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The Pair-Formation Displays of the Western Grebe

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The Pair-Formation Displays of the Western Grebe THE CONDOR JOURNAL OF THE COOPER ORNITHOLOGICAL SOCIETY Volume 84 Number 4 November 1982 Condor &X135&369 0 The CooperOmithological Society1982 THE PAIR-FORMATION DISPLAYS OF THE WESTERN GREBE GARY L. NUECHTERLEIN AND ROBERT W. STORER ABSTRACT. -In this paper we describeand illustrate the pair-formation displays of the Western Grebe (Aechmophorus occidentalis),examining their structural variation, social contexts, and probable evolutionary origins and functions. Most displaysare performed mutually by two or more individuals and occur in elaborate and predictable sequencesor “ceremonies.” One of these, the “Rushing Cere- mony” is performed by either two males, a male and a female, or several males and a female. The “Weed Ceremony,” on the other hand, occurs later in the pairing sequenceand always involves a male and a female. Finally, the “Greeting Ceremony,” used by pairs coming together after being separated, appears to be an abbreviation of the above two ceremonieswith the energetic, core displays left out. We examine the temporal and spatial coordination between individuals involved in each of these ceremonies. Most displays and ceremonies of Western Grebes differ greatly from those described for other grebe species, which supports morphological evidence for retaining the Western Grebe in a separate genus. The similarity of the Weed Ceremony to that of some speciesof Podicepssupports morphological evidence for consideringthe Western Grebe more closely related to that genusthan to any other. The spectacular courtship displays of the description of a typical performance is fol- Western Grebe (Aechmophorusoccidentalis) lowed by comments on variability from one have aroused considerable attention, yet, sur- performance to the next, the degreeand nature prisingly, no detailed description of them has of coordination and orientation among dis- been published. The only available summary playing individuals, and its social contexts. for the species(Palmer 1962: lOO-102) though Probable functions, evolutionary design, based largely on excellent information pro- origins,and comparisonswith displaysof other vided by R. W. Nero (1959), is much con- grebesare then discussed. densed and includes many inaccuracies from the earlier literature. DISPLAY TERMINOLOGY Western Grebes are gregarious at all times A display is usually defined as a behavioral of the year and usually nest in colonies. On the pattern or posture that is ritualized or adapted breedinggrounds, however, unpaired birds are for signal function (Moynihan 1955). There- highly aggressiveand malesfrequently usetheir fore, there is a basic functional dichotomy long, pointed bill to stabother individuals from between display behavior and other daily below the water surface (“torpedoing”). Two activities such as feeding, locomotion, and courting males often perform elaborate mutual comfort movements. This definition, how- displays in front of nearby females. Many ever, does not specify how far behavior in a observers have misinterpreted these displays, display should be combined or subdivided. In mistaking such rival males for courting male- this paper we define a “display” as the largest female dyads (but see Nero 1959). unit of ritualized behavior that, in all contexts, The aim of this paper is to name, describe, is typically reproduced intact from one per- and illustrate displaysused by Western Grebes formance to the next (barring outside interfer- during pair-formation. Under each display the ence). The beginning and end of each display, [3511 Rushing by a pair of Western Grebes. The male is on the left. 352 G. L. NUECHTERLEIN AND R. W. STORER therefore, are determined by natural breaks and squirting them with dye from a pressur- occurring within longer chains of courtship ized tree-sprayer. Black (Nyazol) or yellow activity. For example, if components A and B (picric acid) dyes were used. Other birds were of an observed sequence of ritualized move- captured in funnel traps set in channelsleading ments (ABCDE) are, in other contexts, reor- from the colony to open bays of the marsh. In dered or broken apart (e.g., observedsequences most instances sexes could be distinguished, of ACDB, BACD, or AE), they should be con- both in the hand and at a distance (aided by sidered separate displays. On the other hand, a spotting scope)by the male’s larger body and if D follows C in all contexts, the two com- stouter, more massive bill (Palmer 1962). ponents should be combined and considered Where morphological differenceswere not evi- a single display. This provides a useful defi- dent, sex differences in the Advertising call nition of a display as a “basic unit of ritualized were used. behavior,” somewhat analogousto Williams’ Details of display structure were extracted (1966) and Dawkins’ (1976) definitions of a from approximately 110 h of sound recordings gene. A display frequently incorporates both and 12,000 m of 16-mm film. From thesesam- vocal and visual components, and we often ples, segmentsof particular interest were ana- include the vocalization in the display name lyzed usinga SpectralDynamics real-time ana- (e.g., “Tick-pointing”) and illustration. This lyzer, Kay Electric Sonographand L-W Analyst avoids having separate terms for visual and projector. Interactions not recorded on film vocal components. Grebe displays are often were dictated into a cassetterecorder and later organized into highly stereotyped sequences transcribed onto “time-line” sheets. Most of called “ceremonies,” here defined as formal- the quantitative temporal data in this paper ized interactions involving two or more birds are derived from films and from these sheets. and a combined performance of two or more Storer observed Western Grebes mostly in displays in a predictable sequence (Smith Utah, North Dakota, Oregon, and Saskatch- 1977). Names of displays and ceremonies are ewan. His data were obtained from direct capitalized to distinguish them from non-sig- observation, 2,000-3,000 m of 16-mm film, nal action patterns, as in Moynihan (1955). and several hours of sound recording. Between The photographs,drawings, and sonograms 195 2 and 1976, he also gained field experience should not be considered representative of all with all but 2 of the 19 known speciesof grebes; species members or display performances. not seen were the Madagascar Little Grebe Variations in the form, frequency, and dura- (Tachybaptuspelzelni) and the Atitlan Grebe tion of displays occur and many of these vari- (Podilymbus gigas). Comparative data, drawn ations appear to have functional significance from his observationsand the literature, range (Beer 1975). from extensive for most of the speciesof Pod- icepsto negligible for some of the speciesof STUDY AREAS, METHODS AND Tachybaptus. MATERIALS Nuechterlein studied Western Grebes in Man- SOCIAL COURTSHIP DISPLAYS itoba (1973-l 979) and in Utah, Oregon, and California (1978-1979). He observed display ADVERTISING behavior most intensively at Marshy Point Typicalperformance.While swimming or rest- (50”30’N, 98”5’W) and in the Delta Marsh ing on the water, a Western Grebe raises its (50”15’N, 98”15’W), Lake Manitoba. Both are crestand emits severalloud, one- or two-noted extensive,wind-tide marshes,free from human calls: “creeet” or “tree-creet.” The posture disturbance during the breeding season.Dur- is variable and often differs little from the usual ing three seasonsat Marshy Point (1976-l 978) swimming posture (Fig. 1). The display usually the senior author lived in an abandoned trap- includes from one to six calls 0.5-1.0 s apart. per’s cabin located at the outer edge of a West- During extended calling bouts, the bill remains em Grebe colony numbering several hundred open and the crest is kept raised throughout. individuals. A tower constructed on the roof Advertising calls of different birds are often so of the cabin provided a broad view of the col- distinctive and consistent that one can easily ony. learn to recognize individuals by their calls. Nuechterlein used a small floating blind Spectrographic analysis of call variations (constructed from an inner tube) disguisedas showsdistinct sex differences,correlation with a muskrat house to observe, photograph and color phase, individual variation, and varia- tape-record courting birds at closerange inside tion with respect to pairing status (Nuechter- colonies (Nuechterlein 1980). Incubating birds lein 198 la). In both color phases(Storer 1965) in part of the colony were individually marked callsof males are longer and of lower frequency by slowly approachingthem in a floating blind than those of females. Calls oflight-phase birds DISPLAYS OF THE WESTERN GREBE 353 usually consist of a single note (“creeet“) whereas those of dark-phase birds have two notes (“tree-creet”) separatedby a 20-200 ms gap (Fig. 1). Paired birds tend to give fewer callsper bout than unpaired birds (paired birds: x = 1.7 calls/bout; unpaired birds: K = 3.4 calls/bout; Nuechterlein 198 1a). Orientation and coordination.Lone, actively courting birds repeatedly answer (within l-5 s) Advertising calls given by other unpaired birds in the vicinity. Usually, two birds calling to one another are not in visual contact, and they may be 100-200 m apart. Calling often DARK MALE DARK FEMALE occurs in bursts, apparently caused by many I males respondingsimultaneously to Advertis- ing by a female. Social contexts. This is the only courtship display that is regularly performed by lone individuals. Unpaired males and females Advertise “spontaneously” while swimming LIGHT MALE LIGHT FEMALE restlesslyfrom one open water area to the next t in searchof a mate. On hearing the call, inter- ested birds of the opposite sex respond by approaching and Advertising in reply. Back- and-forth calling continues at intervals until eventually two or more individuals come 0.5 1.0 together and engage in mutual display SECONDS sequences. FIGURE I. Advertising by a male. Calls of females are Later in the season, paired birds use the shorterand higher pitched. Advertising callsof dark-phase Advertising call for individual recognition birds are two-noted; those of light-phasebirds, one-noted. when mates or parents and offspring become separated.
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