What are at least four reasons for birds to fly?
1) Bird migrations are an example of instinctive behavior.
2) The signal for the flight is a decrease in the length of daylight hours and a decrease in the amount of habitual food.
3) Some birds fly to their nesting sites.
4) The ice cover of the reservoirs prevents the free movement of waterfowl and the search for food.
5) Snow cover makes food inaccessible or inaccessible.
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