Bird Migration - High-flying Globetrotters 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 pre-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.

How Do Birds Know When to Migrate? 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, wherein 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 drastically 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 altitudinal 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.

Why is the Timing Important? 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.

How Do Migrating Birds Find their Way? Long-distance migratory birds cover a lot of distance plying 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.