What primary environmental event forces migratory populations from the Northern Tier south for the winter?

The freezing solid of all surface water for extended periods

The distribution of the Belted Kingfisher is heavily resource-dependent, with open water being a critical factor. Populations that breed in the northern reaches of their range, including New England and the northern plains, undertake migration when local conditions become unfavorable for hunting. The text specifies that when winter arrives and temperatures drop significantly, these northern waterways lock up with ice, causing all surface water to freeze solid for extended durations. Since the bird relies on access to liquid hunting grounds for its diet of small fish and invertebrates, the complete cessation of open water is the direct driver forcing these individuals to move south to where unfrozen water bodies remain accessible.