Described and discussed by MacRoberts and MacRoberts (47). A brief summary follows.
Waka. Given primarily as a greeting display by 1 or all members of a social unit landing near each other; also given during intergroup agonistic interactions and boundary disputes. Usually repeated several times in quick succession. The most characteristic display given by this species at all times of year.
Work by Yao (92) has shown that the Waka calls of birds in central coastal California are individually distinct but failed to find evidence of group signatures. Subsequent experimental work by Pardo et al. (93) has begun to elucidate the information derived by birds from their vocalizations. Using playbacks, these authors have demonstrated that birds were able to distinguish callers belonging to two different groups compared to callers from the same group, thus demonstrating “triadic awareness” under natural circumstances.
Karrit-cut. Usually given in association with conspecific or interspecific territorial defense or the detection of predators such as hawks and owls. Often repeated several times in succession and accompanied by a conspicuous bobbing motion of the head and body in synchrony with the call. Aseasonal. Develops ontogenetically from the juvenile Trtrtr call. Sometimes abbreviated to Karrit only.
Urrk. Given just before flying at a conspecific intruder or, less commonly, by adults when feeding nestlings or young fledglings. Grades into the Waka; a display sometimes starts as a Waka and becomes more and more Urrk-like with each repetition. Often accompanied by raised crest feathers, fluffed body feathers, bill-down posture, raised breast, and hopping forward. Aseasonal.
Garrick. Usually associated with presence of a conspecific intruder; may be given by both the resident or the intruder. Sometimes given by an individual when alone on the territory. Unlike the Urrk, it is not associated with the act of chasing. Given regularly by birds during contests for territorial vacancies or "power struggles" (see Behavior: Agonistic Behavior), often in association with nonstop Karrit-cut calls and drumming (94). Most common just prior to and during the spring.
Chatter. Given just before, during, or after an individual has attacked a subordinate group member, most often a juvenile or younger individual in association with begging for food. The bird being attacked gives the call. Most common during the summer following breeding.
Squee and Trtrtr. Given by older nestlings and juveniles, often in combination, while being fed by adults or while waiting to be fed inside nest cavities. At high intensity, accompanied by quick head-bobbing, rapid wing-flicking, and feather erection, especially on the head. Both calls remain in adult repertoire, but Squee given rarely, Trtrtr occasionally given by adults in a context similar to, and sometime apparently in place of, a Waka. Develops into the Karrit-cut display starting when juveniles are 1-2 mo old.
Tse and Rasp. Given by young nestlings; the Tse may be given almost continuously when nestlings are alone in the nest, whereas the Rasp is given when an adult lands at the nest entrance and enters the nest.
Alarm Call. Typically elicited by a predator, real or imagined, especially flying hawks. In response, birds freeze, quickly hitch to the underside of a limb and remain motionless, or dash for the nearest roost hole. Often elicits comparable responses in heterospecifics, including other birds and ground squirrels (Otospermophilus spp.). Structurally similar to, and grades into, the Karrit-cut call. Apparently develops from the juvenile Trtrtr and does not appear until about 2 months after fledging. Aseasonal.
Scream. Given in association with intraspecific grappling and life-threatening situations such as being handled by humans or captured by predators. Develops about 1 wk before fledging. At Hastings Reservation, approximately 60% of birds give distress screams when captured by humans (95). Tendency to give screams does not vary seasonally or according to age, sex, or status. Screams do not attract conspecific individuals, but occasionally attract secondary predators which may distract or dispute the original predator, thereby potentially allowing the caller to inadvertently escape (96, 95).