Detailed descriptions of song structure are critical to understanding the ontogeny, evolution, and function of bird song, particularly for species with large song repertoires. We provide a first detailed characterization of song organization and variability in a migratory population of House Wrens (Troglodytes aedon) breeding in western Canada, using a sample of 15,608 songs from 15 males. Males sang with high intensity in protracted bouts prior to pairing and often resumed high-intensity singing later in the breeding cycle to attract a second mate. The high-amplitude terminal portion of songs comprised rapid trills of frequency-modulated notes organized into discrete syllable types with a mean of 10 syllables and 4 syllable types per song. The population syllable repertoire was large (n = 27) and mostly shared but was used to produce much larger repertoires of song types, most of which were unique. Individual males sang up to 194 different song types, with no evidence of a ceiling. However, males sang most song types only rarely and, thus, had much smaller “effective” repertoires of ∼25 song types. Within singing bouts, males also repeated song types many times before switching and modified the syllable contents of successive songs gradually. Hence, they combined tremendous global song diversity with limited real-time song variability. This mix may reflect central neural or peripheral motor constraints on short-term song diversity or, perhaps, selection simultaneously favoring both diversity and consistency in song performance.