Migratory birds fly to far away areas during a particular time of a year. Which of the following conditions present in their habitat during that time are responsible for this behaviour?
(i) Unavailability of food.
(ii) Extreme weather conditions.
(iii) Over crowding.
(iv) Lack of nesting areas.
Migration of birds occurs due to extreme weather conditions and unavailability of food in their area so to escape from these conditions they move from one place to another.