How Do Birds Know When to Migrate?

Migratory birds face a unique dilemma; reach early and they have to face the problem of limited resources, reach late and the resources have already dried up.

Migration is defined as regular seasonal movement of species between their breeding and wintering grounds. While quite a few animals are known to migrate, the phenomenon is more often readily associated with birds. That might have something to do with the fact that we often see flocks of birds - geese in particular - flying in elegant formations in the sky.

Basically, these migrating birds follow the same route - at least, the most of them - and go to the same place year after year. The flight path used by these birds is referred to as the 'flyway'. If you keep a track of the birds flocking by your region, you will notice that you get to see them every year at a specific time.

When we talk of migratory birds in the United States, a whole lot of neotropical birds will come in front of your eyes. These birds, which nest in North America, migrate all the way to their winter grounds in Latin America, and back every year.

An interesting case is that of the Arctic tern, which inhabits the Arctic region when it is summer in the northern hemisphere, and migrates all the way to Antarctica when it is summer in the southern hemisphere; that makes it two summers in a year for this species.

Migratory behavior is undoubtedly one of the most interesting facets of avian life, and yet, we are far from understanding it completely. How do these birds navigate and follow the same path year after year, without getting lost? What prompts them to venture far and wide? How do they even realize that the time to begin their journey has come?

When it comes to migration, numerous factors come into play. These include changes in climatic conditions, changes in daytime length, breeding season, food availability, and so on. The migratory behavior of a species is primarily guided by seasonal food scarcity.

In the northern hemisphere, birds spend summers at the breeding grounds in higher latitudes where food is available in plenty. As the food resources start depleting with the onset of fall, they flock south to their winter grounds.

Even breeding is said to play a crucial role in avian migration. When the conditions become unfavorable during the winter season, these birds seek refuge in new areas wherein the conditions are apt to raise their young ones.

While the climate - the low temperature to be precise - does have a role to play in the migratory behavior of birds, there do exist species which can withstand low temperatures as long as food is available in plenty.

Coming back to migration timing, all these factors do prompt the birds to migrate, but they are not the ones that trigger the migration, i. e. prompt them to begin their journey. This is made evident by the fact that these birds start their migratory preparations well in advance.

It's important that the migration triggers set in early, well before the species run out of food or the climate becomes unbearable, or else it will defeat the very purpose of their journey.

Like we said in the beginning, we are yet to understand bird migration completely; especially the trigger part, i. e. how the birds are able to understand that it's the time to begin their journey.

In case of long-distance migration, where in the species in question migrate from one part of the world to another, the timing is primarily based on the changes in the day's length, which is by far the most reliable sign of changing seasons. As with other animals, even birds are sensitive to daylight, and use the same to set their biological clock.

As the days start getting shorter, a bird's brain starts releasing hormones like prolactin and corticosterone. The hormonal changes bring about a drastic change in their physiology and behavior. They start overeating (as a result of which they become fat), molting, and become restless.

All these changes are connected to the migratory journey that they would embark on. These changes are even seen in the birds in captivity. As the time of migration approaches, migratory restlessness or 'Zugunruhe', wherein the bird tries to take flight in the same direction in which it would if it were in the wild, becomes evident in captive birds.

The day length factor plays a vital role when it comes to species inhabiting higher latitudes, wherein the length of the day differs with the season. In tropical areas, the length of the day is more or less the same throughout the year, and therefore, the species have to rely on additional factors, like rainfall, to determine the right time to migrate.

In case of short-distance migration, the timing is mainly based on the changes in weather and availability of food. The breeding factor seldom comes into play in this case.

As with short-distance migrants, even the altitude migrants inhabiting various mountains of the world rely on these very factors to begin their journey. These birds spend the summers at higher altitudes, and return to the base in the winters.

It's very important that the birds get their timing right, as leaving late would mean arriving late at the destination, which, in turn, would mean less food resources at their disposal and less time to establish their territory. More importantly, late arrival is also likely to hamper their chances of finding a good breeding site and potential mate.

Arriving too early is not an option either, as the species is more likely to encounter unfavorable conditions and limited resources upon their arrival.

Climate change is considered one of the major threats for migratory population, as it can affect the migratory pattern of these species. It can prompt birds to begin their migratory journey either too early or too late, and this deviation can put the entire migration pattern into jeopardy.

Warmer climate has also forced several species to shift to new wintering grounds, and this, in turn, has resulted in competition with the already existing species.

Long-distance migratory birds cover a lot of distance flying between their breeding ground and feeding ground. In the course of this journey, they even cross vast stretches of oceans with no land in sight. So how do they manage to find their way to their destination?

Basically, some of these species use the Sun and other celestial bodies as the guiding points in the course of their journey. (Those birds which resort to nocturnal migration resort to stars to find their way.)

It is also believed that the small crystals of magnetite located above the nostrils of these birds can help them detect the magnetic north and navigate their way to their destination. Other than these, the landscape and wind pattern are also known to guide the species as they begin the most crucial journey of their life.

The journey from one part of the world to another doesn't necessarily end on a good note for all the birds. Many birds die of starvation and exhaustion as they cross the vast stretches of water and land. While some birds fall prey to predatory birds hovering in the skies, others are killed by land predators at stopover sites.

Even natural disasters take a toll on the migrating population. As weird as it may seem, some of these birds die after colliding with each other.

An even bigger threat for these birds comes in form of human activities. Migratory birds are more threatened by hunting than the non-migratory species, as they fall prey to this all along their flyway.

Similarly, factors like loss and degradation of their natural habitat, both breeding and winter grounds, have also to be taken into consideration when we speak of the threats to these migratory species that grace the sky.