Like their close relative the western kingbird, Cassin's kingbirds are noisy, aggressive flycatchers with characteristic calls -- a buzzy chi-bew or breet that is very different from the pip pip pip pip chatter of the western kingbird (one image shows a bird in mid-call).  Aside from their voices, the two species differ in subtle details.  Cassin's has a shorter bill, a slightly more contrasty white malar stripe against the gray face, and its tail is uniformly dark brown instead of the western kingbird's black tail narrowly bordered with white.   I took these photos at the Hidden Valley Wildlife Area in Riverside, California, and the nearby San Jacinto Wildlife Area (for the photographically inclined, the image in the center is noteworthy because it required the use of both 1.4X and 2X teleconverters -- not a recommended procedure but it can work).