When migration time was getting close what changes started happening that told the birds it was time to go south or west in the book Frightful Mountain?

In "Frightful Mountain," as migration time approached, the birds began to notice changes in the environment, such as the shortening of daylight and the shift in temperature. These seasonal cues triggered their instinctual behaviors, prompting them to gather in flocks and prepare for the long journey south or west. Additionally, the availability of food became scarcer, signaling that it was time to migrate. These natural signs collectively alerted the birds that it was time to leave their summer habitats.