Frightful's Mountain, Chapter 5 evidence. Why did Frightful migrate?

Text Evidence:

When a light snow covered the Schoharie River Valley, Frightful’s bird sense urged her one last time to leave or die. (pg. 50)

On an unusually warm day in early November, she stepped to the open sill of the bell tower. She had caught no food for two days. She must move south. (pg. 55)