6. Post-copulatory display. On June 25, 1960, after a second attempt at copulation with a stuffed bird in which semen was actually deposited on the dummy's back, male 10 (1960) performed a swaying display. In this instance, however, instead of addressing the dummy from the front, the male alighted one inch to the right of the stuffed bird. When swaying to the left (toward the dummy) the head of the displaying male actually passed above the neck of the stuffed bird. This ritualized behavior could conceivably be derived from hetero-preening.
Discussion
Within the scope of my research it was difficult to detect the over-all sequence of epigamic displays that result in synchronization of the physiological states of the sexes throughout the period of courtship. Possibly all displays, except the post-copulatory one, occur in no particular order in the courtship period. However, each ritualized display seemingly strengthens the pair-bond.
Swaying has been recorded in a variety of situations of a sexual and semi-sexual nature for the Solitary Vireo (V. solitarius; Townsend, 1920:158) and the Red-eyed Vireo (Tyler, 1912:230; Bent, 1950:342). In every instance the body feathers of the swaying birds were sleeked. Courtship behavior in any species of North American vireo seems closely to resemble that of any other; pairing and nestbuilding of a female V. solitarius and a male V. flavifrons as reported by Hauser (1959:383) support the idea of close resemblance.
A marked similarity will be detected between certain basic elements of aggressive and epigamic displays. These basic elements are wing- and tail-flicking, tail-fanning, and high-intensity delivery of the chee. Pouncing and supplanting attacks are essentially similar. Such similarities suggest either a common origin for certain aggressive and epigamic displays or the derivation of one from the other.
High-intensity cheeing is obviously a function of excitement, whether in conjunction with hostility or sexual behavior. According to Andrew (1956:179), flicking of wing and tail in passerines are intention movements of flight. These actions have been emancipated from incomplete take-offs and incorporated in ritualized courtship and agonistic behavior. In incipient courtship behavior the male is governed by three conflicting tendencies; to flee, to attack, or to behave sexually before his mate (Tinbergen and Hinde, 1958:256). When pairing, Bell Vireos interrupt sexual chase with "greeting ceremonies," the male's tendency to attack and the female's tendency to flee are momentarily reduced, and the forming bond is strengthened. Thus, the intention movements become an integral part of courtship.
In situations where attacking and fleeing are the two conflicting tendencies, wing-flicking and tail-flicking are incorporated into threat display, but do not lose all of their original function, for they facilitate attack. Tail-fanning, as a display element, increases the awesome aspect of the threatening bird and in courtship presumably makes the sexes more attractive to one another.
Courtship feeding has not been recorded for the Bell Vireo. In general, it is unknown in North American vireos, with the exception of the red-eye (Lawrence, 1953:53). It would serve no "practical" purpose in the Bell Vireo since the male regularly relieves the female during incubation, thus allowing her ample opportunity to forage. In the Red-eyed Vireo, only the female regularly incubates, and courtship feeding is definitely functional. Nolan (1960:228) described a brief pecking or pulling with their bills between pairing birds. This may be incipient "symbolic" courtship feeding, or perhaps mutual preening.