Social Behavior of the Black-Crowned Night Heron

Vol. 55]

SOCIAL BEHAVIOR OF THE BLACK-CROWNED NIGHT HERON

BY G. K. NOBLE, M. WURM AND A. SCHMIDT Plates $-4 THE Black-crownedNight Heron (Nycticoraxnycticorax hoactli) is a commonAmerican bird and its life historyhas beenintensively studied by Gross(1923), Bent (1926) and others. Neverthelessan analyticalstudy of its socialbehavior has not previouslybeen attempted. RecentlyLorenz (1934,1935) has madesuch an analysisof the socialbehavior of the Euro- pean Black-crownedNight Heron, which is only subspecificallydifferent from the Americanrace. One would naturally assumethat the behavior of the two formswould be very similar if not identical,but variousdetails of the behaviorof the Americanrace as givenby Gross(1923) do not agree with Lorenz'sdescription of the Europeanbird. It has thereforeseemed advisableto recheckthe life historyof the Americanr•ce, consideringnot only the socialbehavior of the adult but also the ontogenyof thesebe- havior patterns in the immature. This investigationwas supported by a grant from the Committeefor Researchin Problemsof Sex, National Re- search Council.

THE PROBLEM ;•heBlack-crowned Night Heron is a speciesofespecial interest because Lorenzconsiders that the greetingdisplay of this bird illustrateshis term 'releaser' in an ideal manner. The 'ornamentalplumes' arisingfrom the crownof the adultsserve, according to Lorenz,not for purposesof mutual stimulationas Huxley (1921)would assume but to suppressa strongdefense responsewhich is supposedlyevoked in this heronby the approachof any fellowmember of the species.Immature Night Heronslack the dark crown and white plumesof the adultsbut Lorenzassumes that as they growolder they displaythe bowingmovements toward their parentsin order to sup- pressattacks which their parentsmight make. The movementsappear in ontogenybefore the plumeswhich are destinedto have functionsnever pre- viouslyattributed to the plumesof any bird. Night Heronshave been assumedto exhibit a peckingorder (Schjelderup-Ebbe,1931), that is, a socialhierarchy which has beenintensively studied in the hen, pigeonand a few other domesticatedbirds (Allee, 1936). Lorenz has not considered what rgle, if any, these plumesor gesturesmight have in regulatingthe peckingorder, nor has he consideredthe courtshipof the Night Heron in full, althoughhe deniesthat the plumeshave any functionas adornments at this time. In studyingthe socialbehavior of the Night Heronwe have 8 NOBLE,WvR• A• SC•mT,The Black-crowned NightHeron [Jan.Auk paid especialattention to (a) the ontogenyof socialbehavior, (b) the pecking orderand (e) the courtshipbehavior.

FIELD OBSERVATIONS ON TIlE IMMATURE Field observationswhich we made on immature herons at Orient, Great Neck and Massapequa,Long Island, during 1936 and 1937,showed at once that the socialbehavior of this race is far •nore complexthan Lorenz has describedin the Europeanform. As the young grow older they readily leavethe nest and take a positionclose to the tree trunk a few feet above the nest. If approachedat this stage,they usuallyelirob higher in the tree or make their way clumsilyto anothertree. Most of the youngof the Orient colonywere in this stageJuly 8, 1937. Approximatelya weeklater, as the youngbegin to fly, a profoundchange takes place in their socialbehavior. Sucha stagewas seenin the majority of the youngin the Massa- pequa colony,July 9 and 10, 1937. The young are now widely spaced verticallyon the boughsof the nestingtrees. Otheryoung birds are making short flights to thesetrees and there are numerousdisputes and vigorous thrustsof bills as a newcomerlands near a residentbird. Gross(1923) has interpretedthese disputes as primarily over food. He states: "When the youngwere hungry, they werealso irritable, and the leastdisturbance by a neighborwould cause them to rendera defensivethrust accompaniedby a ghastly, sharply accented'Sque-e-e-e-e-e-ak'." If the birds are in this early flying stageit will be readily observedthat the disputeis over territory and not food. Exactly the samebehavior is found amongthe adults whendefending nesting areas. Whenthe youhgare in pairsthe resemblance of their behaviorto that of paired adultsis evencloser. The frequentoccurrence of groupsof three on mutual goodterms defendingtheir immediatevicinity againstnew arrivals,leads one to suspect that theseare youngof the samebrood which have not left their natal tree. This interpretationleads to the question,Do the young recognizetheir brothers or sisters when not on their home tree? Do the numerous eases of singlebirds defendingtheir territoriesagainst all comersrepresent broods whereonly onesurvived, or hasthe family groupdisintegrated leaving each bird fightingfor its own perchingsite? The fact that one bird can move closeto anotherwithout resistancewhile anothermay be attacked,shows that there is individualrecognition. Neither Lorenznor Grossfound evidenceof suchrecognition and neitherinterpreted the behaviorof the young in terms of juvenile territory defense. It was primarily to secureevldenee of this territorialismof the youngheron that we have rearedthirty-eight individualstaken from nestingtrees before they couldfly and havefollowed the ontogenyof behaviorpattern for over a year. Sincefourteen of these young heronsbred the followingspring we have observednearly a full Irol.193855] J NOBLE,WURM AND SCHMIDT, The Black-crowned NightHeron 9 juvenilecycle in thesebirds. The juvenilebirds in the fieldwere not banded and their sexwas unknown. We shall thereforebase our descriptionof the ontogenyof socialbehavior on the birdsreared in captivity. Many of the detailsof behaviorseen in the laboratorywere alsowitnessed in the field and referenceto thesedetails will be madebelow under various headings.

TERRITORIALISM AMONG YOUNG I-IERONS We have studiedthe developmentof the type of territorialismdescribed above,in a seriesof youngherons maintained in largecages. Twenty-nine youngranging from five to fifty daysof age,to judgefrom the descriptionof Gross(1923), were dividedinto two lots, of which onewas placedin indoor and the otherin outdoorcages. Both groupswere given artificial nests and markedwith coloredbands. Very soonthe olderbirds began to leave the nestsand to establishwell-defined territories which they defendedagainst the encroachmentof other youngbirds. The four outdoorcages were provided with boxesand irregularlyarranged logs, boards and potted shrubs. Cage 1 measured199 by 85 by 88 inchesand was equippedwith perches from oneto six feet from the floor. Cages2 and 3 wereboth 206 by 105 by 75 incheswith most of the perchesthree and a half feet from the ground. The groundplan of Cage2 is shownin text-fig.1. All cageswere provided with large water baths. Cage4 was constructedafter most of the young birdshad left the artificialnests. It measured218 by 132 by 134 inchesand was equippedwith perchesfrom one and a half to sevenand a half feet from the ground. The oldestbirds (Nos. 17, 18, 19) on later dissection,proved to be males. They wereapproximately fifty daysold at the beginningof the experiment and soonafter securedthe mostisolated and highestperches in Cages1 to 3. Other birdswhen approx'maately seventy days old movedaway from the artificialnests and formedthe first pairs(Nos. 21-22, 20-14, 9-12, 1-4, 2-3). A youngergroup, approximately thirty daysold (Nos.6-7-9-10-11-13-16), remainedgrouped near the nestsalong the wall while the older birds were securingterritories. Each of thesegroups, either as a largegroup, a pair, or a lone bird, defendeda territory severalfeet in diameteragainst the encroachmentof any bird of any of the other groups. The oldestand most aggressivebirds (Nos. 17, 18, 19 and pairs 1-4, 2-3) guardedthe largest areas,often the whole width of the cage. When the observerentered the cagethe birdswould leave their territoriesand form a highlyagitated group in onecorner of the cage. Within a minuteafter he had left, the birdswould return to their respectiveterritories and defendthem by vigorousthrusts and squawksagainst trespassers. The territorieswere not securedsuddenly but often by slowadjustment to the otherbirds in the cage. ]7orexample, the birdsa.t the time of leaving 10 Norr,•:,WrrR• •m) SCHMWT, TheBlack-crowned NightHeron [zAanu•.

GROUND PLAN OF CACE I[ .SHOWING- TERRITORIES OF IMMATURE NIGHT HERONS (OUTLINED IVITll DA2tl

•oXE$.

TEXT-FIG. 1. Vol.1938 55]I NOBLE,WURM AND SCHMIDT, TheBlack-crowned NightHeron 11 the nestseemed to form five major divisions:(a) Nos. 9, 12, 14, 17, 18, 20; (b) Nos. 6, 7, 8, 10, 11, 13, 16; (c) Nos. 1, 4, 5, 23, 25, 26, 27; (d) Nos. 2, 3, 19, 21, 22, 24a, 28; and (e) Nos.24, 29, 31, 33, 34, 35, 36. Asstated above, Nos. 18 and 19 early isolatedthemselves from thesegroups but No. 17 was first groupedwith 14. Other pairs which formed and broke were Nos. 20-14, 20-9, 10-8 and 10-7. The immediatecause for the splittingup of the largegroups into smaller lotswas the greedof the youngbirds. At aboutfifteen days of agethe young heronsbegin to snatchat foodbrought them insteadof merelyshaking their headsand bobbingwith widely openmouths in the mannerof very young herons. In the latter, discriminationis poor and they will attempt to swallowpieces of woodor the hand of the observer. Often two youngbirds will attempt to swallowthe wings, neck or feet of a third. Undesirable objectswhich can be swallowedare later regurgitated. The young birds stuff themselvesuntil part of the foodis protrudingfrom the mouth. Even then the sight of more food will induce new feeding movements. At ap- proximatelyforty daysof agethe birdsprefer to pickup foodfor themselves and are usuallyreluctant to feed from the hand. With the changeof behavior there arisesa type of intimidation call which is directed toward drivingother youngbirds away from the food. The food cry of the youngheron at the time it leavesthe nest is a per- sistentcackle which may be written kak-kak-kak-kak-kakrapidly repeated and persistingwhile the observeris present. It is accompaniedby a slight waving of the head sidewaysor vertically. The intimidation call is a con- tinuouschesty gurgle given while the body is held low and the wingsare slightly raisedand spread. The intimidatingbird stepssideways toward the offendingneighbor, gently pushingwith the spreadwings. This intimidation displayis directedonly toward one of the bird's own group. When the birds have becomefurther separateda very differentre- sponseoccurs which is a clearindication that oneor both birdshave separate territories. Againthe body is held low but the wingsmay or may not be spread.The head is drawn back over the bodyand the mouthis openedwide while the feathersof crown,neck and back are erected. Then, as the head is dartedforward in a viciouspeck, a high-pkchedscreech, or frighteningcall, is emittedand the mouthagain is openedwide. In a lessextreme attitude the body is held erect and the feathersof head and neckare raisedwhile a shrill but lower-pitchedsquawk, or fight call, is given. In any groupof youngherons there is one bird that is dominantin the sensethat duringbilling it holdsits headhigher than the othersand in any disputeamong themselves the otherseither withdraw or lower their heads in a subservientmanner. A secondcause for splittingof the originalgroup is that one of the subordinatebirds will no longerlower its head with the 12 NOBLE,WURM AND SCl•MIDT, TheBlack-crowned NightHeron [J•n.[Auk resultthat the usuallyobserved gentle billing gives way to moreaggressive thrustsand finally to typical territory-defensemovements. Approximatelyseventy days after hatching,the heronsbegin to fly and much preliminarywing flappingis indulgedin. Two pairs (Nos. 21-22, 2-3) wereformed at this time and two three-memberedgroups (Nos. 6-7-11, 24-35-36). The intragroupstruggle became more intense. In group24-35- 36 birdsNos. 24 and 36 eliekedtheir bills loudly while billing one another and held themselvestense and erect. PreviouslyNo. 36 had submittedto 24 by graduallylowering its head whenthe two billed. The failureof No. 36 to assumethe subordinateposition of headsoon resulted in an exchange of viciouspeeks between the two and No. 36 was driven from the territory. Consequentlypair 24-35 formed. Similarly one bird was observedto be driven from the territory of Nos. 1-4-5 and 6-7-11 with the result that pairswere produced. All the pairsof immatureherons, excepting those resultingfrom the forcedpairing to be discussedbelow, were derived from the nestlinggroups to whichboth birds belonged. Oncea bird had beendriven from a groupit retainedsome memory of its early associatesbecause much later when frequentchanging of the bird's territory by the experimenterstended to break down territory distinctions, someregrouping occurred. Thus pairs 5-28, 27-25, 6-11 and 7-11, which were well definedfor severalweeks, later formed the groups5-27-28 and 6-7-11. The regroupingof the first of thesetwo lots may have beenfacili- tated by the fact that Nos. 27 and 28 werepartly spayed. The pairsof youngbirds were usually as well definedas that of breeding adults. Introducinga pair into a cagefull of strangebirds invariablyre- sultedin their keepingtogether and establishinga commonterritory in the newcage. Further,these young birds remembered their old territorieswhen returnedto the originalcages some weeks later. Thus pair 23-26 recog- nizedand securedtheir old territory in Cage2 after havingspent 17 daysin Cage 1. Pair 21-22 securedtheir old territory in Cage4 after beingre- tainedtwenty daysin Cage2. Pair 5-28, after three weeksin Cage1, attemptedto reestablishtheir old territory in Cage2. In this they did not succeedbecause their old territory had beenoccupied by pair 29-32. Simi- larly pair 2-3 regainedtheir old territoryin Cage2 afterfive weeksabsence, but No. 19, after five weeksabsence from the samecage, was badly beaten by the other birds while attemptingto regainits old quarters. The strongterritory defenseof youngherons in a cagewhere competition for spaceis great,will sometimesinduee the formationof new pairs. For example,birds 5 and 28 were transferredto Cage2 and birds 25 and 27 to Cage3 beforethey were paired. Thesebirds, driven by the others,were foreedinto a eommonarea and beingfrightened by the new situationthey adopteda subservientattitude which facilitatedthe formationof perma- Vol.1938 55]J NOBLE,Wc•m AND Scamp% The Black-croumed NightHeron 13 nent pairing. Similarly, pressureof environmentcaused birds 29 and 32, resultingfrom the inducedbreaking of pairs 29-31 and 15-32, to form a permanentpair. Pairing off is not accomplishedby the merejuxtaposition of two fright- enedbirds. One bird must acceptsubordination to the other and, further, the bond between them is frequently strengthenedby gesturingand billingof the pair. In this behaviorthe subordinatebird apparentlyalways takes the lead. The head is lowered and weaved back and forth while the bill is directed toward the side or feet of the dominant bird and elieked. We have calledthis gesturethe 'overture.' It is answeredby the dominant bird whichusually keeps its headhigh but may occasionallylower it to the same position as that of the subordinatebird. Overtures are made at frequentintervals. During a period of twenty-onedays fifteen pairs of immature birds were observedfor approximatelysix hours a day. The subordinatebird wasfound to initiate the ceremony1,406 times. In thirty- one easesthe dominantbird was recordedas making the first move,but it is highly probablethat the observer,with thirty birds under observation, failed to noticea still earlieroverture on the part of the subordinatebird. Thus each pair of birds made overturesapproximately five times during six hours of daylight, with the subordinatebird usually, if not always, taking the lead. A modificationof this overtureis the actual graspingof the latter bird by the first bird with its bill. Another modificationis the strokingof the breastfeathers of the parmerwith the bill. A third move- ment we have describedas 'billing.' It consistsof the birds'opening and closingtheir billswhile they are heldin contact. During this movementthe dominantbird alwaysholds its head higherthan that of the subordinate. These movements,although started by the subordinatebird, induce a similarmovement by the partner. Oncea pair has formed,neither billing nor overrating is necessaryfor the mainteneneeof this conditionas we shall point out below. A variety of callsmay be givenby either memberof a pair and as we shallshow, these aid in the retentionof a group. The onemost frequently employedis the recognitioncall, whichmay be written askrwawrk-krwawrk- krwawrk. The secondis a throaty and prolongedvariation of the familiar quawkor flight call of the species. This occursbetween birds which have recentlydriven off a strangeror with membersof a pair whichhave come together after a separation. The third call is a eombinatlonof food and flight callsand occurswhen one bird of a pair movesoff and its partner wishes to recall it. A fourth is a combination of the food and intimidation cry. It is usuallygiven when two heronsare standingbeside each other and is usuallyfollowed by a bout of billing. 14 NOBLE,WURM AND SCHMIDT, TheBlack-crowned NightHeron [•aunk.

RELATION OF SEX TO PAIR FORMATION AMONGJUVENILE HERONS Sexwas determined at the closeof our studiesby a directexamination of the gonadsof the paired birds. As shown in Table 1 there were thirteen male-female,six female-femaleand two male-malepairs. The gonadswere rudimentaryin all eases. Sectionsrevealed some spermatids but no sperm in the testes. No yolk had yet formedin any of the ova. As stated above, no pairs of young birds formed without one bird be- comingsubordinate to the other. Oncea bird had taken sucha positionas shownby (a) the lowerposition of its headduring mutual billingand (b) its initiating overturesto its partner, this positionwas never lost while the groupingwas maintained. In the heterosexualpairs the male wasusually the dominant bird. The three eases of reversed dominance find an ex- planationin the specialhandling the birds received. That is, male No. 11 wasa very tame bird whichwas frequentlypetted by visitorsto the cage. This would account for his taking a subservientattitude to femalesNos. 6 and 7. Pair 29-32 was a forcedpairing following the experimentalbreak- ing of pairs 15-32 and 29-31. Sucha forcedpairing of formerlypaired birds probably never occursin Nature. Our observationsat three different coloniesindicated that when competitionfor spacebecame very severethe bird losingthe fight movedon to new territory. Hencewe may conclude that in the sexuallyimmature heron the male has a decidedtendency to dominatethe female. This dominancehas no relationto bodyweight; it is a characteristicof malenessin the Night Heron.

TABLE 1 Relation of Sex to Pair Formationin Immature Night Herons Heterosexual Pairs Homosexual Pairs Dominant Subordinate Dominant Subordinate Male Dominant Female Pairs 23 9 269 59 289 Br49 Br,G 9 129 99

129 149

Female Dominant Male Pairs 69 11c• 2• 3• 79 11c• 1 • 4• 32 9 29 Vol.1938 55'1] NOBLE,W•'l•r• .•ND ScsMrv?, The Black-crowned NightHeron 15

SENSORYMODALITIES EMPLOYEDIN •)ARTNER tiECOGNITION Young heronsremember not only their old territoriesbut their partners for longperiods as the followingexperiments show. (a) The two heronsof pair 5-28 wereseparated for 72 hours. Whenthe birds were placedwith many other heronsin their originalcage they re- formed within twelve hours. (b) Birdsof the samepair separated96 hoursand thenplaced together with manyother herons in a cagepreviously unoccupied by them,again re- formed within twelve hours. (c) The two birdsof pair 23-26 isolatedfor seventeendays and thenre- placedin their originalcage with otherherons, re-formed their groupwithin twelve hours. (d) The two heronsof pair 21-22 separatedfor twenty daysand then placedin a cagestrange to themre-formed their groupwithin three hours. Whenreturned to the originalcage they attemptedto retrievetheir old territorybut afterbeing driven out by the heronsoccupying it, theymarked out a new territory in the old cageand continuedto defendit. It is thereforeclear that youngherons can recognize their old partners after a separationof at least twenty days. The sensorymodalities employedin this recognitioncould not be determinedfrom observationalone. Somepreliminary experiments were directedtoward modifyingthe re- sponsesof one bird to its partner by coveringthe bill of the latter with bitter substances. Neither an extract of aloe nor concentrated solutions of quininesulphate had an effecton the mutualceremonies. We wereequally unsuccessfulin modifyingthe responseby usingill-smelling substances on the feathers. Attentionwas then directedtoward the visualand auditory modalitiesfor theseare wellknown to be of primaryimportance in the life of mostbirds. Youngherons were found to be muchless disturbed by pro- foundchanges in the appearanceof their partnersthan hasbeen reported for otherbirds. Beaksof variousindividuals were painted brilliant red or blue,the legswere striped with blue,orange and white without bringing any deviationin the reactionof onemember of a pair.to its partner. When, however,the entireface was coveredwith either a bright-redor yellow rubberballoon leaving only the eyesand bills exposed,the maskedbird inducedfright reactions in its partner. But in everyease the maskedbird had merelyto call a few timesto reassureits partnerbefore billing and gesturingbegan again in the normalmanner. Merely coveringthe crown of a youngheron with a pieceof adhesivetape stained with iodinebrought confusionat firstto its partner. Sincesuch a modificationof appearance wasmuch less conspicuous than the paintedbills, it seemedthat the young heronspaid especial attention to the detailedfeatbering of the head. How- ever,when most of thehead and neck feathers of onemember of pair5-28 16 Nos•.•,Wu•>t x•D Setran)% The Black-crowned NightHeron [Jan.Auk wereremoved and replacedby headfeathers from muchyounger herons, no permanent confusionresulted. In these casesit was obvious that the recognitionof the partner'svoice, coupled in somecases with the mutual recognitionof territory werethe bondswhich brought them together. The humanobserver is unableto distinguishany differencebetween the voicesof the differentherons when they wereusing one type of call,or at least we never succeeded. The birds, however, were able to make this dis- tinctionand they require the stimulationof the partner'svoice for a synchronizationof the mutual ceremonies.Plugging their earswith cottoncovered with rubbercement and then bringingthe adjacentskin togetherover the cementwith a singlestitch proved to be an effectiveway of eliminating soundwithout disturbingthe birds. Suchbirds were found not to respond to soundsmade by a hiddenobserver, and their reactionsto their partners indicatedan obviousdeficiency in hearing.

RELATION OF VOICE TO THE BILLING CEREMONY OF YOUNG HERONS As statedabove, it is the subordinatemember of eachpair whichover- tures to its mate and this often leadsto billing. Althoughthe subordinate bird bows,if its partner has its earsplugged, there is rarely any response from the partner. Birdswith pluggedears stand together and defendtheir territow againsttrespassers. Further, if introducedinto a strangecage, they will mark out a new territory and sometimesdefend it together. Hence, overturingand billing are not necessaryfor partner recognitionor mutual territory defense. The responseof the dominantpartner is to the sound and not movementsof the subordinatebird, as the following ex- perimentsshow. (a) Pair 23-26 have earsclosed with cotton,rubber cementand a stitch. When returned to original territory there is no overturing. When trans- forredto a strangecage with otherherons they occupya commonarea and defendit without overturing. (b) Pair 15-32 have ears closedwith cotton and rubber cementonly. When transferredto a new cagethey take up territoW togetherbut do not engagein mutual ceremonies.When earsare openedagain the birds begin at once to overture and bill. (c) Pair 5-28 have earsplugged with cottonand rubbercement only. Head and body of No. 28 sprlnklcdwith blue water color. Pair returned to originalterritory but No. 5 is frightenedby the appearanceof No. 28. By the followingmorning the pair has cometogether again. (d) Pair 23-26 have earsplugged in standardway (as in experimenta). No. 23 is maskedwith a yellow balloonand returned to original cage. Both birdsare drivenby the other birdsin the cage. Within twenty-fourhours the pair hasre-formed. ThE Avco,VOL. 55 PLATE2

YOUNO BLACK-CROWNED NIGHT HERONS Vol.1938JlJ 55] NOBLE, WURM AND SCHMIDT, TheBlack-crowned NightHeron 17

(e) Maskingreversed in pair 23-26. Plugsremoved from earsof No. 23. The latter is no• frightenedby its maskedpartner but when No. 26 calls, No. 23 approachesand they bill. No. 26 seemedto recognizeNo. 23 by its appearance,and was able to attract the latter by callingeven though No. 23 wasobviously disturbed by the appearanceof the maskon No. 26. (f) Both birdsof pair 5-28 have the feathersof their headand neckre- placedby their own breastfeathers. Ear• pluggedin standardway and externalnares plugged with rubber cement. When both birds are returned to their originalcage now emptied of other herons,they standnear together but fail to overture. Normal heronsprogressively added fail to separate pair. (g) Both birds of pair 2-3 have head and neck refeatheredwith own breastfeathers. Ears pluggedin standardway and pair transferredto a cagein whichthey previouslyheld territory. Both birds fight other birds for this territory. Pair transferredto two othercages with otherherons re- form pair both timesalthough slowly (twelve hours). (h) Both birds of pair 23-26 with earsplugged in standardway have head and neck refeathered with own breast feathers and are transferred to a new cage. Other heronsforce them apart, but twenty-four hours later they have cometogether. They fail to overtureor bill. From the above experimentsit is clear that pluggingthe ears greatly reducesor entirelyeliminates the overturingand billing of the pairs. While coveringa bird's face with a rubber mask tends to disturb the partner, neitherthis nor the refeatheringof the head and neckwith the bird's own breast feathersprevents pair formation even in new territory. Young heronswith pluggedears can apparentlyrecognize their partnersby other features than those found on the head or neck. Whether this is head and body movementor merely colorationof the body plumage, our experiments have not shown.

EFFECT OF TIME LAPSE ON RECOGNITION OF REFEATHERED BIRDS

Refeatheringthe head and neck of both membersof a pair of young heronsdoes not prevent partner recognitionafter a lapse of a few hours. This was shownin severalof the casesreported above and alsoin pairs 21-22, 29-31, and 15-32 which in a seriesof tests had their heads and necks refeatheredwith breastfeathers (Plate 2, figs. A and C) but their ears were not plugged. If, however, these refeatheredbirds are isolatedfor six or more days, there is completefailure to recognizepartners, as the followingexperiments show. (a) Birds of pair 29-31 are isolatedin separatecages six days. Color pattern of head is modifiedby the addition of breast feathers. When 18 Nos•,s,WIJRM AND SCHMIDT, The Black-crowned Night Heron [Auk[Jan. birds are placedin Cage 2 where they had previouslyheld territory, the birdsremain apart and showno signof recognition. (b) Birdsof pair 21-22 areseparated for eightdays and thenhave feathers on crown and face replacedby breast feathers. When placedin Cage 4 where they had previouslyheld territory, both birds return to this territory and fight each other for it. There is no evidenceof partner recognition. When pair is moved to Cage 2 where they also had previously held territory they return to it and fight violently with each other. The subordinatebird No. 22 is forcedby No. 21 and the other birds in the cage to a new corner. At no time, in spite of frequentuse of voice,is there partner reeognition. (e) Birds of pair 15-32 are separatedfor elevendays and then have head and neckfeatheirs replaced by breastfeathers. The birds are placedwith other herons,first in Cage2 and then in Cage1, without showingsigns of recognition. In Cage2, whenthe birds contactedin an attempt to occupy their emnmonterritory, they fight eachother. (d) Birdsof pair 2-3 are isolatedfor thirteendays and then have breast feathersadded to headand neck. When returnedto hmnecage they fight between themselvesfor old territory. The subordinatebird, No. 3, is vanquishedand is forcedby partnerto a new positionon the ground. There is no gesturingor signof recognitionby featuresor voice. (e) Pair 23-26 have their headsrefeathered with breastfeathers. When placedin a cagewith other heronsthe subordinate,No. 26, is forcedinto a cornerby the other birds. During a month'sstay in this cagethe groupis neverre-formed even when the numberof birdsin the cageis reduced. Both pairs23-26 and 21-22 had beenpreviously tested for partner reeog~ nition beforethe refeatheringexperiments began. As pointed out above, the latter couplehad recognizedeach other after a separationof twenty days and the former after seventeendays. Young herons,therefore, are able to recognizepartners much longer if the featherlngpattern of the head and neckhas not beenmodified. Evidencehas beenpresented above that young heronsrecognize their partnersalso by differencesin voice. In the refeatheringexperiments reported in this section,the hearing capacity remainedunaltered and yet the youngherons were unable to respondto the auditory cuesafter an isolationof six days or more. Further evidencethat auditory cueswithout adequatevisual ones are unableto holdpairs together was obtained by placingsingle birds of various pairs under screenedboxes in a new cage. When the other membersof thesepairs were introduced into the cage,in no easedid a free bird take up a territory nearits imprisonedpartner even though the latter calledloudly. Hencewhile voiceseems to play the major rgle in initiating and synchro- nizinggesturing, pairs are held togetherprimarily by visualcues. Vol.1938 55]] NOBLE,WURM AND Scm•mT, The Black-crowned NightHeron 19

]•ECKING ORDER AMONG NIGHT HERONS In the fieldwe attemptedto find evidencefor the existenceof a pecking orderamong Night Heronssuch as Schjelderup-Ebbe(1931) described for "speciesof herons." When the young heronsare beginningto fly they frequentlyarrange themselves at differentheights among the branchesof the nestingtrees. Sincethere is an obvioustendency for the youngherons to climb upward it seemedthat the relative height of any heron in a tree mightbe an indexof its superiority.In the laboratory,however, the earliest maturing young securedthe highestperches and defendedthese against youngerbirds. The verticaldistribution in the field appearedtherefore to be an index of age. As many aseleven young herons were found on a single pine at Massapequa,and groupsof six and sevenwere not rare in several othercolonies. As soonas the youngbegin to fly they mark out territories. The ownerof a territoryhas a decidedadvantage in any disputewith a newcomer. In the field at Massapequawe sawseveral instances where a young heronsuccessfully defended this territory againstan adult. In the laboratory, also, adults fled from the onslaughtof a young bird in the latter's territory. It was clear that this infantile territorialismso completely dominatedthe socialbehavior of youngherons that peekingorders, if they existed,would be very difficultto detect. Certainlyin mosteases the bird whichgave way in any encounterwould prove to be the trespasserinto the territory of another. As our experimentswith the pairs of youngherons eontlnued, some un- expectedresults were secured. In pairs29-31 and15-32, the first-mentioned bird wasin eachease dominant (Table 1). Whenthese pairs were broken duringrecognition tests and later crowdedwith other heronsin a strange cagethe new pair 29-32 was producedwith No. 32 dominant. Sincethe latter birdwas a femaleand the formera malethis reversal was an exception to therule that malesare dominant to females.It thereforeseemed possible that there might be presenta peekingorder in whichNo. 32 stoodhigher than No. 29 but belowNo. 15. Attemptsto demonstratesuch an orderby the usualmethods (Masure and Allee,1934) failed. Whenfood is placedin a cagewith the birds they usually fly down to it, one by one, and there is no orderof precedence.If the heronsare starvedfor one or more da:•s severalbirds fly downtogether and foodmay be taken by severalbirds at onemoment. The "socialreflex runway" describedby Murehison(1935) is not suitablefor heronsbecause when forcedto the groundin a strange environmentthey do not run towardone another. Similarlythe social- discriminationcage employed by Murehison(1935) was found to be useless for herons. We weretherefore forced to employnew methodsin our study of the peekingorder in herons. If youngherons are driveninto onecorner of a largecage, the birdsthat 20 NOBLE,W•rRM .•D SCHMIDT,TheBlack-crowned NightHeron [Jan.Auk are strangersin that area tend to flee and thosewhich chance to be resident are the first to revolt againstthe crowdingand successfullyforce all other birdsto yield. In orderto eliminatesuperiority due to territorypossession, a seriesof glass-sidedcages was utilized. Thesehad a floorspace of either 2.8 squarefeet or 4.6 squarefeet, that is, smallerthan the territory of any onebird. Three heronsat a time wereplaced in eachcage and dominance was determinedas in the pairs of unrestrictedherons. There was no over- turlng, exceptwhen pairs wereintroduced, but when two heronsbilled, the bird which kept its head higher was consideredto be dominant. Each group of three heronswas testedfor a whole day and given at least one day's rest beforebeing tested again. The number of billings in any one day by any two heronsvaried from 0 to 147. Every combinationwas testedon at leasttwo differentdays. Birdswhich reacted very little were testedfor as many as ten days. The data securedfrom introducingfifteen birds, three at a time, in all possiblecombinations in the small cagesare showndiagrammatically in text-fig. 2. In very few casesdid a bird which held its head high while billing with one bird, hold its head lower than that same bird when tested again. Thesecases are shownby the symbollight circlein a black square. In GroupA therewas only onesuch pair of birdswhere the right of holding the head higherin billing had not beendecided. Both Group B and Group C (text-fig.2) includedtwo other suchcombinations, 6-29 and 29-36. The chiefcharacteristic of GroupC wasthe lack of billingreaction exhibited by the birds. This group was tested on severaldays and, due to territory- defensereactions and infrequencyof billing,dominance relationships were not completelydetermined. If our criteria of peckingorder are correct thesebirds should be placednear the bottomof the order. Their actual positionwas determinedby the few reactionssecured. Certa/n other exceptionsindicated in the diagramrequire explanation. The two failuresto react in Group A involvedNo. 24 whichwas killed in a fight beforethese reactions could be recorded. This is not to be confused with failure to respond,found frequentlyin Group C. Sincethe birds in the latter grouphad enjoyedthe usualisolation that territory defensegives all Night Herons above a certain age, their failure to react cannot have been due to their previousmistreatment by other herons. It seemsthat birdsat the bottomof the peckingorder described here owed their position to physicalweakness which prevented them from engagingin billing bouts with other heronswhen forcedinto their proximity. In the entire seriesof teststhere was only onesmall groupof birds which engagedin a 'triangle' formation such as Schjelderup-Ebbe(1931) has describedin hens. In GroupA, No. 16 is definitelysubordinate to No. 13 and yet is clearlydominant over No. 27, No. 28 and Br.4. Neverthelessthe i

SEXUAL BEHAVIOR OF BLACK-CROWNED NIGHT I-IERONS Vol.1938 55]] NOBLE,Wvma .•Nz) SCH•IIDT, The Black-crowned NightHeron 21

PECKINC ORDER AMONG IMMATURE NIONT HERONS

__••. ;':zcr '•(••,•o" onTransposeri9h•le•t, andJ'o c[eorreadsolid circles douJn.dob

a" [•)--• G•OUP A

• GROUP •

CROUPC

"• DominoneeReoc•fion UnidirecHonaJ + DominanceReodion bidirectional OnlLjReversed Dominance

_l_I No Confaa•'s Observed

TEXT FIG. 22 NOBL]•,WURM AND SCHMIDT, TheBlack-crowned NightHeron [.Jan.[Auk last threebirds were dominant over No. 13, the 'tyrant' of No. 16. Sucha roundaboutsequence of dominanceif foundin henswould be attributedto one of the socialfactors elucidatedby Sehjelderup-Ebbe(1931) but in heronswith their markedterritorialism other explanationsmust be sought. This groupwas an extremelyactive lot whichnever quieted down entirely in the experimentalcages. It is highly probablethat distractingfactors, suchas the activity of other membersof eachtest group,were responsible for this exceptionalresult. The work on these herons continued from October 24 to November 24, and severaltest groupswere run at one time. The advent of molt in some of the birds made it inadvisableto continuebeyond this point. Although the work wasincomplete it at leastshowed the presenceof a peekingorder whichowed its existenceonly partly to previoussocial experience. That is, GroupA includesthree pairsof birds (5-28, 13-16 and 27-28) whichhad previouslyheld territory togetherin the large cages;Group C, three of thesepairs (36-33, 6-11 and 7-11). The positionof headin billing in the smallcages would be merelya repetitionof the thoroughlyingrained habit begunin the nestingarea beforethe aggregationsof young acquiredthe territory drive. But this wouldnot accountfor the equallywell-fixed order of dominanceexhibited by birds living continuouslyin well-markedterri- toriesand havingno socialcontacts other than territory defense. It seems then that the birds recognizephysical differences unapparent to our eyes and that these determinethe order of dominancewhen any two birds engagein mutual billing. There are, however, some birds which through physicalweakness or temperamentfail to engagein soelalcontacts and are therefore,as stated above, assignedto the bottom of the order.

COMPARISON WITn• Tn•E DOMINANCE AND TERRITORIALISM IN PIGEONS The sequenceof dominancein herons,which we have called a pecking order,regulates the formationof pairs but it seemsto be of little practical valuein the sociallife of captiveherons once the territorydrive has isolated variouspairs of young birds. The questionarose, Might not territorial rights overshadowthe operationof dominancereactions in other groupsof birds? In pigeonsthere are well-markedterritory claims (Taylor, 1932) although neither adults nor young fight for these rights in the vigorous mannerof herons. Masure and Allee (1934), who have investigatedthe peckingorder in pigeons,found some evidence of th{sterritorialism. They state," BB alwaysseemed to do the peckingwhen she was at the entranceof the roost: when BY tried to enter she would be peckedand would retreat. On the ground, however, BY was usually dominant." Schjelderup-Ebbe(1924) pointed out that a bird introducedinto the territory of a strangerwill becomesubordinate to the latter. Hence it Vol.1938 551J NOBLE,WURM AND SCHI•{IDT, TheBlack-Frowned NightHeron 23 seemedto us possiblethat if we studiedpigeons in their own territories,the peekingorder would be maskedin the same manner as in Night Herons under normal conditions. For this study four pa/rs of male and female pigeonswere introduced into smallcages measuring 23 by 11 by 15 inchesand 25 by 19 by 16 inches andtwo additionalmales were placed in singlecages of the samesize. Each cagewas screenedfrom the other and prov/dedwith one or two perches. The pigeonswere well fed for two days and then tested by transferring each male bird into one of the other occupiedcages. In every easethe resident male chargedthe newcomerand drove him in frantic distress againstthe screeningof the cage. The experimentwas repeatedwith the four adult femaleswith exactlythe sameresult. An adult femaleintroduced into a small cage whleh has containedan adult female for two days is vigorouslyattacked by the residentbird. When the attacked female is returnedto her own cageand the formertyrant introducedinto this strange territory the residentbird is exactly as dominantas the tyrant was in her own territory. When the experimentwas repeated with immaturemale pigeons,six weeksof age,nearly the sameresults were obtained. That is, in testswith six youngpigeons only onebird (R-G) whenintroduced into the cagesoccupied for two daysby the other youngpigeons, was able to domi- nate the residentbirds and he did so in only two eases. Sincethis bird swelledits throat while driving, its superioritywas probablydue to an incipient sexdrive. Repeatingthe experimentwith the adultsbut recordingthe heterosexual contactsgave many more exceptions. That is, someresident females were successfulin driving introducedmales but oftenthey retreatedand in some easespermitted the introduced males to tread them. The sexual drive of adult pigeons,therefore, prevents the femalesfrom exhibiting strong territorial claimswhile in the presenceof males.

MECHANISM OF SEX RECOGNITION Male pigeonsrecognize females by their behavior,as Craig (1908)pointed out long ago. What cuesmale Night Heronsmight utilize in sexrecogni- tion havenot beendescribed in spiteof the relativeabundance of the species. We thereforeplanned to study the breedingbehavior both in the field and in the laboratorywith a viewto comparingthe mechanismof pair formation in immatures with that of adults. Eighteenwild adults were securedfrom the National ZoologicalPark throughthe kindnessof Dr. W, M. Mann. Thesebirds were first testedin the large outdoorcages where we kept the younguntil it had beendemon- strated that each bird defendedits immediatesurroundings against the encroachmentsof other heronsand that there were no two birds tending 24 NOBLE,Wl•M .•ND SCUMID?, TheBlack-crowned NightHeran [Jan.Auk to occupythe same territory. They were then transferredthrough the kindnessof Dr. Frank M. Chapmanto one of the live-bird roomsof the Museum,which has a floorspace of 442 squarefeet and a high overhead eelling,allowing us to arrangeperches from three to fifteenfeet from the floor. The birdswere maintained in thisroom at an approximatetempera- ture of 70ø F. andfed porgiesonce a day throughoutthe winter. The first pair courtedFebruary 15, and the last eggof their first set waslaid April 3. At this time anotherpair beganto breedand theirlaying ran synchronously with the earliestovulation we observedin the field, namely,at the Great Neck colony. Four pairs of white adults laid eggs,four in numberat intervalsof from forty to fifty hours,in nestsof their own constructionand at leastattempted to rear young. The eggshatched in from twenty-twoto twenty-fourdays and not twenty-fourto twenty-sixas stated by Gross (1923). Thiswas probably due to ourhigher temperature in the birdroom. Onepair laid twice, eachtime in a differentnest. In addition,seven pairs of first-yearbirds kept in the outdoorcages built nestsand laid eggs. Thesepairs were formed with new partnersindicating clearly that pairing off at the breedingseason involves other factors than thosewhich regulate pair formationbefore the breedingseason. One groupof three first-year birdsbuilt a nestand took turnsat incubatingthe eggs. This groupcon- sistedof Nos. 5, 28, 27, birdswhich had formeda three-memberedgroup during the earlier observations.Later, this group of one male and two femaleswas split at differenttimes into variouspair combinations.The mechanismof mate selectionat breedingtime operatesto break up juvenile pairsbut in this groupof threebirds in a cage150 by 300 by 108 inchesnot only did the pair 5-27 not break but an old partner of both birds was able to join the nestingpair without interference. The exceptionalbreeding behaviorof this groupof birdsis probablydue to partial spayingof Nos. 5 and28. Furtherstudy on thisaspect of the problemis in progress. The behaviorof thesebreeding herons in both the indoorand the outdoorcages was checked against the breedingbehavior of adultsin the field, especiallyat GreatNeck. The latter birdswere not bandedand they could not be capturedfor the verificationof sex. Neverthelesstheir behaviorwas nearly identical with that of the captive birds. We found it therefore possibleto identify sexin the field by the followingcriteria, first worked out with the captive birds. (a) Plumelength.--In any matedpair the maleusually has the longer plumesand sometimesone or two morethan the female(Plate 3, fig. G and Plate 4, fig. B). This was true for six of the sevenpairs of adultsin the laboratory. In all of the first-yearbreeders the malehad a shortplume and the femalenone at all. In the oneexception, the femalehad a plumethree incheslong and the male none. A carefulcheek of fifty-sixbreeding pairs Vol.1938 55]J NOBLE,WUR51t AiNU SCHMIDT, TheBlack-crowned NightHeron 25 at Great Neck, usingtwo or moreof the criterialisted below,revealed fifty- two with longerplumes in the male,three with them equallylong and one with the femalehaving an advantagein plume length. (b) Twig ceremony.--Themale standingover a crude nest platform, or at a distancefrom it, holdsa stick in its bill and loudly snapsits bill on it whileits headis movedrhythmically up and down. Frequentlythe neckis stretchedvertically upward to nearly its full extent and the bill broughtin closeto the neck while the snappingcontinues. Although the stick may eventuallybe placedin the crudeplatform it is moreoften dropped. (c) Snap-hisscereraony.--The unmated male while standingalone on a nest platform but more usuallywhile movingalone about a tree, takes two or threesteps forward, halts, arches the back, lowersthe headuntil the bill is nearlyas low as its feet and then whileraising one foot producesa click or snappingsound in its throat, immediatelyfollowed by a prolongedhiss. The performanceis repeatedwhile either the sameor the oppositefoot is raised. From eight to ten performancesmay be given a minute and the serieslasts over two minutes,to be followeda minutelater by anotherseries. In the laboratorywe werepresent when this call wasproduced by maleRB of pair F, male BH of pair G and by two other maleswhich remainedun- mated throughoutour studies. The call was alsogiven by oneof the first- year birds (RB-G) immediatelybefore it secureda mate. In the field the call was heard many times and on four occasionswe followed its author for longperiods, making sure the bird had no mate. It is thereforeassumed to be characteristic of the male before he secures a mate. As a modificationof this call may be listedthe peck-hisswhich represents a combinationof the usual peck of territory defensewith the snap-hiss ceremony. It is given by the male soonafter a femalehas joined him and beforethe pairedcondition is fully established.It wasbest seen among our birdsin pair F but was alsoheard in the field. (d) Overtureand display.--Whena femalearrives in the vicinity of the breedingmale he overturesand displays. The overtureis identicalwith that of the juvenile bird exceptthat the male often turns his headuntil one cheekis parallelwith the ground(Plate 3, fig. C). At the sametime the maleutters a greetingcall moreguttural than that of the femaleor immature and lessprolonged. The head is then raisedand the featherson crown, neckand back are raised. At the sametime the pupil is contractedand the eyeballactually protrudedfrom its socket,exposing the red iris to its maximum extent. The plumesaxe erected and may even fall forward over the headas the malebows again to the femaleand eitherrepeats the greetingor turns his partly open mouth toward her. The femalein turn repeatsthe salutationincluding the overture, feather raising, plume erection,pupil contractionand eyeball protrusion,but her lower-neckfeathers do not 26 NOBLE,WUI•M AND SCHMIDT, TheBlack-crowned NightHeron [JanAuk protrudeor extendso far laterallyas thoseof the maleand her higher,more prolongedsalutation distinguishes her at oncefrom the male (Plate 3, figs.D-G). Only whenboth birdscall loudlytogether do their voicesseem alike. The male usually overturesfirst and he holdshis head lower than doesthe female. When, however,he displayshe usually holds his head higherthan doesthe female. If the overturingis followedby billing the male still holdshis head higher. At the beghmingof courtshipthe female may not displayat all. (e) Copulatio,n.--Thisact is not immediatelypreceded by any displayor soundon the part of eitherbird. The femalemerely stoops and the male stepsforward on her backand with shufflingmovements of his feet secures a grip on her humerior shoulderswhile bringinghis tail sharplydown and turnedtoward the female'scloaca. He alwaysassumes this dorsalposition exceptin the caseof homosexualunions to be describedbelow. The formationof pairswas witnessed both in the fieldand in the laboratory. In the latter the femalescame directly to maleswhich were guarding crudelybuilt nestplatforms chiefly of their own construction(Pairs C, D, E and F). In the field, althoughthe malesmay take up positionsnear old nests,the femalesseem to be attractedby the snap-hissceremony and not by the adjacentnest. The followingobservation made at Great Neck, April 19, 1937,will illustrate:"A bird, apparentlya male,moves from limb to limb of a tree while giving the snap-hisscall. Betweencalls he would break twigs and fumble them in a modifiedstick ceremony. A secondbird with very pink legsflies to the tree and movestoward the performer. A third bird fliesin but is immediatelydriven off by the secondbird whichis content to remain near the performer." In the laboratorymost of the maleswhich securedmates engagedin a long twig ceremonyover a periodof severaldays beforefemales entered their territories. In the field many singlebirds were seenengaged in this ceremonybut we couldnot be surethat no femaleshad yet enteredtheir territories. The behaviorin the field differedfrom that in the laboratory chiefly in that birds flew about more to the differenttrees. There was considerablesettling down in other birds' territories and a hasty retreat when the residentbirds returned. A residentbird, seeinga strangeron its territory, may fly low and give a raspingcall which appearsto serveas a warning. This is the only callgiven by birdsin the fieldwhich we did not hear in the laboratory. On the basis of the above eriterla for the recognitionof sex we may describethe sequenceof eventswhich takes placein the courtshipof the Black-crownedNight Heron. This representsa compositepicture of many observationsmade at Great Neck during April and May fi•terpretedby meansof data securedin the laboratory. When flocksof heronsreturn Vol.1938 5.51J NOBLE,W•R•i aND SCU•tmT, The Black-crowned NightHeron 27 from winter quartersearly in springthey settledown on or near treeswhich containlast year'snests. ]•ach bird selectsa certainterritory in accordance with the territorialrequirements found even in immatureherons. The males soonmake themselves conspicuous by developingtwo newtypes of behavior pattern: (a) the twig ceremonywhich may be considered'symbolic' of nest building;and (b) the snap-hissceremony which borrows no elementsfrom any other behavior. The breedingfemale is attracted by one or both of theseperformances and settlesdown on the tree occupiedby the male. Out of the breedingseason the male would not tolerate this approachbut the sexdrive hasreduced him to an apparentlower position in the pecking order. Instead of attacking,he adoptsthe attitude of a subordinatebird and overturesto the newcomer. At the sametime his changedphysiology has modifiedhis greetingto a guttural call which further revealshis sex. As the female lingersin this unguardedterritory the male returnsto his stateof dominanceby an elaboratedisplay. This consistso• raisingfeathers of crown,neck and saddleas well as the longplumes of the head. He bows, his pupils contract, eyes bulge and mouth gapes. The latter behavior patternsare availableto the femaleand the male'sgestures stimulate her to respondin kind. Neither her plumesnor neckfeathers equal thoseof the male in extent of spreadand the male graduallyraises his head with frequentdisplays until he has regainedthe sameadvantage in head posture seenin the dominantbird of immaturepairs (Plate 3, figs.C-G). The methodof pair formation,while totally differentin adult from that in immaturecompanion pairs, results in the male of heterosexualbreeding pairs being always dominant. The display,however, has other functions besidesreturning the male to his dominantposition. Observationsin field and laboratoryindicate that the act of copulationoften fails becauseof lack of cooperationof the female. It is obviousthat the male'sdisplay stimulatesthe femalefor she replieswith highervoice and lesserdisplay. It would seem,therefore, that the secondfunction of the displayis to raise both partnersto the sameemotional level in order that the femaleshould bend her body parallel to the nest and permit the deliberate, shuffling movements of the male. The male's twig ceremonychanges gradually to nest-buildingactivity (Plate 3, fig. B). As the nestprogresses the male may bring twigsto the femalewhich remains in the nest and activelybuilds. This stick-passing probablyfunctions as anotherbond to hold the pair together. The male is greatlystimulated by the sight of an eggin his nest,even one taken from anothernest beforehis mate has laid. He at oncebegins to brood and in the laboratory cagesonly males broodedduring the daytime of the first few days of incubation. Later the female assumedmost of the daytime broodingduties and, in a few casesin the laboratory,would remain on the nest until touched. 28 NorL, WURMAND SC•ID•, TheBlack-crowned NightHeron [Jan.Auk

HOMOSEXUALITYIN ADULT'NIGI•T I-IERoN Since the male heron, in order to attract a female into his territory, adoptsthe gesturesof a subordinatebird, the questionremains, May not anothermale occasionallypair with him in exactlythe sameway that two immaturebirds forcedinto the sameterritory may form a pair provided one subordinatesitself? In the field this probably never would happen becausethere is a superabundanceof possibleterritories and the snap- hissingmale while callingoften moves from oneto another. In the laboratory cagesthis is a real dangerbecause of the restrictionof space. One of the reasonswhy only four pairsof our white adultsbred was that two other pairs which formed,were male-malegroups. The formationof both of thesehomosexual pairs was observedin detail Bird R4, drivenby other adultsof the colony,was forced into the territory of B4. This latter bird had beenengaging in the twig ceremonyand when R4, a male, enteredhis territory he loweredhis head and overturedin the mannerof a subordinatebird (Plate 3, fig. C). Bird R• stooda moment in the territory of B4 but when the latter displayedin exactlythe manner he wouldto a female,R• movedaway. Only two other pairshad formedin the bird room at this time and there was still great competitionfor space. Bird R• was forcedagain and again into the territory of B• but he never playedwith the sticksof that bird'snest or evenengaged in twig ceremony. After repeated displays.B4 mounted Ra but the latter immediately flew away. The next attempt was also unsuccessful.The third attempt was initiated by R• which had never displayed. B• remainedin his territory and permitted himselfto be trod. The pair was now formed but B• was unsatisfiedwith his nest and accompaniedby his male mate flew a few days later to a new groupof brancheswhere he begana new nest. The pair remainedtogether and defendedtheir territory againstother birds throughout the breedingseason. Bird R• was observedto tread B4 twenty-one timesand only oncedid the latter avoid theseadvances. On the other hand B• trod R• eleventimes and was avoidedby the latter five times. It will be notedthat B•, the originalowner of the territory, althoughdisplaying, did not succeedin dominatinghis male mate whichmounted him approximately twice as many times as he did the other. Here was a casewhere two males formeda pair becauseone was forced into the other'sterritory. The former malebeing in a strangeterritory showed no aggressionand the latter male becauseof its sex drive was forced to adopt a subservientattitude to the newcomer. In time neither succeededin completelydominating the other, althoughthe newcomerhad an advantageover the residentbird. The secondpair of homosexualwhite maleswas alsoformed as a result of crowding. The two birdswere holding separate territories until drivento the floor of the bird room by a pair seekinga favorablenesting site. One ERECTIONOF PLUMAGEIN NIGHT I-IERONS Vol.1938 55]J NOBLE,WURM AND SCHMIDT, TheBlack-crowned NightHeron 29 bird overturedto the otherand displayed. This subordinationof the overturing bird permitteda further approximationof the couple. A few days later a bondhad formedbetween them and they flew togetherto a nesting site on the top of our observationblind. The one pair of homosexualfirst-year males which formed was not so clearlythe result of crowding. Male RB-B was observedto overtureand displaytoward any bird whichentered his territory. Almostinvariably the newcomerwould becomefrightened and move away. Male RY-Y ap- pearedto be stimulatedby this performanceand remainedto respondwith a similardisplay. Here againthere was no territory defenseby eitherbird againstthe other but strong territory defenseagainst other birds. Soon after the pair had formedthey flew to a new territory whichthey defended against intruders.

I•Ducr•Mr•? Foa The threehomosexual unions reported above gave further evidencethat the malerequires no stimulusfrom the femaleto attempt copulation. The chief requirementis that the one bird of the pair will for the momentat least take a subservientattitude. We frequentlysaw normalcopulations fail merelybecause the femalelifted her headinto the dominantposition. Proof that the femaleis actually subordinateto the male at the time of copulationis shownby the fact that she frequentlyovertures to him immediately after he dismounts. As shown in the study of the immature bird thisis oneof the criteriaof a subordinateposition. We frequentlyobserved both in the field and in the laboratory,copula- tionswithout any preliminaryceremony. In the few caseswhere there was someintroductory behavior this emphasizedthe fact that the male must regainhis dominantposition relative to the femalebefore he will mount. In somecases the femalemay hastenthis process by overturingto the male. The followingobservation made at Great Neck, April 25, will illustrate: "Two birds are standingside by side. The onewith three long plumesis attemptingto break twigs from a branch. The one with shorterplumes overturesto the other, presumablythe male, and attemptsto bill. The presumablemale at last engagesin billing while holdinghis head higher than doeshis mate. Wlfile billing the apparentfemale raises her head until it is higherthan her mate's. This causesthe apparentmale to turn away, but his mate gently pusheshim with her bill and overtures. A minutelater the apparentmale mounts mate in typicalcopulation." On the otherhand if the femaleis not readyfor copulationthe act may not be completedeven though both partnersdisplay. Anotherobservation madeat Great Neck the sameday as the abovewill illustrate:"Male bird, to judgefrom the three longplumes, picks up twigsfrom incompletenest 30 NOBLE,WURM AND SCHMIDT, TheBlack. crowned Night Heron [•aUnk. and engagesin typical twig ceremonywhile erecting feathers of lowerneck. He displaysand apparentfemale, with two muchshorter plumes, responds by erectingfeathers of crownand neck. Male continuesto build nest and then walksto end of limb to breakoff a twig. He bringsit to her but drops it beforereaching her. They stand closetogether, he mountsbut copulation is not effected."

SEXUAL SELECTION IN TFrE NIGFrT HERON It seemedto us a rather significantfact that the two malesof our series of twelve adult malesand five adult femalesthat had damagedcrowns and broken head plumes,were amongthe first birds to build nest platforms, and were the most persistentin both twig and snap-hissceremonies and yet neither securedmates the entire season. Apparently their unusual appearancecaused females to avoidthem. This raisedthe questionof the functionalsignificance of plumes. Lorenz was of the opinionthat these structurescould not be consideredornaments. It may be noted, however, that they are erectedprimarily during the display. They serveno function in territory defensefor they are held flat againstthe neck at this time. Althoughthese feathers increase in lengthduring the breedingseason, their changeis not as dramaticas are two other modificationsof the breeding adult whichhave apparently entirely escaped the noticeof ornithologists. In both our first-yearherons and in our white adultsthere was a marked changein the color of the lore and most of the bill at the beginningof the breedingseason. In both sexesthis turns a dark blue, purplishand finally black (Plate 2, fig. B; Plate 3, fig. F). In our seriesof first-yearbirds the femalesdeveloped blacker lores than the malesbut in our olderbirds, while the blackwas equally intense, the malesdeveloped these dark tonesearlier and held them longerthan did the females. As the loresdarken the upper and lower mandiblesof the white birds alsodarken until at the height of the displayperiod the mandiblesmay be entirelyblack both insideand out. As the pupilscontract and the eyeballsprotrude during the display,these dark loresgive a sharpcolor contrast to the red irises. With the onsetof incubationthey beginto fadeuntil shortlyafter the eggshatch they change to a pale greenishor yellowishin first-yearbreeding birds and to a chalky white in the older adult birds. The secondchange occurred in the pigmen- tation of the legs. Hickey (1937),in discussingthe variationin leg colorof variouseastern herons, states that someLong Island Night Heronsoften have pink legs. Most of our old Night Heronsdeveloped pink legs and feet at the heightof their courtship. Thesewere distinctly redder in the males of two pairs and equallyred in two others. In none of the first-yearbirds was this color attained althoughsome underwent a slight changein leg color. In the field at Great Neck a checkwas made of the leg color of Vol.lO38 55]J NOBL•,WVR• AN•) SCUMX•)T, TheBlack-crowned NightHeron 31 over forty-three adult mated pairs. In thirty-eight both sexeshad pink legs,in four the femalehad yellowetlegs than the male, in onepair of white birds both sexeshad equally yellow legsand one lone male seemedto have yellow legs. Hence, although the laboratory white adults demonstrated a changeof leg colorduring the breedingseason, field observationsindicated that not all breedingbirds attain the full pink color. It may be notedthat the tendencyfor malesto attain the full colormore often than the femaleis correlatedwith the greateruse the male makesof his legsduring courtship. During the snap-hissceremony the limbs are rhythmicallylifted and ex- tendedas if to catch the eye of a female. We observedin the laboratorythat courtshipactivity was"contagious" in the sense that the movements of one bird started the others to court. Similarly, the courtshipmovements of the male when repeatedby the female serveto stimulateher. When one bird of a pair of innnatureherons fliesto its territory it givesa greetingcall whichserves to identifyit. The partnerresponds and they bill. We showedthat wherethe partner could not hear there was tolerationbut few contacts. Similarlyin the adult pairs the greeting identifies the arrival and makes possiblefurther contacts. Paired birds, even after the eggshave beenlaid, when returningto the nest combinea displaywith the greeting. Sincethe male's displayis greater than that of the femaleit servesto keep him in a state of dominancein exactlythe sameway that a superiorhead posture functions in the immature. Overturingand billingare muchrarer in both first-yearand white breeding pairs than in juvenile companionpairs. The fact that both sexesof adult pairs display would seem to indicate, however,that both are sexually stimulated.This Huxley(1921) has called 'mutual courtship.' Carpenter (1933) has showedthat the courtshipof p;geonsmay be subdividedinto actsrequiring different thresholds of provocation. These actsin a definite order "serve to synchronizethe degreesof sexual exaltation in the two animalsof a pair." There is, therefore,some evidence, if only indirect, that a mutual selectionsuch as Huxley postulatedmay have taken place to fosterthe genesisof epigamiccharacters in the Black-crownedNight Heron. Thosefeatures, common to both sexesduring courtship, include (a) black loresand mandibles,(b) red iriseswhich can be expandedby contraction of the pupils,and (c) white plumesset againsta blackcrown. :Futurework may showthat other heronsundergo the seasonalchange of lore color which we have found in the Night Heron. Thus the pairs of LouisianaHeron (Hydrana•satricolor) which Huxley (1922) foundto have differentshades of lore colormay not have owedthese differences to any selectionof hrlght-loredbird by a similarvariant. Rather, thesedifferen- tially coloredpairs were in differentstages of the breedingcycle. Possibly the bright loresare attractivebut all heronsof this specieswould tend to have them at the heightof the courtshipseason. 32 NOBLE,WUBM AND SCHMIDT, TheBlack-crowned NightHeron [Jan.Auk

Ge•es•s oF Dom•A•ce The earliestfeeding response which we observedamong the youngherons (fourteendays of age) was an attempt at graspingand strokingof the parent'sbeak. No systemof rotation of either parent'ssupplying food or of feedingyoung was noted. The oldestand largestbird in the nestsecured the greatestquantity of fish. The parent frequentlysupplied large pieces of fish,when a tug-of-warresulted with the oldestbird the victor. At about threeor four weeksit wasobserved that the olderbirds vied for the privi- legeof strokingthe parent'sbeak. In suchinstances each bird attempted to keepits headhigher. Frequentlybills were contacted, opened and shut and the opponent'shead pushedback. Later thesemovements became moreprecise; the smallerand presumablyyounger birds assumeda subservientposition of the headevery time they billed with their olderbrothers or sisters. In this way the billingbehavior between young in a nestseemed to be an outgrowthof the feedingresponses of the young. From approximatelyfive daysof age,young herons indicate a form of antagonismby a continuouserection of crownfeathers during the presence of the observer. At about ten days of age, the oldest bird in the nest initiatesan exaggeratedform of this reaction,namely, spread wings, erected crown feathers,and a peck followedby the screechcall. Parents,when enteringthe nest, issuea recognitioncall accompaniedby an erectionof crownand neckfeathers. Immatureherons, approximately nine weeksold, overtureto their parents,exchange recognition calls and bill with them. The parentsmay erectthe feathersof crownand upper part of the neckbut thoseof the lowerneck never stand out in typical courtshipmanner. Young heronsapproximately nine weeksof age recognizetheir parents and the latter distinguishtheir own young from other immaturesof the sameage. This waswell shownby transferringfour youngof the first-year birds to differentnests having young of the same age in the samecage. During a singleobservation period of six hoursthere weretwenty-seven con- tactsbetween the foreignyoung and residentyoung. The transferredyoung, whetherin the new nest or within a distanceof eight feet of it, billed with residentyoung until one or moreyoung assumed the subordinateposition of the head. There was no attempt to secureterritory by the transferred birds and only the residentyoung gave food calls. Twenty-twocontacts betweenadults and youngin this experimentwere observed. The adults peckedonly the foreignyoung but this attack wasnot as viciousas it would have been againststrange adults. Parentsresponded to the recognition callsand food calls of their own young only. Adult pairs continuedto ex- changerecognition calls while erectingcrown and upper-neckfeathers. Groupingand interchanging the youngof a numberof nestsdid not result in territory-defensereactions between young. It would thereforeseem that Vol.1038 55]J NOBLE,Wul•ltl AND SCHMIDT, The Black-crowned Night Heron 33 the feather erectionof young birds is a defensemechanism developed beforethere is any claimingof territory. The first evidenceof territory defensebegins when the youngleave the nestand climbabout in the nesting tree. In brief,defense responses begin in the ontogenyof the Night Heron longbefore there is any territory defense. Contraryto the viewsof Schjel- derup-Ebbe(1924), a peckingorder, that is a dominancehierarchy, may appearin nestlingsof the heron,but when territorialismlater appearsit completelyobscures this order in all exceptpaired birds or in grouped individuals from the same nest.

DISCUSSION Throughoutour work we have employedthe term "territory" in a broadersense than usually found in the ornithologicalliterature. The term has recently been the subjectof critical review (Friedmann,1935; Tinbergen,1935; Mayr, 1935;Portielje, 1936; Lack and Lack, 1936). The most comprehensivedefinition is that given by Tinbergen (1936). He statesthat territoryis "an areawhich is defendedby a fightingbird shortly beforeand duringthe formationof a sexualbond." Breedingmale heronsdefend their immediatevicinity againstthe encroachmentof aggressivebirds. But if a non-aggressivemale or female enters one's territory, the resident male assumesa subservientattitude and in this way invitesthe newcomerto stay. A courtingmale doesnot loseterritory entirely. If a bird guardinga neighboringterritory should encroachupon his domainhe thrustshis headforward in a vigorousdefense reactionindicating that territory-defensebehavior is still availablefor use against aggressivetrespassers. There is no doubt that the territory of the adult Black-crownedNight Heron is identicalwith that of the immature althoughit may embrace morearea. We haveshown that two immatureherons, strangers to each other,may form a pair if forcedinto the sameterritory while too harassed to exhibit typical defensereactions. Similarly, two adult malesmay form a pair whichwill remainfaithful to eachother throughout an entire breeding seasonprovided they are driven into the sameterrlWry at the time one of them, becauseof its reproductivedrive, is forcedto adopt a subservient attitude. Territory has, therefore,other functionsthan usuallyassumed. It aids the formation of bonds between two birds which are forced to occupya commonarea. In Nature the femaleis attractedto the male by hissnap-hiss or twig ceremony.Being in a strangeterritory, she would not adopt any aggressiveattitude toward the male. The latter in his turn is forcedby his sexualdrive to adopt a subservientattitude. The result is exactlythe sameas if the birdswere forcedtogether in a strangelocality whereneither would developa defenseresponse. 34 NOBLE,WURM ANI) SCItMII)T, The Black-crowned NightHeron [Jan.Auk

Oncea bondhas beenformed, the pair may leave the territory together and build a nestin a new locality. We notedboth in the laboratoryand in the fieldthat suchpairs easily drove resident birds from the territoriesthey had been defending. The display of the heron has no rSle in territory defense. In other birds it has been assumed to make the male dominant to the femalein orderthat copulationmay proceed(Pottlelie, 1936). Our detailedobservations indicate that it has this important function in the heron as well, even though the responseof the female may alsoserve to stimulatethe male. Sincemales giving the snap-hissceremony may have no nestplatform in their immediatevicinity, it is obviousthat nestsdo not play as importanta rSle in the formationof pairsof Night Heronsas they doin someother birds, such as the GreyHeron described by Verwey(1930). It is highlyprobable that otherspecies of Ardeidaewill be foundto have a socialsystem very similar to that of Nycticorax. Many featuresof the behavior of American Ardeldae given by Bent (1926) indicate that the courtshipsof the differentspecies have much in common. Nevertheless, the life history of no other Americanspecies is known in sufficientdetail for a closecomparison with the behaviorof Nycticoraxas describedabove. It is necessaryto turn to Verwey'sdescription (1930) of the EuropeanGrey Heron for these details. Verwey describesboth young and old Ardea citetea as directingvicious thrusts toward othersof their own kind and his description(p. 15) of this behaviorin the field indicatesthat A. cinerea makesthe sameterritory claimsas doesthe Night Heron in fleedom. He statesthat as soonas the young Grey Herons begin to fly they developa greetingcry which suggeststhat individualswithin the family grouprecog- nize oneanother as in the caseof the Night Heron. No attempt wasmade by Verwey to identify bondsbetween the youngherons but the description of Holstein (1927), as quotedby Verwey, indicatesthat there is an even more severecompetition between the youngGrey Heronsof the samenest than amongnest mates of Night Herons. Nevertheless,Verwey raises the questionas to the functionalsignificance of this struggle. It would seem certainfrom Beetham's(1910) accountof the struggleamong nestlings of Ardea purpurcathat age and superiorbilling ability aid in securingfood fromthe parent. The samestruggle was seen by usin youngBlack-crowned Night Heronsand wasshown to give riseto the specificdominance reaction which we call billing. Lorenz(1934) found that youngNycticorax n. nycticoraxare recognized, fed and protectedby their parentsoutside of the nest. We haveobserved no feedingof youngoutside of the nestarea in our Americanrace. Never- theless,parents distinguish their youngfrom other youngand the latter candistinguish their parentsfrom otheradults. In the Grey Heron adults may feed strangeyoung as well as their own, indicatingthat there is less recognitionof youngthan in the caseof the Night Herons. Vol.1938 55]] NOBLE,WURM AND SCHMIDT, TheBlack-crowned NightHeron 35

The Grey Heron exhibitscertain movementsnot found in the Night Heron but otherwisethe courtshipbehavior of the two specieshas much in common. The male Grey Heron attracts the femaleby a modiftedflight call and not by a snap-hissceremony described above. Thereis no typical twig ceremonyin the GreyHeron although the bill eliekingpracticed by the male only might be considereda twig ceremonywithout twigs. The neck of the mate is stretchedforward and slightlydownward, not upwardas in the typlealtwig ceremonyof the Night Heron. De Waard (1936) claims that the female may occasionallyposture and click the bill in the manner of the male. If this is true, the bill eliekingmay be merelyan overture movementpracticed in the Grey Heron primarilyby the male. The stretchmovement of the Grey Heron seemsto correspondto the overtureand displayof the Night Heron modifiedby a vertical posturing of the neck. As in the easeof the Night Heroh'sdisplay, this is at first practicedby the male only but later by both sexes. If our interpretation of this movementis correct,it wouldserve the doublefunction of securing dominancefor the male and stimulatingthe female to sexual activity. When adult Grey Heronsare pairedthey exchangegreeting calls. Verwey foundthat the youngGrey Heronsfailed to retain this call after the young left the vicinity of the nest. Our captivebirds, being kept in closecontact throughoutthe winter, retained their greetingcall and we were able to showhow this call togetherwith the overturewas taken over into the pattern of the adult. The Grey Heron has one call that has no analoguein the Night Heron. After an unsuccessfuleopulation the femaleGrey Heron uttersa distinctive cry, accordingto Verwey. The femaleNight Heron either remainssilent or overtures. The overturing tends to facilitate the secondattempt because it placesthe femalein a subservientposition to her partner. No evidence has been found in our studies of the existence of a 'releaser' suchas Lorenz (1935) described,that is, of an "organ of a peace-making ceremonywithout any sexualmeaning" (Lorenz, 1937). The plumesare held in an inconspicuousposition during the defenseresponse and also duringovertures. Steinfart(1934) describes these plumes as beingerected during the attack of the closelyrelated Nycticoraxn. nycticorax. This occursin the Americanrace only when an attack doselyfollows a display. Only duringthe breedingseason when the displayis combinedwith the overturesare the plumeserected at the sametime that the heronbows low. In the Yellow-crownedNight Heron (Nyctanassaviolacea) Nice (1929) describesa easeof nestrelief with erectionof plumes. In the Black-crowned Night Heronsuch a reliefis usuallyaccompanied by recognitioncalls and the plumedisplay, if it occurs,is part of the courtship. That this display is stimulating,is an inferencebased on the fact that a heronusually responds 36 NOBLE,WURM AND SCHI•IDT, TheBlack-crowned NightHeron [Jan.[Auk to the displayof its mate and alsoon the fact that early in the seasonbefore there has been much displaythe femaleBlack-crowned Night Heron will not allow the male to mount. Two of our maleswith damagedcrowns and plumesfailed to securemates although they courtedfor longperiods in the presenceof unmatedfemales. As we have shownabove, the erectionof the crown featherstakes place in a wide variety of socialsituations while the erectionof the plumesoccurs only aspart of the courtshipdisplay. Whether the failure of thesebirds to securemates was due to their shortplumes or the unusualappearance of the crownscannot be stated. We have shownin the youngherons that pairsform beforeplumes develop but thesepairs are not sexuallyactive. The fact that malesof sexuallymature pairs usually showlonger plumes than the femalesis correlatedwith the fact that the male must securedominance over the female before eopulationmay pro- eeedand, in the processof securingthis dominance,the male displaysmore extensivelythan the female.

CONCLUSIONS 1. The struggle for food among the nestlingsof the Black-crowned Night Heron leads to a hierarchyof dominancewhich is maintainedin older nestlingsby billing reactionstoward one another. 2. As flight developsyoung Night Herons claim territorieswhich they defendagainst other herons. Territorial requirementstend to break up the groupsof nestlingsand a strugglefor dominancein any one territory tendsfurther to disintegratethe group but where dominancerelations are well establishedimmature pairs may remaintogether for monthsdefending commonterritory even in new cages. 3. The new territorial requirementscompletely mask the dominance hierarchyamong birds maintainedin large cagesexcept in the caseof pairs or nestling groups. Crowding the birds into a single strange territory causesthe reappearanceof the dominancehierarchy. 4. Territorialism may similarly prevent the functioningof a pecking order in pigeons. 5. Membersof pairs of immatureherons may recognizeeach other after a separationof twenty days. Details of voiceand featberingserve as cues in this recognition. Separationof membersof a pair for only six days is followedby failure to recognizeif the color pattern of the head has been modifiedby artificial refcathering. 6. Pluggingthe ears preventsa synchronizationof greetingceremonies, indicating that soundis more important than movementin calling forth these responses. 7. New pairsof immaturebirds may be formedby crowdingtwo indi- vidualsfor shortperiods in a commonterritory providedone bird acceptsa Vol.1938 •] J NOBLS,WURM AND SCHMIDT, TheBlack-crowned NightHeron $7 subservientposition to the other. Homosexualmale pairs may be formed duringthe breedingseason by the samemethod. Thesepairs remain stable in new territory. 8. The normal formationof heterosexualpairs in breedingbirds differs from that of the immature. The male attracts the femaleinto his territory by his snap-hissand twig ceremonies.At the sametime the maleassumes a subservientattitude toward birds of either sex enteringhis territory althoughhe still attacksneighboring males which encroachin a belligerent mannerupon his territory. 9. Adult pairs practicethe greetingceremony of immature pairs but duringthe courtshipperiod display while making the gesture. This display differs from the feather erection of other social situations in that in addition to raisingthe feathersof crownand upperneck those of the lower neck are alsospread while the long crownplumes are erected. At the sametime the pupilscontract, eyes bulge and mouthfrequently gapes. The greetingcall of the courtingmale alsobecomes more guttural than that of the female. 10. The plumesof the crownare onlyerected during the courtshipdisplay and have no function in territory defenseor in pacifying approaching individualsas previouslyreported. Sincethe displayof the male induces the femaleto gesturein the samemanner, the performanceis presumably stimulating to the latter. 11. The plumesof the male are usuallylonger than thoseof his mate. Two maleswith damagedplumes failed to securemates, suggestingthat femmeswill not pair with malesof unusualappearance. 12. The displayand billing ceremoniesof the male Night Heron tend to return him to a state of dominancein orderthat eopulatlonmay occur. 13. There is a seasonalchange in colorof lores,mandibles and legsof the Black-crownedNight Heron. The loresand bill of the courtingbird tend to becomebluish black, the legspink. In somebreeding birds of the first yearthe mandiblesmay not attain this colorand the legsvery rarely under- go the completechange. Femalesusually lag behindthe malesin plume growth and often in leg reddening. 14. The tendencyfor the malesto have pinker legsis correlatedwith a greater display of their legs during the courtshipceremonies. Greater plumelength is correlatedwith the necessityof malessecuring dominance over their matesbefore eopulation may proceed. Mutual selectionmay have aided the genesisof striking color contrastsof plumes,eyes and lores becausethe ceremoniesof overtureand displayare mutual. Nevertheless, there are sexual differencesin this display which eventually lead to the male securingthe necessarydominance. NOBLE,WURM AND SCHMIDT, TheBlack-crowned NightHeron [Jan.Auk

REFERENCES ALLEE, W. C. 1936. Analytical studiesof group behavior in birds. Wilson Bull., 48: 145-151. BEETHAM,B. 1910. The home-lifeof the Spoonbill,the Stork and someherons. London. BENT, A. C. 1926. Life histories of North American marsh birds. Bull. U.S. Nat. Mus., no. 135: 197-213. CARPENTER,C. R. 1933. Psychobiologicalstudies of socialbehavior in Aves. Journ. Comp. Psychol., 16: 25-98. CRAIG, W. 1908. The voicesof pigeonsregarded as a meansof socialcontrol. Amer. Journ. Sociol., 14: 86-100. DE WAARD,S. 1936. Blauwe Reiger. De Levende Natuur, 41: 159-160. FRIEDMANN,H. 1935. Bird societies,in: Murchison,C., 1935, A handbookof socialpsychology, Worcester,Mass., Clark Univ. Press,Chap. 5. GBOSS,A. O. 1923. The Black-crowned Night Heron (Nycticorax nycticorax naevius) of Sandy Neck. Auk, 40: 1-30; 191-214. HICKEY, J. J. 1937. Notes on leg colorsof white herons. Condor, 39: 131. HOLSTEIN,V. 1927. Fiskehejren. Kopenhagen. HUXLEY, J. S. 1921. The accessorynature of many structuresand habits associatedwith courtship. Nature, 108: 565-566. 1922. Preferential mating in birds with similar coloration in both sexes. British Birds, 16: 99-101. LACK, D., ANDLACK, H. L. 1936. Territory. Somerecent American work. British Birds, 29: 255-258. LORENZ,K. 1934. Beobachtungen an freifliegendzahmgehaltenen Nachtreihern. Journ. f. Ornithol., 82: 160-161. 1935. Der Kumpan in der Urnwelt des Vogels. Journ. f. Omithol., 83: 137-213; 289-413. 1937. The companionin the bird's world. Auk, 54: 245-273. MASURE,R. H., ANDALLEE, W. C. 1934. The socialorder in flocksof the commonchicken and the pigeon. Auk, 51: 306-327. MAYR, E. 1935. Bernard Altum and the territory theory. Proc. Linn. Soc. New York, 1933-34, nos. 45, 46: 24-38. MUBC•[ISON,C. 1935. The experimental measurementof a social hierarchy in Gallus domesticus. I. The direct identification and direct measurement of social reflex No. 1 and social reflex No. 2. Journ. Genet. Psychol., 12: 3-39. II. The identification and inferential measurementof socialreflex No. 1, Vol.1938 J NOBLE,WURM AND SCHMIDT, TheBlack-crowned NightHcro• 39

and social reflex No. 2 by means of social discrimination. Journ. Soc. Psychol., 6: 3-30. III. The direct and inferential measurement of social reflex No. 3. Journ. Genet. Psychol., 46: 76-102. NINE, M. M. 1929. Some observationson the nesting of a pair of Yellow-crownedNight Herons. Auk, 46: 170-176. PORTIELJE,A. F. J. 1936. Ein bemerkenswerterGrenzfall yon Polygamiebezw. accessorischerProm- iskuitltt beim H0ckerschwan, zugleich ein Beitrag zur Ethologie bezw. Psychologieyon Cygnusolor (Gm.). Journ. f. Ornithol., 84: 140-158. SCHJELDERUP-EBBE,T. 1924. Zur Sozialpsychologieder VOgel. Zeitschr.f. Psychol.,I Abt., 95: 36-84. 1931. Die Despoticim sozialenLeben der VOgel. Forsch.V01kerpsych. Soziol., 10: 77-137. STEINFATT,O. 1934. Ein Beltrag zur Kenntnis der Naturgeschichte,insbesondere des Brutlebens des Nachtreihers,Nycticorax n. nycticorax. Beitr. z. Fortpfianzungsbiol. VOgel, 10: 85-96. TAYLoI•, W. S. 1932. The gregariousnessof pigeons. Journ. Comp. Psychol.,15: 127-131. TINBERGEN,N. 1935. Over de Betekenisvan "Territorium" in her Leven der Vogels. Vakblad v. Biol., 16: 95-106. 1936. The function of sexual fighting in birds; and the problem of the origin of "Territory." Bird-banding, 7: 1-8. VERWEY•J. 1930. Die Paarungsbiologiedes Fischreihers. Zool. Jahrb. (Allg. Zool. u. Physiol.), 48: 1-120.

EXPLANATION OF PLATES

PLATE 2 Young Black-crownedNight Herons Fig. A. Immature, approximately four months old, showing pale-greenishlore, upper bill black shadingto olive and lower bill olive. Fig. B. Sexually mature Night Heron, approximately thirteen months old, showingdark lore, bright black beak and well-developedplumes. Both the brown and the white adults undergoa seasonalcolor changeof loresand beaks. Fig. C. Same bird as in Fig. A, sameage, showingarrangement of its own breast feathers which were attached with rubber cement to crown and side of face for recognition tests. PLAT• 3 Eight stagesin the sexualbehavior of Black-crownedNight Herons Fig. A. Territory defense. The bird on the right with damagedcrown and plumes failed to securea mate the entire breedingseason. Fig. B. Twig ceremony. Although the female will fondle sticks, only the male lifts his head vertically upward and clicksstick loudly. Many sticksemployed in this ceremonyare droppedbut someare built into the nest. 40 NOBLr,WURM AND SCHMIDT, TheBlack-crowned NightHeron [Jan.Auk

Fig. C. Male overtures. If the female remains in his vicinity the male (left) lowershis head and bringscheek parallel to nest while giving a guttural greetingcall not heard outside of the breeding season. At the same time he may fan his lower neck feathers as shown here. Fig. D. Male displays. Erecting his crown feathersand plumes, raising his saddle feathers and puffing up all the feathers of his neck, the male (right) repeats his guttural greeting while bowing to the female. At the same time his pupilscontract exposingthe red irisesto the maximum and the eyeballs protrude slightly from the head. Fig. E. Attempt to rega/n dominance. The male, while continuingto ru•e the feathers of neck and back, raiseshis head toward the level of the female's head. Fig. F. Billing for dominance. The male to be dominant must bill with head higher than that of the female. While fencingfor this position,the male continues to display. Fig. G. Display following successat securingdominance. Both sexesdisplay in exactly the same way. The plumesof male (right) are longer and neck feathers are fanned more extensively(compare Figs. F, E, D; Plate 4, fig. B). Fig. H. Copulation. The male (above) balanceshimself on the humeri of the female, his bill touchesher crown and his tail is brought down and turned sideways.

PLATE 4 The plumes of the Black-crownedNight Heron are erected only during courtship. In other socialsituations other feathers may be raised. Fig. A. Plumagein territory defense. The bodiesare held low while someof the feathers on crown, neck and saddleare erected. The plumesare laid fiat against the back and fail to show while the birds exchangepecks. Fig. B. Plumage in courtship. Male (left) erects his three plumes, while raising feathers of neck and saddle. The female (right) responds with erected crown feathersand singleplume. Her pupils contract and eyesbulge. Fig. C. Plumage in dominance reaction. Male (left) holds his head in the superiorposition when billing with the female, his mate. At the same time he puffs slightly the feathers of head and neck but fails to erect the plumes. Male on the right is standing on extreme left of his territory, while bird in foregroundwith damaged crown and broken plumes maintains the territory he occupied alone throughout the season. The zimericanMuseum qf Natural History New York City