What environmental condition triggers the migration of Yellow-bellied Sapsuckers from their northern breeding grounds?

When the northern woods freeze up and the sap flow ceases entirely.

The migratory habits of the Yellow-bellied Sapsucker are closely tied to the availability of their primary food source, sap. These birds breed in the boreal forests of Canada and the northern US. When the environmental conditions in these northern regions transition, specifically when the ground and trees begin to freeze, the tree sap flow stops. This cessation of the sugary liquid, which is essential for their survival during colder periods, prompts them to travel south. They move to southern locales, such as the southeastern US, where they remain as winter residents until the spring thaw signals the return of sap flow northward.