In the following questions, you have a brief passage with 5 questions following passage. Read the passage carefully and choose the best answer to each question out of the four alternatives.

The bird life of the Tarai region is much the same as that found in the Malabar Coast and Assam, and includes horn-bills, barbets, fruit pigeons, bulbuls and woodpeckers. Along the cultivated areas and on the edge of the forests are found some of the more common birds of the Indian plains, and many wading birds and water fowls spend the winter among its streams and marshes. The cold season stimulates the migration of many species of birds from Tibet such as wild ducks and cranes, which breed in the mountains and descend to the plains for a brief change. The Central Region is described as a clusterous space of mountains varying in elevation from 4,000 to 10,000 feet with a range of temperature varying from ten to twelve degrees lower than the Tarai. It includes the Mahabharat range of mountains which rise to 8,000 feet to form a continuous barrier across Nepal from east to west. At intervals, this mountain wall is pierced by the gorges of the transverse rivers of the Seven Kosis, or the Seven Gandaks, or the noble river Karnali. Between the Mahabharat range and the main Himalayan chain, there are many populous valleys like Dumja, Kathmandu, Pokhara, and many others where the majority of the population is densely concentrated. The fauna of this central zone is characteristically Himalayan. Many of the species occurring in this region are peculiar to Nepal alone. Such animals as the ferret, badger, raccoon, crestless porcupine, etc., do not occur in the Indian peninsula. The whole genera of such birds as yuhina, siva, minla, ixulu, etc., are nearly, if not wholly, restricted to this region. The majority of reptiles occurring in this zone are purely Himalayan species.

What does the cold season stimulate ?