A specialized version of the congested song is associated with pre- and post-copulatory display but differs from the typical squeaky performance in terminating in two ascending notes reminiscent of the ascending phrase of the primary song.
3. Distress call. It was heard only once, when a captured bird was being freed from a net. When the bird was almost disentangled it uttered 10 high-pitched, plaintive notes. The quality of the notes suggested a relationship to the song phrase rather than to other types of vocalization. A nesting pair of Bell Vireos, 10 feet away, became extremely excited when they heard the distress notes. They "scolded" vigorously and flew around my head at a distance of six feet.
4. Alarm note. This is a specialized, three-note call of the male and was heard only from the onset of pair-formation through early nestbuilding. This whinnying, flickerlike call, phonetically eh-eH-EH, each succeeding note of which is louder than the one before, is given whenever the male is disturbed by an unfamiliar object. This call is generally succeeded by the chee, but occasionally blends into an extended "whinny," and is typically given from some perch affording an unobstructed view of the offending object. The male stretches his neck and cocks his head, the wings and tail are not flicked or fanned, and no feather tracts are erected. The bird, nevertheless, flits nervously from perch to perch when uttering the call.
5. The zip. The male has a special "scold" note of his own that is heard when an intruder first approaches the nest. Phonetically it is zip-zip-zip. It is not so loud as the chee, and the delivery is more deliberate than that note. If the intruder remains near the nest, the zip is usually replaced by the chee.
6. The generalized call note or chee. The call notes associated with several situations are combined under this subheading since all can be rendered in English by the same phonetic equivalent—chee. The chee associated with nestbuilding is of moderate pitch and delivered deliberately at a rate of about 40 per minute. The feeding call of the adults is a soft slurred chee, while that of the nestlings has a mewing quality. In general, the chee utilized in signal situations consists of a few repetitions of the basic note emitted at a moderate pitch. The chee associated with hostile and courtship behavior is higher pitched and the delivery is much more rapid, approximately 200 per minute. Nolan (1960:240) reports a continuous rate of 25 per five seconds when an adult Bell Vireo is alarmed. The chee of extreme anxiety is a loud emphatic buzz, phonetically ZZ-ZZ-ZZ-ZZ.
TERRITORIALITY
The Bell Vireo exhibits "classic" passerine territoriality. Within a specific area, a pair of this species carries out pair-formation, courtship activities, copulation, nesting, rearing the young, and foraging. With the cessation of reproductive activities, a pair continues to restrict its other daily activities to the same general area.