Condor, 80:159-172 @ The Cooper Omitlmlogical Society 1978
PAIR-FORMATION DISPLAYS OF THE GREAT EGRET
DOUGLAS W. MOCK
Many displays of the Great Egret (Casmero- MATERIALS AND METHODS &us a&us) have been described recently by Great Egret breeding activities were studied in 1973 McCrimmon (1974), Wiese (1976), and Tom- and 1975 on Hog Island (Redfish Bay, Aransas Co., linson ( 1976). Yet much remains to be under- Texas : 27”50N,’ 97”OOW).’ Egrets, herons, and stood about the displays ’ functions and how Roseate Spoonbills (A@ a@@) nested within lo- these are integrated to serve the egrets’ com- 15 m of my blind at heights of l-3 m in the surround- ing saltcedar ( Tamurix spp. ). To minimize distur- munication needs. In this paper I shall present bance to the birds I lived totallv inside the blind for further information about this species ’ displays periods of 24 days at a time. ’ with an emphasis on description of form, vari- Individual and sexual identifications were difficult ability, and contexts. I shall then attempt to for this pure-white, sexually monomorphic species. interpret signal function and evolutionary I relied on idiosvncracies in bill coloration which, because of hormonally influenced changes through: derivation. This paper is intended to com- out the pair-formation period, had to be checked and plement a study of social behavior of the Great redrawn continually. I consider this method to be Blue Heron (Ardea hero&as; Mock 1976). somewhat less reliable for Great Egrets than for The communication systems of these two spe- Great Blue Herons, where more permanent plumage cies are compared in detail elsewhere (Mock, idiosyncracies can be found (Mock 1976). One dye- marking attempt in 1973 resulted in that egret de- unpubl. ) . serting its nest. Sex was determined by coital posi- Each display will be treated separately with tion, with the male assumed to always take the upper the following format: name, description of position. No reversed mountings were seen, nor have a typical performance, variations in form, so- any been reported for other ardeids. Approximately 1200 min of courtship display se- cial contexts, and a discussion of signal design quences were taped via dictation and later transcribed and evolution. I use a modification of Meyer- onto time-plotted sheets. The quantitative data in riecks ’ (1960) display terminology (see Mock this paper were derived from these samples. 1976, Wiese 1976) with display names cap- Mv svstem of heron breeding chronology (Mock italized ( Moynihan 1955). Synonyms in heron 1976) can be summarized as follows: ( 1) &hStuge’ -from the moment an unpaired male chooses a nest- display terminology are reviewed in Mock territory until an attentive (sutddte) female ap- (1976). proaches him; (2) Bachelor Stage-from the arrival of On the basis of form, context, and probable a satellite female until her acceptance on the males’ functions I distinguish between “reactive” dis- nest (or her departure); (3) Paired Stage-from the plays and “spontaneous” displays. Reactive moment of female acceptance on the nest until the laying of the first egg; (4) Incubation Stage-from displays are characterized by relatively fixed egg-laying until hatching; and (5) Parental Stuge- orientation toward the signal-receiver and are from hatching until independence of the young. apparently performed in response to the re- ceivers’ actions. These signals serve primarily RESULTS agonistic functions (especially as distance- SOFTPART COLORATION increasing threats). Reactive Great Egret dis- plays include Upright, Fluffed Neck, Forward, At the onset of breeding, herons undergo a Stab-Crouch, Bill Duel, Supplanting, and Bill hormonally-mediated change in the coloration Clappering. Spontaneous displays are much of their “softparts” (bill, legs, lores, and irises). less rigid in their orientation, are not obviously In this population the bill changes from dull, elicited by the actions of other birds, and may streaked yellow to bright orange. The legs re- even be performed in the absence of signal- main black, the irises acquire an outer ring of receivers. Typically, spontaneous displays are red surrounding the yellow (not shown in given in long sequences by unpaired males Palmer 1962:366), and the yellow-green (of and seem to function primarily as female- Smithe 1975) lores brighten. Most egrets also attractors. They include Stretch, Snap, Wing have a narrow stripe of this green on the prox- Preen, Bow, Twig Shake, and Circle Flight. imal margin of their lower mandible. About Only one display, the Extended Neck Flight, 25% of all the Hog Islands egrets had solid does not fit well into either category: it com- “spectrum-yellow” (Smithe 1975) lores and a bines properties of both and probably conveys few others were intermediate. The yellow- both kinds of information in its various con- lored individuals had shorter aigrette plumes texts. (not extending beyond the rectrices) that t1591 160 DOUGLAS W. MOCK
FIGURE 2. Bachelor male performing Stretch as satellite female watches. Note the conspicuousness of the males’ plumes.
FIGURE 1. Stop-action illustration of a complete peak, bill angle at peak, and degree of scapular Stretch performance, traced from movie footage at 9- erection. Some performances are incomplete, frame (% s) intervals. The aigrette plumes were ending abruptly after the bill is only partially omitted. raised. These “intention movement” perfor- mances (Daanje 1950) comprised 12% of the were rarely erected, seldom held territory for 965 Stretches in the timeline sample. As in more than 2 hours, displayed more slowly (l-2 the Great Blue Heron (Mock 1976), certain displays per min), and seemed to be discrim- male egrets performed consistent variations inated against during mate-selection. Only a (e.g., Male #l always held his peak position few yellow-lored egrets bred successfully. for an extra fraction of a second; Male #4 did Thus I strongly suspect that yellow-lored in- relatively shallow crouches), dividuals are yearlings and that yellow lores Two uncommon variants warrant special might be a reliable (though unreported) char- mention. Double-crouching performances, acter for that age-class. There are many ac- wherein a second Stretch is continuous with counts of yearling ardeids showing an interest the first, were seen on six occasions. I also ob- in colony activities (e.g., Owen 1959) or even served a few Stretches in which the egret breeding successfully (Palmer 1962, Siegfried clacked its bill once at the displays’ peak. 1966, Milstein et al. 1970, Pratt 1973, Rodgers These features are conspicuous components 1978) though the most common age for first of the Stretch in the Louisiana Heron (Hydra- breeding is two years. nussa tricolor; Rodgers 1977) and Little Blue In 1973 there were two peaks in egret pair- Heron (Florida caerulea; Meyerriecks 1962) formation activity. From 7-28 March, many but have not been observed in the genus Ar- pairs courted successfuly and quickly pro- dea. The Great Egret Stretch display is ob- ceeded with nest building and incubation, viously homologous with the Stretches of The first laying occurred on 15 March. Then a many other ardeid species (reviewed in Mock month passed in which there was virtually no 1976, 1978). Unlike the Great Blue Heron courtship until, on 28 April, a second wave of (Mock 1976) and Green Heron (Butorides unpaired egrets suddenly appeared. The only striatus; Meyerriecks 1960), the Great Egrets difference between the two groups was that on Hog Island did not use the Stretch as a the first wave was comprised entirely of green- mate-greeting signal. Instead the Fluffed lored egrets while the second wave had a high Neck was performed in that context. Interest- percentage of yellow-lored individuals. If my ingly, Tomlinson (1976) illustrated a display suspicion that yellow lores are characteristic he called the Stretch Greeting which appears of younger birds is correct, this temporal pat- intermediate between the Stretch and Fluffed tern would indicate later nesting by yearlings. Neck and which has been reported only from Rhodesia so far. SOCIAL SIGNALS Unpaired Great Egret males perform many Stretch. A typical Great Egret Stretch is il- Stretches, usually from the nest-territory (Fig. lustrated in Figure 1 and described well by 2), but occasionally from a nearby perch. I McCrimmon (1974) and Wiese (1976). Each have no records of female Great Egrets per- performance takes 2-2.5 s. Unlike Tomlinson forming a Stretch. ( 1976), I heard no accompanying vocaliza- Because of its amplitude, suddenness, and tions. A highly stereotyped display, the speed, the downward crouch makes the Stretch varies slightly in maximum height at Stretch the most visually conspicuous display GREAT EGRET DISPLAYS 161 in the Great Egret repertoire. It seems to function in advertisement to the whole colony, announcing both territory-ownership and re- productive willingness. The Stretch may in- vite females to approach by expressing non- hostile intentions. Snap. With scapular aigrettes fully fanned, a Great Egret smoothly extends its neck hori- zontally until the neck is nearly straight. Then, with a sudden deep flex of the legs (crouch), the neck drops well below horizon- tal and the mandibles are clacked together once. As the torso drops, the aigrettes bounce upwards. See illustrations in McCrimmon (1974) and Wiese (1976). The only acoustic component is the mechanical bill-clack. A performance, from first neck-extension to final clack, takes 3-4 s, but the subsequent recovery to a standing position usually requires about
5 s. FIGURE 3. Bow performance. Note extreme height The Great Egrets’ Snap is highly stereo- of aigrette plumes as the egret crouches. typed, showing only minor variability in neck angle (at the time of the clack), degree of casionally sticks are chosen that are directly plume erection, depth of leg flex, and overall beneath the toes or as far away as 60 cm, pro- speed of performance. Of 533 Snaps in the ducing a range of neck extensions. Few Bows timeline sample, 87% had low neck angles (
FIGURE 4. Twig shake by an unpaired male egret.
No other ardeid studied to date has such a highly ritualized Bow but several, including the Great Blue Heron (Mock 1976) and even the Great Egret itself (see below), perform FIGURE 5. Ritualized Wing Preen performance by semi-ritualized Twig Shakes. It is possible an unpaired male egret. that the Twig Shake represents an interme- diate evolutionary stage between the primitive motor pattern (nonsignal nest-building) and high-neck-angle and vigorous performances the most derived signal (Bow). In any case, also increased in the Bachelor Stage. I have I believe the Bow to have evolved from the seen only five Twig Shake performances by “tremble-shoving” movements used in nest females. construction. Twig Shakes have been described for many Tuig Shake. The egret extends its neck out other ardeid species (reviewed in Mock 1976) and down, grasps a stick in its bill, and gently and always vary in form more than other dis- shakes it (Fig. 4). The bird does not appear plays. Some Great Egret Twig Shakes re- to be trying to snap the branch off for nesting semble nest-building motions, others re- material and there is no leg flex component. semble the more highly-ritualized Bow, and The performance takes l-5 s and may lead to still others resemble comfort bill-wiping. The normal nest-building activity (“tremble-shov- tendency to escalate performance vigor in the ing”). Twig Shakes are much more variable presence of a satellite female has also been in form than the spontaneous displays already noted in the Great Blue Heron (Mock 1976). described. Virtually all components vary, es- Wing Preen. With scapular plumes usually pecially neck angle, performance vigor, and half-erect or less, the Great Egret leans over duration. Like the Snap, most Twig Shakes and runs its bill-tip along the entire leading have sub-horizontal neck angles (86% of 260 edge of one wing. The bill is partially open Twig Shakes ) . Vigorous “thrashing” perfor- and surrounds some primaries in most per- mances comprised over 23% of the sample. formances (Fig. 5). Most Wing Preens in- These often had a circular shaking motion of clude holding the wing out and slightly for- the bill that occasionally caused the egret to ward. There are no leg flexes and no acoustic lose its balance. components. The entire display takes 2 to 3 s. The Twig Shake is a spontaneous male dis- The Great Egrets’ Wing Preen is highly ste- play, the frequency of which greatly increases reotyped, though slight variations occur in ori- in the presence of a satellite female. The aver- entation, bill placement on the wing, number age performance rate of .15/min for actively of strokes, and wing position. Displaying signalling solo males nearly tripled to .40/min egrets show a significant tendency to orient when a satellite arrived. The incidence of their bodies broadside to the signal-receiver GREAT EGRET DISPLAYS 163
(529 of 678 Wing Preens) and then to preen Extended Neck Flight. An egret takes off whichever wing is closer to the receiver (326 with its bill pointing 45-60” above horizontal of the 529). Often the bill does not even touch and usually flies with the neck recurved and the wing, but travels parallel to its leading almost fully outstretched. It may fly this way edge, 3-5 cm away. In a sample of 136 per- for a short distance and land, or it may re- formances by one male, 22% were of this non- tract the head against the shoulders and con- contact variety. Less commonly the bill tinue flying some distance. A few flights are brushes higher on the wing, among the pri- accompanied by a kruack call. In landing, the mary coverts. Single-stroke performances egret extends its neck and points the bill up- constituted nearly 99% of 2022 Wing Preens wards as it brakes to perch. in the timeline samples (19 were double-stroke The distinction between this display and performances and only one was a triple- the Circle Flight may be artificial. Descrip- stroke). Incomplete, “intention movement” tions of Circle Flights in other species (e.g., performances comprised only 4% of the total. Green Heron; Meyerriecks 1960) emphasize When actively courting, males perform Wing the circular flight path with return to the point Preens at a mean rate of 2 per min, making of departure. In the Great Egret, however, this display and the Bow the most frequent many display flights are short and straight, displays in the unpaired male egrets’ reper- between two points in the colony. For clarity toire. Satellite females perform Wing Preens I am tentatively separating the types of egret only occasionally while watching a bachelor aerial displays in lieu of broadening the Circle male. Flight definition to include non-circular Circle Flight. The egret takes off in an Ex- flights. tended Neck posture (see next section) which Extended Neck Flights may be used any time it maintains for about the first 10 m, then par- an egret moves within the colony. I have seen tially retracts its neck to form a U-shaped loop. it performed by unpaired males moving to new It flies thus, with legs dangling, in a large cir- display sites, returning to old display sites, cle (25-75 m diameter) and lands on or near visiting nearby nests for stick-theft, or flying its original point of departure. Approximately home with stick in bill. Bachelor males often 10 m before landing the egret again assumes begin aerial attacks (Supplants ) on satellite the Extended Neck posture until reaching the females with an Extended Neck takeoff pos- perch. Some landings are accompanied by ture and satellite females use it commonly loud calls. when Supplanting each other near a bachelor Aerial displays generally are more variable male. Satellite females also use it sometimes than ground displays because they last much when leaving (rejecting) a bachelor male. longer (usually 10-20 s), they must contend Extended Neck Flights are performed by both with air traffic conditions above the colony, sexes but decrease sharply after pair-forma- and they are more influenced by wind. The tion. Circle Flight, therefore, varies in flight path, Like Crest Raising, the Extended Neck circle size, duration, and postures in flight. Flight is both a display itself and a component Sometimes, instead of following a circular of other displays, in this case Circle Flight and course, an egret may fly a figure-8 or double- Supplanting. Its use in these displays is par- lap circle. ticularly interesting because they seem to A few Circle Flights are performed by solo have opposite functions. As mentioned be- and bachelor males during spontaneous court- fore, this display does not fit cleanly into ship but most are flown by satellite females. either the spontaneous or reactive categories. After a Circle Flight a satellite female usually The Extended Neck Flight presumably lands a few meters closer to the bachelor males’ evolved through a ritualization of takeoff and nest. Thus the display seems to be a tactic for landing postures. It is tempting to speculate approaching an aggresive male gradually (and perhaps submissively). that the Stretch, which Daanje (1950) con- Circle Flights are much more common in the sidered to be a ritualized form of crouching Great Egret than in the Great Blue Heron. for takeoff, might be a grounded version of the This agrees well with Meyerriecks ’ (1960) Extended Neck Flight (or vice versa). Ex- body-size observation that smaller, more agile tended Neck Flights occur in several mem- herons rely more an aerial displays than larger bers of the Egretta-Hydranassa complex: herons. In both of these species the display Little Egret (Egretta garzetta) , Intermediate seems to serve similar functions: mate- Egret (E. intermedia) (Blaker 1969b), Snowy attraction for unpaired males and non-aggres- Egret (E. thula), Hydranmsa tricolor, and sive approach for satellite females. Florida caerulea (pers. observ. ). In these spe- 164 DOUGLAS W. MOCK
FIGURE 6. Crest positions of the Great Egret. FIGURE 7. Inter-Display Stance of a territorial Top: normal relaxed feathers. Center: fully erect male egret. Note the extreme erection of the aigrettes. crest. Bottom: partially erect (“fat-head”) position.
feathers-as described for the Cattle Egret ties the bill is usually held less vertically than (Bubulcus ibis; Blaker 1969a) and Great Blue in the Great Egret. Heron (Mock 1976). Crest Raising is less fre- Landing Call. When a paired egret returns quent in the Great Egret than in many other to its nest, it usually utters several low-pitched ardeids. croaking notes (each of which is about 0.5 s Inter-Display Stance. A territorial male long and spaced at 0.5 s intervals), usually Great Egret about to begin a display bout from an Extended Neck posture. Wiese leans over, points his bill 30-45” below hori- (1976) has shown that this signal (which he zontal, erects his scapular plumes into a fan, termed a Greeting Call) is recognized by the erects his crest partially (“fat-head”), and mate on the nest whose reply enables the in- shifts his weight slowly from leg to leg, pro- coming egret to find its nest efficiently. I have ducing a gentle swaying motion (Fig. 7). also seen unpaired egrets give Landing Calls From this posture he can begin any of the upon returning to their nests, especially at the spontaneous displays, usually returning to the end of Circle Flights. Such non-greeting con- Inter-Display Stance between displays. An- texts suggest that the signal may also be a gen- gles of the bill and torso vary. Sometimes eral warning to all birds in the vicinity. males begin displaying first and then erect Crest Raising. Though lacking the special- their scapular plumes after the first or second ized occipital plumes found in many other display. Occasionally I have seen the Inter- ardeids, the Great Egret erects its crown Display Stance adopted while an egret is nonetheless (Fig. 6). As in other species, standing on one leg. Crest Raising is both a signal by itself and a The plumes, which are usually the highest component of many other displays. When body part during this posture, acquire a con- used alone, Crest Raising is mostly a close- spicuous floating motion from the body range signal between mates (or prospective swaying. Yellow-lored males, who have much mates). As a display component it appears in shorter plumes, rarely adopt the Inter-Display the Inter-Display Stance, Stretch, Snap, Bow, Stance when performing; instead they stand Wing Preen, Fluffed Neck, and Forward. quietly with relaxed feathers. Degree of erection is the only noticeable Because Great Egret males display in bouts variable, ranging continuously from a slight (lasting up to 10 min, with resting time be- raising of the feathers that gives the egret a tween bouts ), adopting the Inter-Display smooth “fat-head” appearance to full erection Stance announces the beginning of a series that produces a ragged-looking crown. I did of signals. Thus it provides important infor- not notice Great Egrets selectively erecting mation about the birds’ next actions in addi- the anterior vs. posterior halves of their crown tion to the general statements of territorial GREAT EGRET DISPLAYS 165
FIGURE 8. Upright postures of the Great Egret: silent Upright (right) and vocal Upright. FIGURE 9. Mutual performances of Fluffed Neck as mates greet each other at the nest. ownership and willingness to breed. The Great Egrets’ Inter-Display is far more ritual- to roost for the night, Uprights are frequently ized than the pre-signalling postures of other employed as the birds get settled. Similarly, ardeid species. the Upright is by far the most common dis- Upright. An egret extends its neck out play used in non-colony roosts as new birds straight and slanting about 45” above hori- arrive and find positions. Urights are often zontal. All body and head feathers are sleeked directed at landing birds who frequently re- (Fig. 8). Vocalizations ranging from soft turn the signal. They are also commonly di- gurgling to a rough, staccato rowk-rowk-rowk rected at other (usually smaller) species. On (or even a long, harsh braarrk that can last up Hog Island, Great Egrets often display at to 2 s) accompany about half of all perfor- Snowy, Egrets, Cattle Egrets, and Roseate mances. Virtually all Uprights are directed to- Spoonbills, but only rarely at Great Blue ward another bird, usually a conspecific. The Herons. Upright is maintained for l-5 s and occasion- The Upright seems to be a general threat/ ally longer if the opponent also holds the same warning that helps an egret maintain suitable posture. The Upright varies in neck angle, space (“individual distance”) around itself. It movement toward the opponent, wing posi- is employed mostly by non-territorial individ- tion, vocalization, duration of performance, uals; territory-holders rely more on the For- and speed with which the posture is assumed. ward for threatening interlopers. Uprights Often a displaying egret advances deliberately seem to proclaim the senders’ presence and or even hurries toward its opponent. If, in- willingness to escalate the threat (to a For- stead of fleeing, the opponent responds with ward or Supplant) if approached too closely, an Upright or Forward, the charging bird though this is seldom necessary. veers aside at the last moment. On six occa- Fluffed Neck. An egret elevates its head sions I saw egrets hold their wings cocked until the neck is extended about three-quarters over their backs during Uprights: once two of maximum and erects all feathers of the birds held this variant simultaneously while head and neck. The scapular aigrettes are confronting each other. sometimes partially erected. The bill is usu- Uprights occur in many contexts but are ally held about 15” below horizontal and is most commonly performed by satellite females opened slightly (Fig. 9). Some performances gathered around a displaying bachelor male. are accompanied by high-pitched, “squeaky” They use it to maintain a space of about 2 m calls. In this posture the egret usually turns around themselves. Bachelor males sometimes to face the signal-receiver. The display lasts use Uprights to threaten approaching females. 1-3 s before the performer relaxes again. When many unmated egrets join the colony Fluffed Necks vary in feather posture, neck 166 DOUGLAS W. MOCK
TABLE 1. Identities of the signal-receivers for 89 male Forwards and 69 male Supplants.
Signal-receivers
Con- ConY specific M& S$lli~ speaf1c l?on- Other Display stage neighbor neighbor species
Forward Solo 0 9 7 23 Bachelor 36 2 4 8
Supplant Solo 2 1 16 1 FIGURE 10. Stop-action tracings of a Great Egret Bachelor 36 2 9 2 Forward. Each figure represents 3-frame (% s) inter- vals ( total time scale is 1.0 s ) very uncommon (in this species) high-necked curvature, orientation, etc., and are usually Snap which is not directed at the receiver. performed in response to a landing bird. They Most Forwards are performed when the in- are performed by both sexes throughout the tended receiver is 2-5 m from the sender. breeding season anywhere in the colony (but Solo males tend to direct most Forwards to- usually near the nest). They are most com- ward other species (Table l), especially mon when the senders’ mate returns to the neighboring Great Blue Herons, as the egrets nest for incubation relief, but are also per- try to establish nesting territories among the formed by unpaired egrets, especially bachelor already incubating herons. Such threats usu- males after returning in Extended Neck pos- ally provoke retaliatory Forwards from the ture from successful Supplanting. Although Great Blue Herons that cause the egret to cooperative nest-building between the mates retreat. Bachelor male egrets direct most For- seems less highly developed in the Great Egret wards toward approaching satellite females. than in the Great Blue Heron (Mock 1976)) In general, Forwards elicit escape or defen- males do sometimes bring sticks to the fe- sive squatting (“Withdrawn Crouch” of Mey- males. At such times they are usually greeted erriecks 1960:35) in conspecifics and smaller with Fluffed Necks. This display also occa- species. Female egrets fled from 72% of 36 sionally follows Bill Duels between neighbors. bachelor male Forwards but Roseate Spoon- The Fluffed Neck apparently functions as bills and Great Blue Herons moved away from a mild threat/warning, especially during the only 32% of 31 egret Forwards. The Forward mates’ arrivals and departures from the nest. is not a common signal in the Great Egret It is second only to Bill Clappering in the mea- repertoire. It is used primarily by territorial ger signal repertoire used by mated Great males in defending their nest sites and, less Egrets. In addition it is sometimes directed frequently, among satellite females near a at other egrets and other species in the colony. bachelor. Great Egret chicks perform recog- Forward. An egret faces its opponent, nizable Forwards when only a few days old erects all plumage, retracts its neck prepara- and can drive away an adult Cattle Egret tory to striking, and leans its torso forward. when two weeks old. In this position it often advances slowly to- Stab-Crou,ch. An egret erects its crest and ward its opponent-a condition distinguished scapular plumes and strikes with closed bill as the “Full” Forward by Meyerriecks ( 1960). toward an imaginary target 45” above its op- Most performances end with 1 or 2 stabbing ponent (who is generally several meters motions toward the opponent, usually with- away). At the fullest extension of the stab, out physical contact (Fig. 10). During the the legs flex rapidly at the heels, producing a stab the wings flick out quickly (possibly for full crouch (with heel angle of above 50-60” ) . balance) and a call is often given (a harsh The wings are usually flicked out and back skok or, less commonly, a high-pitched eeee-i- during the crouch (not in Fig. 11). Afterwards eeee. Performances usually take l-3 s. the birds resumes a normal standing position. This is a highly variable, probably graded, Most performances are silent but some are signal. It varies in degree of feather erection, accompanied by a mechanical bill-clack dur- presence and quality of vocalization, wing mo- ing the stab. The whole display takes l-2 s. tion, stab motion, and mobility toward the Like the other reactive signals, Stab-Crouch opponent. Many performances are only in- performances are variable in form. The stab complete “intention movements” to strike the itself sometimes has a slight rotary action, giv- opponent. Forwards that include the stab- ing it a rocking appearance, and may end with bing motion but lack the call can resemble the the neck fully retracted against the shoulders GREAT EGRET DISPLAYS 167
ever, the opponent anticipates the attack and meets it in midair. Attacking males quickly routed male opponents in 64% of 22 Supplants and female opponents in 100% of 38 Sup- plants. Occasionally, when a bachelor male flies an unusually long distance (up to 20 m) to Supplant a satellite female, the display ap- pears to draw special attention to him. A few such distantly supplanted females actually i i i moved closer and grew more attentive after FIGURE 11. Stop-action tracings of a Great Egret being attacked. Since females presumably are Stab-Crouch at S-frame (St; s) intervals. evaluating male aggressiveness (among other things ) in their choice of mates, this act could sometimes be viewed as a form of male adver- at the bottom of the crouch. On four occa- tisement. sions the stab was directed horizontally rather Supplanting differs from the Forward in at than upward. least three important ways. It is more effective Though even less common than the For- in eliciting escape, it presumably requires ward, the Stab-Crouch occurs in many of the more energy (for flight), and it is effective same contexts. It is usually performed by over greater distances. Also, Supplanting is di- bachelor males toward bickering satellite fe- rected toward a different class of receivers males or intruding Roseate Spoonbills. To during the solo stage (Table 1); most For- my knowledge this display has never been wards ( 82% ) are directed at nearby birds of described for any ardeid species. The lowest any species (i.e., neighbors with whom the point of the crouch, when the neck is often performer has interacted before) while most retracted and the wings held out, closely re- Supplanting (80%) is directed at conspecifics sembles the static Pfuhlstellung of the Eur- perched farther away, including potential asian Bittern (Botuufus stellaris; Portielje satellites. Supplanting has been described in 1926). every species of ardeid studied. Because an egret typically performs this Bill Duel. If a newly mated (or very close display only after its opponent has begun to satellite) female egret raises her head while a retreat ( or is socially preoccupied) and be- male is facing her, a ritualized attack may cause full attacks do not ensue, I suspect that follow. The male strikes with his bill toward it is only weakly motivated to attack. Such the females’ face and she recoils, avoiding the an egret appears somewhat afraid of its op- contact. Repeated stabs by the male and ponent; this display, with its combination of recoils by the female produce a very rapid see- threat components (stab) and retreat com- sawing motion. Occasionally the female man- ponents (crouch), suggests ambivalence. ages to catch the males’ bill-tip in her man- Supplanting. With a harsh squawk an egret dibles (see photo in Tinbergen 1965:84), in flies suddenly and lands either atop its op- which case she can render him immobile and ponent or on the spot just vacated by the flee- end the duel. Otherwise it ends when the ing bird. Often the attacker adopts an Ex- male ceases his stabbing, usually within 5 s. tended Neck posture for the attack and for Such ritualized Bill Duels are quite rare in the the return flight to the nest. Occasionally Great Egret compared with the Great Blue the attack results in physical contact with the Heron (Mock 1976). receiver, involving grasping feet and thrust- Fights. Unritualized physical conflicts are ing bill (see Fights). The fleeing opponent fairly common among Great Egrets. Neigh- may be pursued briefly and pecked in flight, bors occasionally have stab-and-counter-stab but usually the attacker simply lands on the bouts that superficially resemble Bill Duels. empty perch, remains there for a minute or The most spectacular fights occur when one two, and returns to its original location. egret tries to Supplant another and meets re- Supplanting is usually done by bachelor sistance. The birds may clash repeatedly in males (toward satellite females) and by the air 5 m over the colony, squawking and satellite females (among themselves). An snatching at each other with bill and feet. In attacking bachelor may swoop repeatedly to general, territorial male Great Egrets ’ inter- drive away several attending satellites. If specific dominance relationships appear to de- the attacking egret catches its opponent by pend directly on size; they lose most con- surprise it is virtually assured of immediate- frontations with Great Blue Herons, draw with if only temporary-victory. Sometimes, how- Roseate Spoonbills, and prevail over Snowy 168 DOUGLAS W. MOCK
a male egret does not grasp a females’ neck in his mandibles. Total mount time is usually 20-30 s, but cloaca1 contact occurs only dur- ing the final 10 s. After the male dismounts, both egrets preen: no post-copulatory signals are exchanged. Wiese (1976) reported that rapes, both het- erosexual and homosexual, were common in the Great Egret. On the contrary, I have never witnessed any kind of forced (or even “extramarital”) copulation in this species. It is possible that Wieses’ frequencies of rape were unnaturally inflated by crowding; his primary study colony was situated on a man- made flat bamboo platform. My findings also disagree with Wieses’ that “Males copulate nonselectively with any female landing on their nest platforms. . .” (Wiese 1976:717). In FIGURE 12. Contact Bill Clappering by paired the Hog Island colony, females who landed Great Egrets. on male nest sites were usually attacked and never quickly mounted.
Egrets, Louisiana Herons, Little Blue Herons, MISCELLANEOUS SIGNALS and Cattle Egrets. I have never seen a Great In addition to the easily recognized displays, Egret sustain injury from fighting. I observed three “semi-ritualized ’ motor pat- Bill Clappering. Great Egrets have three terns that may or may not be social signals: recognizable forms of Bill Clappering: Aerial, Tall Alert, Nibble Shoulder, and Treadling. Contact, and Hard Contact (for character- My accounts of similar behavior in the Great izations of these see Mock 1976). Great Egrets Blue Heron (Mock 1976) suffice to describe do not perform Bill Clappering often, but this their form and contexts for this species as well. display is nevertheless the principal com- Three rare vocalizations, performed from ponent of the impoverished signal repertoire regular standing postures, are included for in the paired stage. Contact Bill Clappering completeness. Solo male Great Egrets occa- is the most frequent type, comprising 83% of sionally utter soft pet calls (at approximately 59 performances. This differs from the ten- 60 s intervals) while standing on the nest. dencies of Cattle Egrets who favor Hard Con- Because there are no obvious nearby receivers tact Bill Clappering (Blaker 1969a) but gen- for this faint signal, it is hard to imagine what erally matches the preferences of Great Blue function it might serve. On a few occasions, Herons who use gentle Contact Bill Clapper- I have observed satellite females giving rok- ing for 64% of all performances (Mock 1976). rok and glug calls while watching a bachelor Bill Clappering is performed by both sexes male. but only after a female has been accepted onto a males’ nest (i.e., after the tentative for- SOLO AND BACHELOR STAGES mation of the pair bond). Bill Clappering Unpaired male egrets choose nest-territories often seems to interrupt the senders’ mate and use them as display places during court- from whatever it was doing and causes it to ship. Territorial defense becomes stronger interact with the sender. It is also the only after the first hour or two and remains very display that commonly precedes copulation. high thereafter unless the site is abandoned. Despite a bewildering controversy over the Unlike Great Blue Heron nest-territories, existence of Bill Clappering as a display at all Great Egret territories are often 3-dimensional, (review in Mock 1978)) this signal has now owing perhaps to the smaller birds’ greater been reported for 17 species of ardeids (Mock aerial agility; male egrets frequently fly up to 1976). intercept and chase passersby. * Copulation. A male approaches his mate During the solo stage, male egrets select a with partially raised crown feathers (“fat-head”: prominent perch and perform a lengthy series Fig. 6) and usually performs Contact Bill Clap- Gf displays, pausing occasionally to look about. pering before mounting. Great Egret cop- Bachelor males mix their spontaneous signals ulation closely resembles that of Great Blue with reactive signals, giving an ambivalent Herons (described in Mock 1976), except that message pattern to a female. She is, in effect, GREAT EGRET DISPLAYS 169 beckoned and repelled repeatedly. I suspect TABLE 2. Display rates (per minute) of actively that this stalling tactic gives a male time to signalling Great Egrets during chronological stages of appraise each female (and vice versa). Fe- pair-formation. males often have an assortment of males from Male displays Female displays which to choose; early in the season the Hog Total stage minutes N Rate N Rate Island colony had as many as 50 unpaired males displaying simultaneously. A satellite Solo 586 3197 5.46 - - female can reject one male simply by moving Bachelor 409 2721 6.65 94 0.23 over to another. On the other hand, a male egret exercises his choice by selective rejec- Paired 154 103 0.67 47 0.31 tion. Before accepting a mate on his nest, a male usually has chased off many other fe- males. Wiese (1976) found that the solo + building, and copulating involve truly social bachelor stages of 16 Great Egret males in interactions between the mates. In general, Florida and Louisiana lasted for an average the paired stage is characterized by the fe- of two days. males’ being absent (presumably feeding) or, One might speculate that females are ac- when present, standing quietly beside the in- cepted or rejected on the persistence of their active malt. Displaying between the mates is approaches and that males are chosen accord- remarkably infrequent compared with other ing to some optimal standards of site-tenacity, herons (Mock 1975, 1976). The overall dis- aggressiveness, and display vigor. Exactly play rates for paired egrets are very low what cues are most important is unknown. (Table 2) and most of those signals are threats directed toward non-mates. Intra-pair PAIRED STAGE signalling consists only of occasional Bill Clap- After a male egret accepts a mate on his nest pering, Bill Duels, Fluffed Necks, and at- the level of social activity diminishes quickly. tempts at copulation. For the next 4-7 days prior to initial egg-lay- Nest Building. Although many ardeids sal- ing, a trial pair bond seems to exist. I believe vage whatever old nests survive from the that five major tasks are undertaken during previous nesting season, on Hog Island most this period, the duration of which may be ulti- of these nests are already occupied by the mately defined by the females’ food intake for earlier breeding Great Blue Herons. The egg production. First, the new mates continue Great Egrets have to start anew. The division the mutual testing that began during the bach- of labor by the sexes during nest building elor stage. The pair bond is fragile and “di- varies from species to species. In the Great vorces” occur frequently (I estimate that Blue Heron, for example, males do not collect about l&20% of all pair bonds dissolve before new sticks for the nest until they have ac- egg-laying). Second, the mates engage in quired mates. Then a nearly equal division of semi-cooperative nest building (see below) labor develops in which the male brings the that prepares the nest for laying and incuba- sticks and passes them to the female who tion. Third, both sexes defend the nest- then inserts them. This cooperative effort is territory though most defense is still done by accompanied by considerable displaying by the male (in part because the female is absent both sexes, which apparently helps to coordi- much of the time while feeding). Mated fe- nate their efforts (Mock 1976). Cattle Egrets males direct most of their defensive efforts show a similar nest building pattern (Blaker toward the remaining satellite females, sug- 1969a). On the other hand, solo male Green gesting that they are defending not so much Herons and Louisiana Herons start collecting the nest as their interest in the male. Fourth, and inserting sticks soon after choosing a nest the pair engages in repeated copulations, be- site. After pair-formation males of these spe- ginning on the day of first acceptance and cies switch to a cooperative system and be- continuing at least until there are two eggs gin passing the sticks to their mates for inser- in the nest (i.e., throughout the paired stage). tion. Again, there is much signalling between Estimates of 20 copulations/season made for the mates (Meyerriecks 1960, Rodgers 1977). other ardeids (Blaker 1969a, Mock 1976) seem The Great Egrets on Hog Island showed yet reasonable for the Great Egret. Finally, it is a third pattern. Solo and bachelor males col- likely that males devote part of their energy lected sticks and began building or repairing, during the paired stage toward “extramarital” but after pair-formation the cooperative sys- courtship (Mock, in press). tem seen in other species did not develop fully, Of these tasks, only pair bond-testing, nest Males continued to collect sticks, but passed 170 DOUGLAS W. MOCK
TABLE 3. Contexts of Great Egret displays.
External disturbance X X X Nest defense x x x x x x X 8 Advertisement x x x x x x x X 0 - 0 Interactions X X x x X Greetings at nest x x X x x Intra-pair appeasement X X Intra-pair aggression X X
fewer than half of them to the females. The DISCUSSION AND CONCLUSIONS rest they inserted themselves. The stick-pass The display repertoire of the Great Egret, like itself was poorly developed; often a male that of the Great Blue Heron (Mock 1976), merely laid the stick on the nest and the fe- relies primarily on vision, with acoustic cues male inserted it. There was almost no dis- serving mainly to keep the visual channel playing between the mates when a stick was open. Great Egrets court and nest in open brought to the nest. Females sometimes per- nest-sites; this permits efficient transmission formed weak Fluffed Necks at such times, but of visual signals. To enhance the optical ef- more often they just watched. Even during fects of basic ardeid displays, Great Egrets stick insertion Bill Clappering was uncommon. have evolved rapid performance speeds and Great Egrets of Rhodesia are reported to the most spectacular set of scapular (“ai- show a much sharper division of nest-building grette”) plumes in the family. These feathers labor, in which one unspecified sex (presum- are held high during courtship and sway or ably male?) brings 93% of the sticks while the bounce with every body movement. All the other sex does 98% of the insertions. These spontaneous displays (Stretch, Snap, Bow, birds also perform more displays in the process Wing Preen, Twig Shake, and Inter-Display ( Tomlinson 1976). Stance) are characterized by bent-over pos-
EXTRINSIC FACTORS tures, widely fanned plumes, and/or sudden crouching actions that animate the plumes. Many weather features affect the level of so- These six displays comprise 96-97% of all dis- cial activity in a heronry. Although there is play performances by unpaired males; an ac- often enough light to see well before sunrise, tively courting male averages one of these many birds start displaying as the first rays plume-bouncing displays every 10 s. Follow- of sunlight strike the colony. On partly over- ing pair-formation, the spontaneous displays cast days a sudden sunbeam can trigger wide- virtually disappear and the scapular plumes spread displaying or preening in a group of are rarely erected. It seems, therefore, that This stimulating effect previously idle birds. the spontaneous displays and scapular plumes of sunlight may be primarily responsible for are used primarily to advertise a males’ avail- the greater display synchrony during the ability for mating. morning peak social period than in the eve- For herons in general, Mayr (1972:97) ning peak. At dusk some individuals become posed a “tranference” hypothesis to explain quiet early but others, especially bachelor the existence of female plumes. According to males, may continue courting until after they his scheme, males evolved plumes by sexual have become invisible to me (light meter selection to reinforce male courtship displays. readings of 0.17 Lux) . The extensive mutual signalling necessary to Rain, hot sun, and especially wind strongly test and maintain the monogamous pair bond inhibit social activity. The birds sleek their led to “transference” of the male character to feathers during rain, ruffle them (presumably the female. I see no reason to invoke sexual se- for convection cooling) and gular-flutter in lection at all. In species where both sexes the sun, and brace themselves horizontally in perform spontaneous displays, natural selec- high wind. When wind speeds exceed about tion should favor whatever characteristics en- 30 km/h courtship usually halts. hance efficient communication. Thus, in GREAT EGRET DISPLAYS 171
Green Herons (Meyerriecks 1960)) Great Blue #5 TO1 GM01779) and the Rob & Bessie Welder Herons (Mock 1976)) Louisiana Herons (Rod- Wildlife Foundation of Sinton, Texas. For assistance in constructing the blind I thank C. Barkan, S. Der- gers 1977), and Boat-billed Herons ( Coch- rickson, J. Eldridge, M. Howe, K. Mock, J. Rappole, lea&s cochlea&s; Mock 1975), where both 1. Reagan, W. Reagan, and S. Winckler. For edi- males and females display extensively through- torial criticism I thank my graduate committee, es- out the breeding cycle, natural selection has oeciallv D. F. McKinnev. H. B. Tordoff. and R. E. produced plumes in both sexes. Phillips. This study was part‘ of a Ph.D.dissertation‘ submitted to the Department of Ecology and Be- The Great Egret, however, is apparently an havioral Biology, UniGersity of Minnesota; Minneap- exception to the “normal” ardeid condition olis. It is contribution number 190 of the Welder insofar as females scarcely use their plumes Wildlife Foundation. at all. Perhaps in this species female dis- playing has been secondarily reduced but the LITERATURE CITED unused female plumes have not yet been dis- BLAKER, D. 1969a. Behaviour of the Cattle Egret, carded via “streamlining evolution” (Regal Are&la ibis. Ostrich 40:75-129. 1977). Indeed the pair bond of Great Egrets BLAKER, D. 1969b. The behaviour of Egretta gar- is unlike that of other herons in several re- zetta and E. intermedia. Ostrich 40:150-l%. spects already described (and see below). D~ANJE, A. 1950. On locomotory movements in birds and the intention movements derived from The comparatively small amount of overt them. Behaviour 3 :48-98. signalling that occurs after pair-formation MAYR, E. 1972. Sexual selection and natural se- raises some interesting questions: Why do lection, pp. 87-104. In B. Campbell [ed.], Sex- paired Great Egrets not interact with each ual selection and the descent of man 1871-1971. other as much as other ardeids? What is dif- Aldine Publ. Co., Chicago. MCCRIM~~OX;, D. A. 1974. Stretch and snap dis- ferent about this species ’ mating system and play in the Great Egret. Wilson Bull. 86:165- reproductive behavior that allows egrets to 167. maintain such “asocial” pair bonds? What MEYERRIECKS, A. J. 1960. Comparative breeding are the parental investment patterns and re- behavior of four species of North American productive options of breeding egrets? I be- herons. Publ. Nuttall Ornithol. Club No. 2. MEYERRIECKS, A. J. 1962. In R. S. Palmer [ed.], lieve we will need to know much more about Handbook of North American birds, Vol. I. Yale the ecological constraints on egret breeding Univ. Press, New Haven, Connecticut. before these questions can be answered. MILSTEIN, P. LES., I. PRESTT, AND A. A. BELL. 1970. As in Great Blue Heron communication, The breeding cycle of the Grey Heron. Ardea Great Egrets seem to possess redundancy in 58:171-258. MOCK, D. W. 1975. Social behavior of the Boat- their repertoires and multiple functions for billed Heron. Living Bird 14:185-214. many displays (Table 3). This may allow the MOCK, D. W. 1976. Pair-formation displays of the best compromise between maximum informa- Great Blue Heron. Wilson Bull. 88:185-230. tion flow and minimum ambiguity to the re- MOCK, D. W. 1978. The comparative approach to wading. bird behavior. In A. Snrunt. 1. Ogden. ceivers (see fuller discussion in Mock 1976). <, _ 1_1 and S. Winckler [eds.], Wading birds. -Nat11 Audubon Sot. Res. Report 7:17-25. SUMMARY MOCK, D. W. Display repertoire shifts and “extra- Pair-formation behavior in the Great Egret marital” courtship in herons. Behavior. In press. was studied for two seasons at the Hog Island MOYNIHAN, M. 1955. Remarks on the original colony in Texas. The 16-17 displays used by sources of displays. Auk 72:240-246. OWEN, D. F. 1959. Some aspects of the behaviour the egrets are described and classified as of immature herons, Ardeu cinerea, in the breed- either spontaneous or reactive. Several dis- ing season. Ardea 47: 187-191. plays are illustrated here for the first time. PALMER, R. S. [ed.]. 1962. Handbook of North Each display is described for form, variability, American birds. Vol. I. Yale University Press, contexts, and probable function. The pair- New Haven, Connecticut. formation process, sex roles, and characteris- PORTIELJE, A. F. J. 1926. Zur ethologie bezw. psy- tics of the pair bond are discussed. Follow- chologie von Botuurus stellaris. Ardea 15: l-15. ing pair-formation Great Egrets perform very PRATT, H. M. 1973. Breeding attempts by juvenile Great Blue Herons. Auk 90:897-898. few displays with their mates. In general it REGAL, P. J. 1977. Evolutionary loss of useless appears that Great Egret communication is features: is it molecular noise suppression? Am. strongly visual and that specialized plumes Nat. 111:123-133. and derived display forms have evolved to RODGERS, J. A., JR. 1977. Breeding displays of the enhance the optical effects. Louisiana Heron. Wilson Bull. 89:266-285. RODGERS, J. A., JR. 1978. Display characteristics ACKNOWLEDGMENTS and frequency of breeding by subadult Little Blue Herons. In A. Sprunt, J. Ogden, and S. Financial support for this research was provided by Winckler [eds.], Wading birds. Natl. Audubon the National Institutes of Health (Training Grant Sot. Res. Report 7:35-39. 172 DOUGLAS W. MOCK
SIEGFRIED, W. R. 1966. Age at which Cattle Egrets VERWEY, J. 1930. Die paarungsbiologie des fisch- first breed. Ostrich 37: 198-199. reihers. Zool. Jahrb. Abt. Allg. Zool. Physiol. SMITHE, F. B. 1975. Naturalists’ color guide. Am. Tiere 48: l-120. Mus. Nat. Hist., New York. WIESE, J. H. 1976. Courtship and pair formation in TINBERGEN, N. 1965. Animal behavior. Time-Life the Great Egret. Auk 93:709-724. Books, New York. Department of Zoology, 730 Van Vleet Ozjal, Room TOMLINSON, D. N. S. 1976. Breeding behaviour of 222, The University of Oklahoma, Norman, OK 73069. the Great White Egret. Ostrich 47:161-178. Accepted for publication 8 August 1977.