What environmental stress acts as the biological imperative forcing the buzzards to migrate south?

Answer

The Arctic freezing over, leading to a dramatic drop in food availability

The decision to leave the tundra is triggered by environmental stress; as the Arctic freezes and ground solidifies, the availability of small rodents and birds that sustain the buzzards declines sharply.

What environmental stress acts as the biological imperative forcing the buzzards to migrate south?
habitatbirddistributionbuzzard