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Internal signals are being sensed by animals when it is time for them to migrate. The most usual cue that they make use is when their pituitary gland,the one which controls their sex characteristics,starts to release hormones that stimulates their craving for food. This makes them eat more than their usual appetite. Their observation of external changes like scarcity of food follows.Once they've gathered enough strength to consider them physically fit for a journey,they then start to migrate.
During journey,some animals just rely on natural forces to guide them into their destination.An example is a baby eel which just glides through in the water currents to lead them in rivers. Other animals' sense of direction are instinctive. They just try to memorize or recall the surroundings that they happen to pass by on their way to their destination. Once it's time for them to go back,they rely on these familiar places to guide them back home.Animals also make use of their senses like their sense of smell. They try to recall the odor of their homeplace on their way back home after going to their destination.
In almost all animals,they have the ability to know the length of time that passes. This knowledge is called a biological clock. Together with this ability,celestial bodies also guide them in going to a certain direction. An example is when an animal that travels at daytime uses the position of the sun in knowing where it is located and the direction that it must follow. This is the same in the case of animals which journey during nightime except that they make use of the stars in navigating.
However, some species of animals make use of the route they had learned to memorize while travelling. They rely on the knowledge of their elder members as they travel together in groups.Once they're old enough,they can then remember these familiar routes and just follow it in going to their destination and also on their way back to their homeplace.
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