Both Eastern and Western Kingbirds are common in Wyoming. The Eastern Kingbird is very dark, while the Western Kingbird has a yellow belly. In the photo below, the Western is on the left, while the Eastern sits on the right.
Kingbirds belong to the genus Tyrannus, probably because--in spite of their size--they are little tyrants who are extremely feisty and aggressive in chasing off intruders who stray into their territory.
When the Bald Eagle was chosen as America's national symbol, Benjamin Franklin was not happy. In a letter to his daughter he wrote "Besides he is a rank Coward: The little King Bird not bigger than a Sparrow attacks him boldly and drives him out of the District. He is therefore by no means a proper Emblem . . . ."
I saw an example of this recently north of Powell. While the images leave much to be desired, they do capture the little kingbird boldly attacking a golden eagle and driving him off.