The following account is based on Post (1981):
Primary Song (Growl). Both sexes sing throughout year, most commonly in close quarter encounters with conspecifics, usually near the nest, but also in feeding flocks and roosts, when predators are in vicinity of nest, when mated birds meet near their nest, and during interspecific agnostic encounters. Birds usually sing from elevated perches, but also from ground. The rate of singing varies from 1.2 to 2.3 songs/minute by birds perched near their nests. Singing is common in evening and early morning when birds are in communal roosts (12-17 songs/minute).
The song lasts 1-2 s. It is composed of a short introductory note and a buzzy trill. The click-like introduction extends beyond 16 kHz, and resembles a chwip or check call (below). The introductory figure is probably imperceptible to humans, as it is overlapped temporally by the more audible buzz component, which has a frequency range of 1.5-5.0 kHz, with energy concentrated at 4-5 kHz (Fig. 8). It is not known if song repertoire varies between individuals.
Both sexes utter growls during the song-spread and during wing-raise. With song-spread the growl is given as the wings reach their full height, the sound continuing as the wings are lowered. Song-answering occurs among birds in nesting areas: a song (growl) from one bird was usually followed within an unexpectedly short interval by one or more growls from other individuals.
Song development: Juveniles rapidly utter a short, muted growl accompanied by wing-raises. These may be examples of subsong (Thorpe 1961). In April, a juvenile sitting alone gave an abbreviated growl 55 times in 4 minutes while wing-raising. This may have been an instance of practice singing.
A flight series, usually uttered when birds are first air-borne, appears to have no unique components, but rather is a composite of different calls, such as cut-zee, pee-puu, and chwip (below).
Calls. The Yellow-shoulder has at least 10 distinct calls, given by both sexes throughout year. Most calls are related to intersexual communication; several related to predator mobbing.
Rasp. This call, rendered vvvt, is used during mobbing and aggressive interactions, is structurally similar to song, although its fundamental is lower, about 1.5 kHz, with discernable overtones at 3 and 4.5 Khz. To humans, the call sounds thinner and less resonant than a growl.
Cut-zee is a common alarm call, composed of two parts: a short introduction followed by a slightly longer, falling element. There is temporal overlap in the two figures. Both sexes utter cut-zee when mobbing predators around nest sites, and also throughout year, in feeding and roosting areas. Cut-zee is given with or without wing-raise and tail flip. Wing-trailing birds also give cut-zee.
Queea is composed of two figures, the fundamental of the first being about 3.5 kHz, with two discernable harmonies. The second note, which temporarily overlaps the first, falls slightly in pitch, and has its fundamental at about 5 kHz (Fig. 10). Queea is frequently given by birds mobbing predators. Average rate of calling was 18 calls/minute. Queea is given in association with other sounds such as check. Queea-type calls appeared to be used most often as alarms.
Chwip is simple in structure (Fig. 10). Young in the nest give chwip when they are 6-7 days old, so after begging calls and pink (below), it is one of the first vocalizations young Yellow-shoulders give.
Check is more structured than chwip. It shows a distinct harmonic, and the call lasts about twice as long as chwip. The energy of check is centered around 4 kHz, but the call extends over a wide frequency (Fig. 10). Check is given by birds mobbing predators, are engaged in agnostic encounters, or possibly are disturbed in some way.
Pee-puu is composed of two elements: the introductory pee is a clear, slightly rising note, while puu, also unslurred, falls in pitch. This vocalization, for which I have a spectrograph for only the pee component (Fig. 10), probably functions mainly in communication between paired individuals. I frequently heard it on nesting grounds, and pee-puus from females were usually answered by growls from their males.
Pink is a contact call between young and parents. The call was given by nestlings when they were 6 days old. Free-flying young gave pinks as they followed their parents about, either singly or as a double call. Adults leading their young in flight also gave the call. Rate of utterance by one just-fledged young was 122 in 3 minutes.
Scream. Fig Similar in structure to homologous vocalizations given by other blackbirds (Orians and Christman 1968). Given by hand-held birds or those captured in nets or traps.
Greeah. Given in nest vicinity by both adults, most often when young about to fledge. Also heard when birds mobbing. Associated with wing-trailing and moth flight.
A flight series is uttered when bird takes flight, most often when leaving nest. It is a composite of different calls, such as cut-zee, queaa, pee-puu, and chwip.