Development. Vocalizations of young very similar to those of Bay-winged Cowbird: nestlings peep at 4-8 kHz; calls of older nestlings comprise 2-9 similar elements which have wide frequency range, 2-9 kHz, with nasal tone; begging consists of loud elements, repeated (up to 9 times) like a fast nasal rattle (Fraga 1986). Young birds in first Pre-basic molt give recognizable versions of adult calls in peculiar, squeaky voice and likely equivalent to subsong (Fraga 1986).
Vocal array. Song, by male only, is dzeee pe tzeeee, used in courtship display (Friedmann 1929); or zilít-zchä (Sick 1985, 1993); or grrr-shhhleeee; the grrr is soft, low, audible at close range; the shhhleeee short, high, sharp and explosive; sometimes preceded by a couple of truuk or puk calls (Jaramillo and Burke 1999).
Call notes are a harsh, grating chuck (similar to a call of Bay-winged Cowbird, higher and harsher than Shiny Cowbird; actually similar to chuck of Red-winged Blackbird (Agelaius phoeniceus) but of higher pitch; Friedmann 1929); mostly given when flying. Another note is a hollow, internal, slightly metallic "plunk," similar to sound of 2 pieces of metal hitting each other; rarely given. Give a "harsh scream" when about to alight and also when about to start flying (Friedmann 1929).
No female "rattle" heard (Friedmann 1929).
"The hunger cry of the young M. [= Agelaioides] badius is short, somewhat strident, tremulous, and uninflected. [The]... resemblance of the young M. rufoaxillaris to its foster-brothers in language and plumage ... " is great and "quite different from that of M. bonariensis ... [which has a] shrill two-syllabled note, the last syllable being lengthened out into a continuous squeal ... " (Hudson 1874: 163, 1920: 105).
Geographic variation. No variation described.
Phenology. Little information on annual pattern.
Daily pattern. No information.
Places of vocalizing. No information.
Repertoire and delivery of songs. Little information. Paired female usually respond to male's song with screech in a simple duet (Fraga 1998).
Social context and presumed functions. No information.