bird is mainly only seen in its breeding grounds on the arctic tundra but will migrate from these sites all the way to antartica,a 49,700 mile journey that they partake in annually. And with an average lifespan of 30 years, an individual can expect to have migrated approximately 1.5 million miles, or 2.4 million kilometers during its lifetime. That’s equivalent to a trip from Earth to the moon and back over 3 times.
Although there are still many things that are a mystery about migration, it is believed to be triggered by a combination of different things like daylight, food, temperature and genetics.
Several factors can be used to allow for such an incredible navigational journey.
Some species are able to migrate thousands of miles for the first time all by themselves which has led to the understanding that migration routes are in a sense ‘programed’ in their dna. Species like the yellow billed cuckoo follow this genetically obtained route. Another interesting example of this can be found in the swainson’s thrush, or two of their subspecies to be exact. An experiment was performed that showed a coastal subspecies flew a route along the west coast to reach its wintering roost in Mexico while the inland subspecies flew through east-central North America to reach the same point. The interesting bit was that the hybrids of these two species actually took a route in the middle of these two established routes.
Although migration routes are traditionally led, where older individuals may lead allowing for the first year birds to learn the paths that need to be followed.
It is also believed that many species that migrate at night use the stars and moon to aid them. They also can sense the magnetic field of the earth, and along with the stars and moon, they can use these as a form of compass. Nocturnal migration can be beneficial for many species as it can help avoid predators, overheating and then have time to feed during the day.
In some species, like the homing pigeon, a sense of smell can also play a role in their navigation.
Others tend to follow specific pathways for their migration, waterfowl are a good example of this style of migration. This is generally used when their migration is timed with certain availability of food. These key stopover points are vital for these species to fuel up and get the energy and fat reserves to complete their long range migration.
A perfect example is the spring migration of some shorebirds is timed for when the horseshoe crabs lay their eggs, an easy plentiful source of food on their way north.
Some large congregations of birds can actually be seen leaving their roosts on some radars.
But this is also a deadly time for bird species. During their migration they can face a vast number of threats. Besides just the physical stress, possible shortage of food, predators and even weather, there are more human caused threats than can be counted.
Threats like power lines, wind farms, hitting windows and night lights causing disorientation. Since nocturnal migrants use the light from the moon and stars to find their way, lights from buildings can disrupt their paths.
There are many ways you can help keep migrating birds safe on their travels.
Since windows can appear to be open spaces due to their reflections, as many as 1 billion birds die a year from window collisions. There are several versions of uv stickers that can be applied that will make it more visible and help prevent strikes and fatalities.
And keeping porch lights off from dawn to dusk can help birds that migrate by night. These lights can cause disorientation and cause birds to hit windows, walls and buildings. Studies have found that by simply turning off an individual buildings lights can reduce bird deaths by up to 80%
Another great way to help these birds is to advocate for more bird friendly architecture guidelines and telling others these steps that anyone can take to help protect birds not only during their migration, but all year long. As bird lovers it is imperative that we try our best to reduce threats to our avian neighbors to help conserve and protect them now and the years to follow.