We're starved for thrashers in Pittsburgh right now. Of the eight species in North America only one, the brown thrasher, occurs in the eastern U.S. and he's away on migration. All the rest are western or southwestern birds, several of which occur in California.
This one has "California" in his name. He doesn't migrate -- in fact he hardly moves away from his birthplace -- so if you want to see him you have to be in California or northern Mexico.
The California thrasher loves dense desert chapparel but is sometimes found in scrubby or suburban habitat where he encounters a bird whose habits are quite similar.
Northern mockingbirds eat the same food and forage in the same way as California thrashers. Both are highly territorial so when a mockingbird moves into a thrasher's territory constant warfare ensues.
Imagine the two contestants hopping and lunging.
Hey, Mr. Mockingbird, watch out for that beak!
Fortunately for northern mockingbirds, few of them like dense chaparral so these species are usually in separate places.
Good for the thrasher too. What a waste of energy to be constantly thrashing it out!
(photo by Alan Vernon from Wikimedia Commons. Click on the image to see its original)