Mechanical Sounds and Sexual Dimorphism in the Crested Doradito

Abstract

We observed the Crested Doradito (Tyrannidae: Pseudocolopteryx sclateri) in southeastern Bolivia and southern Paraguay. We describe aspects of natural history and morphology not previously reported, which include mechanical sounds, flight display, and microstructural modifications of flight feathers. Mechanical bill-snaps precede and are embedded in the song of P. sclateri. Introductory bill-snap notes may be functionally homologous to vocal notes in the songs of other Pseudocolopteryx species. Males differ from females in their head and bill coloration, intensity of breast coloration, and their shortened and pointed primaries 6 and 7. Males also exhibit microstructural modifications of the bases of primaries 6–9 in the form of open external vanes. Further research on P. sclateri and its congeners is needed to uncover the proximate causes, function, and evolution of some of the unusual tyrannid characters described herein.

Sonidos Mecánicos y Dimorfismo Sexual en Pseudocolopteryx sclateri

Resumen. Se observó Pseudocolopteryx sclateri en el sureste de Bolivia y en el sur del Paraguay. Se describen aspectos de historia natural y morfología no reportados previamente, incluyendo sonidos mecánicos, despliegue en vuelo y modificaciones microstructurales de las primarias. Golpeteos introductorios del pico preceden y forman parte del canto en P. sclateri. Golpeteos introductorios del pico en P. sclateri pueden ser funcionalmente homólogas a sonidos vocales en los cantos de otras especies de Pseudocolopteryx. Los machos difieren de las hembras en la coloración de su cabeza y pico, intensidad del color del pecho y sus primarias 6 y 7 acortadas y puntiagudas. Los machos también exiben modificaciones microstructurales de las bases de las primarias 6–9 en la forma de lámina externa abierta. Se requiere más investigación sobre el P. sclateri y sus congéneres para encontrar causas, función y evolución de algunas de las características aquí descriptas que son inusuales en Tyrannidae.

Pseudocolopteryx is a tyrannid genus of four species: P. sclateri, P. dinellianus, P. acutipennis, and P. flaviventris. All are small, bright-yellow-bellied, olive-backed flycatchers commonly found in grassy marshes, primarily in southern South America (Ridgely and Tudor 1994). With the exception of nest descriptions (Narosky and Salvador 1998), little has been published regarding their natural history (Hudson 1920, Johnson and Goodall 1967, Canevari et al. 1991, Sick 1993).

One previously known, interesting aspect of Pseudocolopteryx is that three of the four species, P. sclateri, P. acutipennis, and P. dinellianus, exhibit a peculiar sexual dimorphism in wing feathers (Hellmayr 1927, Meyer de Schauensee 1970). Specifically, the sixth and seventh primaries of males are miniaturized and attenuated, and are substantially shorter and narrower than adjacent primaries. This sort of feather modification is often associated with mechanical sounds in other species, but documentation of mechanical sounds in Pseudocolopteryx has been minimal. P. sclateri reportedly makes whirrs with the wings (Wetmore 1926, Sick 1993), and wing sounds have been recorded in P. acutipennis (B. Whitney, pers. comm.). Although vocalizations of all species are now known (Straneck 1990, Ridgely and Tudor 1994, Mayer 2000), our review of the literature uncovered no mention of other mechanical sounds in this genus.

We report observations of bill-snaps in P. sclateri as an additional form of mechanical sound production in the genus. We compare the song of P. sclateri with those of its congeners to show that bill-snaps exist in a temporal position in the song occupied by vocal notes in P. acutipennis and P. flaviventris. We also describe the song, a flight display, sexual dimorphism, and microstructural wing-feather modifications found in this species.

Methods

We observed Pseudocolopteryx sclateri at three locations: (1) Río Negro: Departamento Santa Cruz, Bolivia, on the east bank of the Río Negro, 8 km north of its confluence with the Río Paraguay (20°6′S, 58°9′W); (2) Yabebyry: Departamento Misiones, Paraguay, at Estancia Santa Ana, ca. 15 km northeast of Yabebyry (27°19′S, 56°56′W); and (3) Cerrito: Departamento Ñeembucú, Paraguay, at Isla Martín García in the Río Paraná, 2 km south of the Cerrito marina (27°20′S, 57°37′W). The Yabebyry and Cerrito sites are separated by less than 65 km and are referred to as the southern sites. They are ca. 825 km due south of the Río Negro site.

At Río Negro, we observed a pair for approximately one hour on 30 September 1999 in grassy savanna with scattered islands of palms. We used playback after the first 20 min to bring the male closer for recording. At Yabebyry, we found birds in vast, inundated sedge marshes, and at Cerrito they were in tall (ca. 2 m) grass on sand dunes. We recorded vocalizations using a Sony TCM 5000 recorder and a Sennheiser ME 80 microphone and deposited them at the Library of Natural Sounds (LNS), Cornell University, Ithaca, New York. We obtained recordings of P. acutipennis and P. flaviventris vocalizations from LNS. We analyzed, measured, and produced spectrograms of vocalizations using Canary 1.2.1 (Charif et al. 1995). Frequency and duration of songs are presented as averages ± SD. Eight specimens of P. sclateri were collected at the southern localities during 20–28 October 1999; four each were deposited at the University of Kansas Natural History Museum, Lawrence, and the Museo Nacional de Historia Natural de Paraguay, Asunción. Drawings of primaries were made using a camera lucida attachment on a Nikon SMZ-U stereomicroscope. Sexual dimorphism is described based on examination of the plumage, bare-part colors, and gonad conditions of the collected birds.

Results

Vocalizations and Mechanical Sounds

Songs at Río Negro were different from the southern sites. Songs from both areas incorporated bill-snaps. Bill-snaps were identified as such by the visual observation of the bill rapidly snapping closed in conjunction with production of sound. Analysis of eight songs by one individual revealed that the Río Negro song was composed of four notes and was often preceded by 2–10 bill-snaps (Fig. 1). The pace of the introductory snaps increased until the vocal part of the song began. The first vocal note of the song was brief (35 ± 3 ms) and even-toned with harmonics (5.6 ± 0.1 kHz fundamental, 11.3 ± 0.2 kHz harmonic). The second note was a brief bill-snap (∼6 ms) with a broad frequency distribution (∼1.2–7.6 kHz), and identical to those produced prior to the first vocal note. The third note was the same as the first (31 ± 2 ms, 5.5 ± 0.1 kHz fundamental, 11.1 ± 0.1 kHz first harmonic), and the last note was again brief (18 ± 2 ms), but different in frequency structure from the preceding notes, slurring upwards (6.7–7.9 kHz; Fig. 1). Intervals between notes were relatively consistent between songs (159 ± 8 ms between first and second, 202 ± 24 ms between second and third, and 136 ± 13 ms between third and fourth notes).

Songs at southern sites were similar, but somewhat abbreviated, being two-noted instead of four-noted and not preceded by bill-snaps. The two notes were indistinguishable from the first vocal note and bill-snap of the song described above (10 of 15 songs recorded from one individual). Southern birds used bill-snaps frequently and intermittently between songs, almost as if they were functioning as contact calls; one 3.5-min recording included 68 bill-snaps independent from songs. We did not observe females producing bill-snaps, nor did we hear or record any wing-produced sounds at any of the sites.

Males popped and jerked their heads back and forth as they sang. The act of bill-snapping also produced a visual display; the crest was raised while the head was thrown back and the lower mandible was dropped, creating a huge gape just before the bill was snapped shut. The bird's whole body appeared to be convulsing from a rhythmic series of powerful hiccups as it snapped and sang. We witnessed flight displays in both the northern and southern birds. The display was simple, consisting of a short (2–6 m) flight with light, butterfly-like wingbeats in which the body hung vertically suspended between the wings. One bird was observed to vocalize while performing the flight display, jerking and bobbing the head during note production.

All individuals collected in the south had enlarged gonads, indicating they, like other birds in the region, were breeding. During 21–23 October 1999, we observed a pair in the final stages of nest building. Except for strong response to playback, we found no evidence that the Río Negro birds were breeding. Observations at the Río Negro site were made nearly one month earlier than those in the south and the region was suffering a severe drought.

On 7 November 1999 we observed P. flaviventris vocalizing (Otamendi Reserve, Buenos Aires Province, Argentina; not recorded) and noticed introductory vocal notes similar in temporal placement to the introductory bill snaps in P. sclateri. Recordings from LNS showed that both P. flaviventris and P. acutipennis have numerous, brief, vocal notes preceding their songs (Fig. 1). We detected nothing that resembled bill-snaps in these recordings.

Sexual Dimorphism

The sixth and seventh primaries of males are substantially shorter and attenuated relative to adjacent primaries, primaries of females, and “normal” tyrannid primaries in general (Traylor and Fitzpatrick 1982). Contra Wetmore (1926), females' primaries show no obvious signs of modification, with the sixth and seventh primaries of nearly equal length and width to adjacent primaries (Fig. 2).

Unreported in the literature is microstructural modification of male primary feathers 6–9. Each of these feathers has modified barbs on the proximal end of the anterior vane (Fig. 2 inset). These modified barbs diverge from the rachis at a greater angle than more distal, unmodified barbs. As one moves from the base to the tip along the length of a modified barb there is an abrupt loss of barbules. Depending on the particular barb and feather, the loss of barbules occurs at 10–50% of the barb's length. These modified barbs create a distinctive looking anterior proximal vane that is closed pennaceous near the rachis, but becomes open pennaceous (corresponding to where barbules are lost) at its outer edge. The modified barbs end in free, spiky filaments, that side-by-side appear comb-like.

The species also exhibits subtle but distinct sexual dichromatism in plumage and skin (soft) parts, such that males appear more boldly contrasting overall than females. In males, the head is nearly solid black; black feathers extend from the base of the bill through the lores to the back of the auricular patch, and around the eye (somewhat more extensive than illustrated in Ridgely and Tudor 1994). The crest is also primarily black, with pale-yellow edges on the most medial feathers, forming a dispersed or weak yellow central stripe. The throat, breast, and belly are bright yellow, and the bill is solid black. In females, the head is much paler; facial feathers are washed with pale yellow, and form a distinct pale-yellow superciliary line. The crest feathers are brownish-black centrally, but broadly edged with pale yellow. Females are also paler yellow ventrally than males, and have a pinkish-orange mandible. Belton (1985) mentions pale mandibles in immature birds, but we found this in all females, at least two of which were reproducing.

Discussion

The use of mechanical bill sounds, upright display flights, microstructural modifications of the primaries, and sexual dichromatism have not been previously reported for Pseudocolopteryx sclateri. Further research is needed to determine if any of these features are shared with other members of the genus.

P. sclateri produces a bill-snap as an element of its song. Although many tyrannids and tyrannoids produce mechanical sounds, including bill-snaps, the placement of a bill-snap inside an otherwise vocal song is unprecedented. Sounds that are introductory vocal notes in two other species of Pseudocolopteryx are bill-snaps in P. sclateri. Behavioral homologies can be identified in much the same way as morphological ones; characters such as temporal placement in a series, function, anatomical structures used, and phylogenetic congruence can be used to create hypotheses of homology (Wenzel 1992). The similarity in quality and timing between the brief, sharp, broad-frequency, and repeated introductory notes given immediately preceding the full song suggests that the mechanical notes of P. sclateri are functionally homologous to the vocal notes of P. flaviventris and P. acutipennis.

Whether bill-snapping is shared among all isolated populations of this broadly distributed species remains to be determined. Verbal descriptions of song structure of P. sclateri from Trinidad indicate that these vocalizations are similar to those described herein and that some of the notes interpreted as vocal may in fact be bill-snaps (ffrench 1991, Andrews and Manolis in Ridgely and Tudor 1994).

Some authors (e.g., Hellmayr 1927, Meyer de Schauensee 1970) list primaries 4 and 5 as modified. This is merely an artifact of counting primaries proximally (outermost to innermost) rather than distally. The primary feathers of P. sclateri are modified beyond what has been described previously. Modification of barb and barbule morphology exists on male primaries 6–9. We believe the brevity or seasonal timing of our observations may have prevented us from witnessing how these modified feathers are used. Wetmore (1926:318) observed that “before alighting [P. sclateri] males at times produced a sudden whirr” and he attributed this sound to the modified primaries. Audio and video recordings of sound production will be needed to begin to understand the functional significance of the feather modifications.

Unlike all other members of the genus Pseudocolopteryx, we easily distinguished male and female P. sclateri in the field by their subtle but distinct dichromatism. Ridgely and Tudor (1994) mentioned the female may have a pale supercilium, and ffrench (1991) suspected that birds with duller-yellow underparts, browner faces, and distinct superciliary lines might be females. We verified with voucher specimens that males indeed have blacker heads, including an all-black mandible, and are brighter-yellow ventrally, as compared with females that have much more yellow in the facial feathers (forming the pale-yellow supercilium), pale lower mandibles, and ventrally are not as bright yellow.

Knowledge of the nearest relatives of Pseudocolopteryx would be of interest for assessing polarities of some of the unusual characteristics of P. sclateri. Unfortunately, relationships within the genus are unknown, as is the relationship of the genus to other tyrannids. Although historically considered similar to Tachuris rubrigastra (Wetmore 1926, Johnson and Goodall 1967, Canevari et al. 1991), Lanyon (1988) placed the genus in the Elaenia group, which does not include Tachuris, and he did not hypothesize a sister taxon.

With regard to the evolution of mechanical sound production, the only other member of the Elaenia group known to produce bill-snaps is Suiriri suiriri (Sick 1993), and no species in this group are reported to have modified primaries. Similarly, Tachuris and other genera (Euscarthmus, Polystictus, Anareites) potentially closely related to Pseudocolopteryx are not known to produce bill sounds or wing sounds, or to exhibit wing modifications. Mechanical sound production is therefore most likely an evolutionary novelty derived twice in the genus, once as wing sounds, and once as bill snaps.

Sexual dimorphism, display flights, and two forms of mechanical sound production suggest that P. sclateri, and possibly its congeners, may have an unusual social system promoting the evolution of some of these characteristics, which are otherwise uncommon in tyrannids. Closer examination of all Pseudocolopteryx species for behavioral and phylogenetic clues to the evolution of displays, wing sounds, and bill sounds is well warranted.

We thank the Museo Nacional de Historia Natural del Paraguay, the University of Kansas Natural History Museum, and Burroughs Audubon Society for funding and logistical support. Additionally, we thank the Library of Natural Sounds at Cornell University's Laboratory of Ornithology, and Bret Whitney in particular, for providing recordings of Pseudocolopteryx species. We are also grateful to Mario Cohn-Haft and an anonymous reviewer for improving the final manuscript.

Literature Cited

Author notes