Some warbler species call between song phrases, notably Blackburnian and Black-throated Green Warbler (and others in the virens group). These calls are typically heard when the more aggressive territorial song is being sung. What's interesting about these calls is that in many, perhaps most, cases, the calls are the same as the juvenile begging calls of the species. In other words, an adult male is using a vocalization from early life when defending territory. The calls seem to be acting as "filler" between phrases, perhaps accentuating the vocal impact of the male.
Black-throated Green Warbler song (July, ME)
Black-throated Green Warbler fledglings (July, ME)
The normal adult chip is much lower (Oct, NJ)
Blackburnian Warbler singing at dawn (July, ME)
Blackburnian Warbler fledglings (July, ME)