I'll suggest an answer to my own question about where the recent Cassin's Kingbirds are coming from. I suggest this is a northward movement, similar to the "reverse" migration of Tropical Kingbirds in California, and now one from CO. Looking at eBird patterns for Wyoming and Nebraska to the north, and CO counties that are north of or adjacent to the main CO breeding range of Cassin's (e.g., Jefferon, Boulder, Larimer, Weld, Adams), there is an increased frequency of occurrence in September that is not a continuation of a late summer presence but seems to result from migrants moving during September (and into early October). Since there is not really a pool of birds to the north during September or late August, it seems to me most likely that the Cassin's we are finding over the last few weeks have moved north in a "reverse" pattern... maybe from afar away.
-- Maybe this pattern is already obvious to others or established? Thoughts?
David Suddjian
Ken Caryl Valley
Littleton, CO
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