One of the extraordinary natural phenomena that disappeared as the size of America forests decreased was the occasional mass emigration of the gray squirrels. Caused by an excessive build-up of their populations, these movements involved millions of the animals - a half - billion were estimated to have migrated across southern Wisconsin in 1843. Moving in hordes, they devoured crops and even managed to struggle across such mighty rivers as the Ohio, thousands drowning in the effort. Currently, the squirrel population, although smaller, is on the increase, emigrations still occur but they are not often noticeable.
1. Grey squirrels usually emigrate because of _______ .
1. danger from hunters
2. an increase in the number of squirrels 3. disruption of river traffic
4. major weather changes

2. According to the passage, the squirrel emigrations have caused problems for people because of ______.
1. harm to children
2. an damage to home
3. loss of farm production
4. disruption of river traffic

3. What caused a great number of squirrel death during the emigrations.
1. The hunters
2. The Ohio River
3. Lacking of food
4. The weather changes​