Passage: This little bird is native to Canada, Mexico, and the United States.Their breeding habitat is marshes with tall vegetation such as cattails across North America. In the western United States, some birds are permanent residents. Other birds migrate to marshes and salt marshes in the southern United States and Mexico. their non-breeding range is in the southern United States going into Mexico and their breeding range is in the northeastern United States going into Canada.These birds forage actively in vegetation, sometimes flying up to catch insects in flight. They mainly eat insects, also spiders and snails.

Question: If the birds are not permanent residents, where do they go?
southern United States and Mexico