Function of Crest Displays in Royal Flycatchers (Onychorhynchus)

Royal flycatchers (Onychorhynchus coronatus and 0. mexicanus) of the neotropics have large, brilliantly colored, fan-shaped crests that are usually concealed but can be erected perpendicularly to the axis of the head in both sexes. Hand-held birds spread their crests and slowly wave their heads from side to side with open bills (Bangs and Barbour 1922, Wetmore 1972, Dick and Mitchell 1979) in a manner convergent with the "snake display" of hole-nesting tits (Sibley 1955). Within the family Tyrannidae (375 + species), both the details of crest elaboration and the head-waving behavior of royal flycatchers appear to be unique. In this paper, I examine hypotheses concerning the adaptive function of crest displays in royal flycatchers and report new field observations, including the simultaneous display of a hand-held pair. The crest display of hand-held royal flycatchers was aptly described for a Panamanian bird (0. mexicanus) by Wetmore (1972, p. 499), "as I held the bird in my hand it spread the crest widely and threw it far forward, at the same time compressing the nape feathers laterally into a straight sharp edge down the back of the neck. The bird rotated the head from side to side in a swinging motion during which it turned through an arc of 1800. With this the bill was opened widely, and the tongue flicked...." On 1 July 1977, I mist-netted a male and female O. coronatus together in the forest understory on the alluvial plain of the Rio Heath, Department of Madre de Dios, Peru. When removed from the holding bag and held together facing one another, the birds began immediately to perform a synchronous but out-of-phase head-waving display (Fig. 1). After 15-20 sec, the birds were held closer together at a distance of a few centimeters. They locked bills and synchronously rotated their heads back and forth 1800. After several minutes, neither individual could be induced to perform. The display was not accompanied by vocal hissing, which is often produced by hole-nesting birds to discourage predators (Sibley 1955). Bill locking during courtship is unknown in tyrannids (Smith 1966), which suggests that it could have been a mutual attack. However, the birds did not peck at extended fingers or the camera lens. The crest displays of hand-held royal flycatchers have also been observed in mist-netted birds of both sexes in the Guyanas (Dick and Mitchell 1979), on the Rio Xingu in eastern Brazil (pers. observ.), and in northern Peru and southeastern Paraguay (M. Foster, pers. comm.). Geographic or sexual variation in the behavior, if any, and its ontogeny are unknown. Dick and Mitchell (1979) interpreted this stereotyped behavior as an instance of Batesian mimicry; potential predators are repelled by the snake-like appearance of the bird. Wetmore (1972) suggested that while the crest display may represent a threat function, it might be the result of a "nervous reaction" to being handled or presented otherwise in normal courtship display. Despite the extensive geographic ranges of royal flycatchers, published observations of crest displays in free-living birds are limited to Skutch's (1960) anecdotes of O. mexicanus in Costa Rica. He observed crest erection several times by preening birds of both sexes, by males during agonistic interspecific encounters, and by males displaying to females building nests or attending eggs. Skutch (1960, p. 525) noted one such display, "the [male's] crest was spread widely while he preened. In the middle of the morning he came with his mate when she returned from an excursion for finding food. Alighting on a vine near the nest, he spread his scarlet diadem to the full and turned his head from side to side so rapidly that the feathers quivered. At the same time, he shook his half-opened wings, fanned out his yellowish tail, and uttered a rapid series of peculiar notes, somewhat like the usual piping call but sharper. This superb display, the only one I have ever seen in many hours of watching Royal Flycatchers, lasted only a few seconds." Skutch (1960) concluded that the principal function of the crest was in courtship displays. Skutch's observation includes two of the principal elements of the crest display of hand-held birds: (1) crest erection; and (2) the side-to-side motion of the head. This suggests that the "snake-like" behavior of hand-held birds may be derived from courtship behavior. Concealed crown patches occur widely within the Tyrannidae, including genera closely related to Onychorhynchus (Traylor 1977). Crown feathers are raised most frequently during intraand interspecific agonistic encounters. Smith (1966) suggested that the exposed crown feathers of Tyrannus spp. mimic the open gape and convey a message of aggression by "doubling" the number of open mouths seen by the signal recipient. Although the elaborate crest of the royal flycatcher might provide a supernormal "gape mimic" during intraor interspecific interactions, most of which are aerial, there 'Received 19 July 1989. Final acceptance 15 December 1989.