We describe vocalizations of Ruby-throated Hummingbirds (Archilochus colubris) recorded during agonistic confrontations at feeders. Calls were composed of 1–5 different note types. A sixth distinct note type, the W note, was not given within a call sequence. Similar to those of Black-chinned Hummingbirds (Archilochus alexandri), Ruby-throated Hummingbird calls were complex, and exhibited a nonrandom organizational pattern as analyzed using a Markov model. The two closely related yet allopatric species shared similarities in the acoustic structure of note types, syntax, and call length. Slight differences occurred in the opening note types of calls in the two species. We investigated how vocalizing is associated with the outcome of encounters at a feeder and found that the vocalizer usually was the winner.