Eastern Kingirds are one of the latest neotripical migrants to arrive in Washington State. I know that once they start to be seen in Eastern Washington that our spring migration is winding down.
Another cool bit of information about the Eastern Kingbird is that they are a very strong flier in comparison to the warblers, vireos, and many other migrants that fly directly across the Gulf of Mexico to the mainland of the U.S. in spring migration. Local birders know that when the migrants leave to the Carigean coast of Mexico in the evening, fly all night and into the day, that the fastest and stongest fliers, Eastern Kingbirds and Barn Swallows, arrive first. In the early afternoon, if the flight has been difficult, these two species arrive and land at the first sighted land. When I was visiting Dauphin Island, Alabama years age locals would watch a beach and parking lot area in early afternoon, and if Eastern Kingbirds and Barn Swallows started to land and feed eagerly that it was going to be a good afternoon for birding, maybe even a "fallout" where large numbers of birds "drop in" after the flight. If it was an easy flight many continue overhead to points farther inland, and it may be a slow day. The birders suspect this because of few to no Eastern Kingbirds at the indicator parking lot and beach.
Kingbirds are flycatchers, and feed usually by hunting from a perch and making short flights to catch larger flying insects. They are very vocal, and often use prominent perches, so are pretty easy to see.
Good birding.