Vocalizations are probably a primary means of communication especially in individuals and breeding pairs that restrict their activities to relatively dense woodland vegetation where visual contact is limited (1, 169). Vocalizations could conceivably have a reduced role in urban birds where sightlines are less diminished, but this suggestion is not apparent in urban versus rural Wisconsin breeders (RNR). Breeding females in Tucson, Arizona, were less likely to give whaaa calls (possible food-begging calls, see below) at city nests perhaps because of lower food stress compared with rural sites that have smaller food reserves (153). Silent much of the year, but fairly vocal during breeding season (39, 101). Individually color-marked males utter kik calls (see below) when apparently alone on their breeding territories in Wisconsin in autumn and winter months in Wisconsin (1, RNR). Most comprehensive description of vocalizations included 42 different calls by females, 22 by males, and 14 by young (39). Larger repertoire of calls by females attributed to greater need to convey more information; females, the larger sex, may control male-female interactions (169). Male's voice generally higher than female's, also slightly faster and not as harsh, but certain calls lower than hers (170). Pitch said to differ with age in breeding birds of both sexes; in females nesting as yearlings, for example, calls resemble those of older females but higher in pitch (39). Although no spectrographic analyses done, no age-difference in frequency noted in adults in Wisconsin (RNR, JB).
Young may give weak cak-cak-cak or ki-ki-ki adult-like alarm calls when about 3 weeks of age (RNR, JB).
The cak-cak-cak, or “alarm” call, is probably the best known—heard most often (Figure 7b, c; variously spelled as kak-kak-kak or cac-cac-cac). Typically given when nest threatened (24, 39, 171). Duration 2–5 s (169). When alarmed, often given many times in sequence by either sex on wing and/or when perched in vicinity of nest. During courtship, often prefixed by or interspersed with one or more kik notes in females, especially at dawn when both sexes begin daily mating activities (169). At this time, the cak-cak-cak call may convey presence and also signal aggressive intent to reassert female dominance on a daily basis. May be given while perched, when at the nest while female is nest building, or while in flight (169). Call may indicate excitement and/or irritation by females (39). Males said to give this call during dawn courtship periods (39), but only one of 50 males studied did so during such period (RNR, JB).
The kik is a male's most frequent call. Used during nighthawk-like, courtship flight display (see Behavior), to announce his presence and location to mate near (about 100 m) the nest, either upon perching after returning with food for mate and/or young, or on daily basis at dawn during courtship (39, 169). At both times, females is probably unable to see male's arrival, especially in more forested areas where lines of sight are diminished. Males also give kik when at nest during nest-building (169), which construction typically occurs (but not always) in the presence of a female (1). Kik call also used during a postural bowing display (see Figure 8), which may convey readiness to nest-build and simultaneously to inhibit aggression and signal submissiveness (1, 172). Alternatively, bowing may be ritualized prey plucking by males as bowing resembles the motion of their plucking of prey presented to females (A. C. Stewart, personal communication). Females also give kik calls, but less so than males. Call may be given when female tries to locate male (39). During pre-incubation, females give kik notes mostly at dawn when in flight toward nest, while collecting twigs for nests, and while perched near males when latter nest-building (169). This call may signal presence and location to mates (169).
The whaaa (Figure 7a; variously spelled as whaa or wa) call is primarily given by females (24, 39, 1, 173), when flying toward males for food, during food exchanges between mates, when plucking prey delivered by males, and while flying back to nestlings with food, but not while feeding young at the nest (39, 169). Before incubation, given in clusters of 1–4 notes, almost exclusively by females, when perched near males while latter nest-building, when in flight to or from nest, from perches preparatory to collecting twigs, but less so when loafing near nest while males away hunting (169). May be a nonaggressive or submissive signal of female's willingness to suspend potential for aggression and join in breeding, thus reassuring her smaller mate that it is not dangerous to be near her (169). May also function as a female food-begging call to social mate (sensu 174, 169), or food-begging to nonresident, extra-pair males who might offer food in trades for copulations when social mate hunting away from nest during pre-incubation period (1).
Multi-noted, abbreviated (2–6 s) whaaa calls given by both sexes during copulation, always by females, but not so for males (figured in 169).
Cheep or chirrp is first call of young, given when eggs are pipped (39, RNR, JB). Chip by young at least 1 week old is seemingly given when irritated as by direct sunlight; not given when young are ≥ 3 weeks of age (39). Eeeeeeee, eeeeeeee-oo, or speeeeeeeeoo given when young have fledged and apparently food-begging (39, RNR, JB). Fledged young in Florida gave weee-uur; a high-pitched, squeal whistle weee-tear; and rapidly-repeated cucks, which supposedly indicated increasing level of excitement (175).
Daily Pattern of Vocalizing
Little information. Before incubation, females more vocal in presence of mates, especially in early morning, but marked variation in frequency of calls, especially females, among 10 marked pairs; age and previous breeding experience might influence the timing and frequency of vocalizations (169).