What triggers birds to start off on their journey?
Migrating in Autumn allows birds to move to a different location so that they will continue to be able to find food. In Spring they return to these places where they will breed and raise their young. It may be the slant of the sun's rays, hormonal changes, the change of the weather, unavailability of food or other factors that contribute to the birds' urge to migrate to their other home.

How can they travel such long distances?
The distances of Migration often cover a good portion of the globe, consequently the process of preparing for the journey is very important. Many small birds may even double their body weight before migration storing enough fat that many can fly non-stop for unbelievable lengths of time - sometimes up to 90 hours! These fat reserves are often created by a change of hormones at certain times of year. However, many species migrate over shorter distances, or migrate over routes where they can often land and refuel before continuing meaning that they are less dependant on adequate and abundant food prior to the trip and the need to store the large fat reserves.

However, many species have learnt that fuel-saving methods of flying (such as in V-formation) are equally as effective as storing large fuel reserves. Flying together in formation cuts down on the drag of air currents making it possible to glide more.

How do birds know which way to go and how to stay on course?
a) A number of studies have suggested that birds use the sun or stars as guides. Star Navigation is rather complex though as it constantly changes throughout the journey as new constellations appear on the horizon as the birds move north or south. However many birds can recalibrate their magnetic compasses against their star navigation during their rest stops along the migration route. Sometimes though, conditions may be overcast suggesting a more complex set of factors help them.
b) Birds may also use the geographic features of the land below, such as following rivers, shorelines, foothills and valleys as guides.
c) It is believed Birds also monitor the Earth's magnetic field, apparently with their visual system and with tiny grains of a mineral called magnetite in their heads.
d) There is also the "homing" instinct and studies where birds have been taken in closed boxes to remote locations and released have been done showing that the birds found their way back to their colonies.
e) Homing pigeons are thought to use smell to help navigate.
f) By following their neighbours, as many fly in large flocks.

Birds do get lost sometimes though being thrown off course by huge storms, fierce winds, or other weather conditions. Young birds, perhaps those who fledged late in the season and unable to migrate with the older birds, may become confused and stray. Sometimes, a mechanism which can lead to individuals turning up as vagrants thousands of kilometres out of range is known as Reverse-Migration, where the genetic programming of young birds fails to work properly.

Problems of Migration
The downside of migration is the hazards of the journey, especially when difficult habitats such as deserts and oceans must be crossed, and weather conditions may be adverse. Also, there is a high risk of predation for species Migrating during the day. The Eleonora's Falcon which breeds on Mediterranean islands has a very late breeding season, timed so that autumn migrants can be hunted to feed its young.

Routes
Some large broad-winged birds rely on thermal columns of rising hot air to enable them to soar. These include many birds of prey such as vultures, eagles and buzzards, but also storks. Migratory species in these groups have great difficulty crossing large bodies of water, since thermals can only form over land, and these birds cannot maintain active flight for long distances. The Mediterranean and other seas therefore present a major obstacle to soaring birds, which are forced to cross at the narrowest points. This means that massive numbers of large raptors and storks pass through areas such as Gibraltar, Falsterbo and the Bosphorus at migration times. Commoner species, such as the Honey Buzzard, can be counted in hundreds of thousands in Autumn. Other barriers, such as mountain ranges, can also cause funnelling, particularly of large diurnal migrants. In spring, the males of many species of songbirds precede the females, presumably because it is to a male's advantage to arrive in its breeding area early and stake out its territory before its rivals do. There seems to be a premium on getting to nesting areas as soon as possible. In fact, many kinds of waterfowl begin move northward as soon as the lakes and ponds are released from the grip of ice. But at the same time there may be a heavy price to pay if the birds arrive too early -- weather is less dependable in early spring.