Chickadees are non-migratory birds, so you will see them at your feeders year-round. It is important to keep your feeders full during the winter as Chickadees eat 10 times the regular amount in the colder seasons!
The more "dee" notes at the end of a chickadee call indicate increasing levels of agitation. For instance, a chickadee may end their call with just one "dee" when a known person fills a favorite bird feeder. An owl roosting near the feeding station would warrant many more "dee" notes.