Sympatry of Laysan and Black-Footed Albatrosses

SYMPATRY OF LAYSAN AND BLACK-FOOTED ALBATROSSES

HARVEY I. FISHER

IF we assume that albatrossesarose in the Southern Hemisphere where the greatestnumber of formsoccur today, the three speciesin the North Pacific are derivativesof southernstock. They are isolated from sig- nificant ingressof genotypesfrom the southernspecies by the broad band of warm, unsuitablewaters near the equator. Few southernspecies are known even as vagrants north of 20ø S. The extreme southern recordfor a North Pacific speciesis 8ø N (Thompson,1951), and most individuals stay north of 20ø N. The three speciesof North Pacific albatrossesare: the Laysan, Dio- medeaimmutabilis; the Black-footed,D. nigripes;and the Short-tailed, D. albatrus. The Short-tailedAlbatross is outsidethe scopeo.f this paper becauseits closeaffinities appear to lie with the great albatrosses,D. exulansand D. epomophora,rather than with the Laysan and Black- footedspecies. It probablyrepresents a northwardimmigration separate from that of the other two species. The purposeof this discussionis to present someviews of the origin and subsequentsympatry of the Laysan and Black-footed albatrosses over virtually all their breedingrange. These two specieshave much in common, despite the major difference in coloration. The more sig- nificant similaritieslie in the bloodproteins (Brown and Fisher, 1966) and the courtship display, which is more complex than in Southern Hemisphereforms; both sexes regularly participate and exhibit the dotsumrather than the ventrumof the wing to the partner. The species are close in body size, breedingseason, incubation period, pattern of sexualparticipation in incubation,and vocalization.And in many of thesefeatures they alsoshow important variations from southernspecies.

PRESENT HABITAT

Knowledgeof the habitat requirementsof the two speciesis necessary for the rationale of the original separationinto two forms. The data presentedrelate solely to the breeding grounds,and primarily to the major coloniesin the western Hawaiian Islands, for virtually nothing is known of their ecologicaldistribution or behavioron the high seas. Both specieshave, or had within historical times, a similar if not identicalbreeding range extending from the Hawaiian Islandsto Torishima (Bird Island) south of Tokyo and lying approximatelybetween 20 and 30ø N (Rice and Kenyon, 1962a). Breeding has been limited to half

351 The Auk, 89: 381-402. April 1972 382 HARVEY I. FISHER [Auk, Vol. 89

Fibre 1. Bree•ng colones of albatro•es on Middy Atoll. Top: La•a• M the interior of E•tern •land. MMdle: B•ck-foote• on the beach of Sand Is•nd. Bottom: M•ed colony on E•tern Island where wave erosion destroyed B•c•oot colony. April 1972] Laysan and Black-looted Albatrosses 383 a dozen small islandsin the eastern part of the range and even fewer volcaniccinder conesin the west. Where the two speciesbreed on the sameislands, and this was true in nearly all instancesprior to the inter- vention of man, the coloniesare basicallyseparate and the Laysansare much more numerous than the Blackfoots. The difference in numbers may be related to two factors; the total habitat suitable for Blackfootsis less extensive.and they appear to nest in less densecolonies. The Black-footedAlbatross utilizes the exposed sandybeaches and adjacentfringes of islands(Figure 1, middle). Seldom does a Laysan Albatross nest in a wind-blown sandy place on the edge of an island, and when it does the nest is usually unsuccessful.The Laysan nestsin the brush-protectedinteriors of islands (Figure 1, top). Black-footedAlbatrosses nest where there is little or no vegetation at or surroundingthe nest site. Even when they breed successfullyin the vegetatedinteriors of islands, they do not use grassyspots and do not build the conicalnests typical of colonieson sand beaches.The Laysan nestson grass,coral rubble, and patchesof sand and dirt betweenclumps of vegetation. Both speciesprefer sites that provide loose material the birds can drag together to form the nest cone. This ecologicalseparation of breeding colonies,except where their edges may meet, is evident on undisturbed atolls such as Lisianski in the Hawaiian Chain. On disturbedislands the habitat of both species has been altered, and care must be taken in assessingthe requirements for successfulnesting. Military constructionhas removedthe vegetation in many instancesand providednewly exposedsandy rubble. The Black- footed Albatrossesoften pioneer such areas. Furthermore dredge de- positssometimes provide new shoreoutside the old Black-footedcolonies. On Sand Island of Midway, someBlackfoots nest a quarter of a mile inland, at the site of the original shoreline and separated from the more recently established colonies near the. current shoreline. Ap- parently they are only occupying long-held breeding grounds where, as photographsshow, the vegetationhas appearedin the last 30 years. The shorelineof Eastern Island of Midway was less disturbed,and only one pair of Blackfoots nests more than 250 yards from the sea. Constructionof the concreterunway and its macadamaprons along the south shore of Sand Island preemptedsome space of former Blackfoot colonies and forced the birds to retreat to the nearest suitable habitat farther inland. Secondaryevidence of this is the presentcrowded condi- tion of thesenew colonies;nests are much nearer each other than in the seaside colonies. The natural habitat of the two speciesis best indicated by the un- disturbedconditions on Midway at the turn of the century. Old accounts 384 H^Rw¾ I. FXSH•R [Auk, Vol. 89 refer to Eastern Island as Green Island becauseof its heavy vegetation, and they indicate that Laysansfar outnumberedBlackfoots. Early in- habitants also noted that many Black-footedAlbatrosses and but few Laysan Albatrossesnested on Sand Island, which had little vegetation. The Laysan populationon Sand Island apparently grew as the vegetative cover increasedthrough the efforts of employeesof the Pacific Cable Companysubsequent to 1903 (Rice and Kenyon, 1962a).

DIVERGENCE OF LAYSAN AND BLACK-FOOTED ALBATROSSES

As Hutton (1903) and Richdale(1950: 26) point out, the Blackfoot showscloser affinity than the Laysan with the SouthernHemisphere speciesof albatrosses.The adult Blackfoot is brown to black in color, and the young are dark-downed. Blackfoots raise both wings simul- taneouslyin the courtshipwing-lift. The specieshas a more widespread breedingrange than the Laysan. It nestssuccessfully in places where the Laysan fails, and the Blackfoot is known to be successfulin oc- cupyingLaysan habitat. The Black-footedAlbatross appears to be the more adaptable,generalized, and older form. For these reasonsit is postulatedthat D. nigripesis closer than D. immutabilis to the original stock of albatrossesthat formed the Blackfoot-Laysan line. Once this original stock becameisolated on two or more islets, the smallpopulations could have divergedsimply on the basisof accommoda- tion to breeding on bare sand islands (Blackfoot) or on isles almost completelycovered with vegetation (Laysan). Bare sand precedes vegetatedsand in the terrestrialsuccession in atolls, and the geological history of the Hawaiian Islands indicates the continuouspresence of low sandy atolls. Thus albatrosseswith the habitat preferencesof Blackfootscould alwaysfind suitablenesting places. The Laysan type of nestinghabitat in the North Pacific appearedlater and initially must have been limited in extent. No albatross except the Laysan nests at sea level, on flat terrain, and in tall, dense vegetation, much less heavy, woody brush. The Laysan thus utilizes a nestingsituation at least partly new for albatrosses.Although both speciesof this albatross line are relatively small-bodied,the Laysan's body is the smaller, perhaps as an adaptationto movementin the brush. Consideringthe geographicalisolation of the entire breeding range from invasion by additional forms of albatrosses,the isolation of the atolls within the range, the originally small populationsof breedingbirds, and the proclivity of the birds to. return only to. the natal colony to breed (Fisher, MS), divergenceand evolutionof the two forms may have proceededrather rapidly. As the birds exploitedthe predator-free land and the rich energysources of the northern waters, the populations April 1972] Laysan and Black-looted Albatrosses 385 may have outgrown the space originally occupied and forced younger birds to seek other suitable islands. Some islands occupiedsecondarily may have provided satisfactoryhabitat only for a Laysan-typeor for a Blackfoot-typealbatross. Thus further reproductiveisolation took place and, as populationgrowth causedadditional emigrations,other islands were used, islandssuitable for coloniesof both forms. In this way the Laysan and the Blackfootcame into near sympatry; the colonieswere contiguousperhaps but not overlapping. As wave action eliminated the narrow seashoreareas occupiedby Blackfoot coloniesand forced inland the birds bound by tradition to return to that island and colony, and as the vegetationalsuccession began, along with probableincreases in the breedingpopulations of one or both species,the originallysharp borders betweencolonies of the two specieson any singleisland became obscured. Black-lootedAlbatrosses nested side by side with Laysan Albatrosses (Figure 1, bottom),and on smallislands this zoneof overlappingcolonies constitutedsignificant sympatry. By this time the two forms were suf- ficientlydistinct that interbreedingseldom occurred.

INTERBREEDING

Today the amount of interbreedingis almost negligible; reports of hybrids are few. Rothschild (1893-1900) found one on Laysan Island, and a member (Palmer) of his party saw a Laysan Albatross feeding what appearedto be a Black-footedchick. Hadden (1941) saw an adult hybrid 25 November 1938, and Blackman (1941) noted one. Five (four on Sand and one on Eastern Island of Midway) were indicated for 1949-50 by Grenville Hatch and Earl and Willadean Sawyer on their joint banding schedulesubmitted to the U.S. Fish and Wildlife Service. Frings and Frings (1960) saw one in 1960. There is one specimenof a hybrid in the B. P. BishopMuseum in Honolulu. In some 24 years of interest in these species,including 36 months of intensivefield study of the LaysanAlbatross in the last 10 breedingsea- sons,I have found only slight evidenceof interbreedingamong the ap- proximately 180,000 pairs nestingon the 1,600 acresof land in Midway Atoll. In 1945 I saw one younghybrid and anotherin 1961. A Laysan and a Black-footedwere feeding the 1945 chick (Fisher, 1948); the parents of the 1961 bird were not seen. I recordednonbreeding hybrids in 1946 (one on Eastern Island), in 1961 (two on Sand Island), in 1964-69 (same bird on Eastern), and in 1964-69 (same individual on Sand). The latter two are banded; one is now (July, 1970) at least 8 years old and the other is at least 10. We see them regularly; each wan- ders in a limited sectorin a colony of Laysansand does not appear to 386 H•vEY I. Fxsm [Auk, Vol. 89

Figure 2. Hybrid albatross: Blackfoot X Laysan. be paired. Their areas of activity are less than 100 yards from nesting Blackfoots. No hybrids have been found nesting. Some hybrids reported in the literature and by local residents are probably simple aberrationsor age-relatedchanges in plumage. All reports of "hybrid Blackfoots"made to me by residentsof Midway were of individuals with unusual amounts of white on the abdomen,at the base of the tail, at the base of the bill, and scatteredwhite feathers among the contour feathers on the body. In Black-footed Albatrosses these areas may becomeincreasingly white with age. Two albinisticLaysans were found on Laysan Island in 1891 (Roths- child, 1895-1900; Munro, 1941a, 1941b), and Munro (1946) reported an albino on Eastern Island. We found one albino Laysan chick on Eastern in 1961, one in 1969, and another in 1970, but never have we seenan adult albino of either species. Plumage aberrationsin the Lay- san Albatrossoccur most frequently in the gray mantle and on the dorsum of the wings. Examinationof six such living specimensand one col- lected by Eugene Kridler on Pearl and Hermes Reef in February 1969 producedan impressionof specklingon the back and wings; the Kridler specimenalso had an unusuallyextensive spread of pastel gray over the side of the head. The speckleis producedin different ways: 1) white or buffy baseson otherwisenormal gray major and minor coverts of April 1972] Laysan and Black-looted Albatrosses 387 primary, secondary or tertial remiges; 2) all-white feathers scattered in the scapularand dorsalspinal tracts; 3) a one-eighth-to one-half-inch white trim on the edgesof the flight feather coverts; and 4) silvery, prismatic-coloredfeathers in the mantle, some with brown edges. (The bird showing this last variant was the only albatross with aberrant plumage found nesting.) As in all instances the color pattern was typically Laysan Albatross,it appearsunlikely that these birds were either primary or secondaryhybrids. The two true hybrids we have observedare pearl gray, darker on the back, and almost white in places,particularly the ventral surfacesof the neck and belly. The smokey,pearl gray is superficial; the slightest ruffling exposesthe underlying white. The shape of the bill is intermediate; its color is black in one bird, not the rosy-yellowof the adult Laysan, and it is part black and part gray in the other hybrid. Both hybrids have the Black-footedpattern of white at the base of the bill, but the white is more extensive,moving onto the foreheadand supra- orbital area. One bird has one black leg and foot and one light gray leg and foot (Figure 2); the otherbird's legs and feet are typicallythose of the Blackfoot. The hybrid thoughtto be the older is browner,less duskygray than it was 6 yearsago. The body is upright, more the posture of the Laysan Albatross, and the walking gait is more that of a Laysan than a Blackfoot's head-down waddle. The "sky-moos" of these hybrids are intermediate in general, but at times the soundsare clearly thoseof a Laysan or a Black-foot--a low clear "ooo" versusa hoarseand throaty "aaw," respectively.The "sky- moo" was the only soundheard from either hybrid, perhapsbecause neither one had an associate. Neither hybrid could be sexed positively and neither maintained a station or territory as a typical male of either species does at a com- parable age; both wanderedover an area perhaps 100 feet in diameter. Both rather obviously "thought" of themselvesas Laysan Albatrosses for they ignoredseveral Black-footed Albatrosses within range of their movements and attempted to socialize only with Laysans. However they seldomcould proceedbeyond a touch of the bill, the initial greeting of the two species,before the Laysan moved on. The hybrid sometimes followedthe Laysan for a few steps. Rejectionof the hybrids by Laysan Albatrossesis also indicatedby the solitary state of the hybrids, for both are severalyears beyondthe age when a normal juvenile begins to associate with another bird. Where the nesting colonies overlap, a Laysan and a Black-footed Albatrossmay sit side by side, as membersof a pair customarilydo. They may even preen each other. Close follow-up observations have 388 H^•w¾ I. Fxsi• [Auk, Vol. 89 failed to showthat thesecouples became more intimately involved; most likely they werejuveniles, which are knownto seekcompany, especially at night. Nor is the presenceof a memberof one speciesincubating an egg with a memberof the other speciesstanding alongside indicative of a pair in the crowdedzone of overlappingcolonies. Only oncehave I seena Laysan and a Blackfootattempt a courtshipdance, although not infrequentlya memberof one specieswill bow to, or circle,a dancing twosomeof the other species.These are the only indicationsI have of more than casual social relations betweenmembers of the two species. One might speculatethat a few hybridscould result from successful rape attempts,a behaviorobserved in both speciesand that I recently (1971) describedfor the LaysanAlbatross. This is not impossible,but no actualinterspecific rapes have beenseen. A 2-weekinterval separates the peak seasonsfor copulationin the two species,and I found no evidenceof inseminationduring rape attempts by Laysan males. Conse- quently it is assumed,from the evidencepresented, that mixed pairs do occur. Why are they so rare? And why are so few hybrids found?

POSSIBLE ISOLATING FACTORS DURING SYMPATRY

SPATIAL Althoughgeographical separation was an importantisolating mecha- nism originallyand is still operativefor most individualsof the two species,physical separation is not complete.The 5- and 6-year-old juvenilesdo return to congregateand to socializewithin a colonyof their own kind rather than on its edges,and these mate-seekingbirds are, therefore,most often in contactwith conspecificalbatrosses. But this can be operativein actual pair formationonly when there is room within the colony for the young male to establisha territory. If the colonyis crowded,and the zoneof overlapof the coloniesis frequently the most crowded,he may be forced to establishhimself in a place partly or whollyoccupied by the other speciesand thus exposehimself to morefemales of that species.Even if he is insidea colonyof his own kind, an itinerant femaleof the oppositespecies may approachhim in her meanderingbetween males o.f her own species.As eligible females apparentlypick the malesto dance.with, the femalesare probablymost responsiblefor the formation of mixed pairs.

BreedingBlack-footed Albatrosses arrive on the islands2 or 3 weeks beforethe breedingLaysan Albatrosses, but this differencecannot be a primary isolatingfactor. Their pair bondswere establishednearly 2 yearsbefore, in mostinstances. From February through April, unpaired April 1972] Laysan and Black-)•ooted Albatrosses 389 juveniles of both speciesand of proper age to pair are on the island at the same time. Older birds, surviving membersof pairs broken by death, begin seekingmates in late egg-layingtime (mid-November for Blackfoots,early Decemberfor Laysans) and would be more likely to form mixed pairs. But the paucity of such pairs indicatesan isolating mechanismother than differencesin times of arrival of different age groupsof the two species.

BEHAVIOR In many ways the behavior of the two speciesappears to be suf- ficiently similar to permit more than the observed infrequent interbreeding. For example, on 1 December 1962 we exchanged25 eggs between nests of the two species. Fledging successin both instances equaled that of undisturbed coloniesof both species. Also, chicks up to the age of 4 weeks were adopted interspecificallywhen exchanged between nests. It is evident that differencesin the behavior, voice, and appearanceof chicks do not always present secondarybarriers to continuedintrogression. However Black-looted Albatrossesare more aggressiveand intense. They are quick to strike vigorouslyand repeatedlyat avian or human intruders, which a Laysan Albatrossseldom does. Laysans seem not to defend the mate, only self, territory, nest, or egg. An incubating Blackfoot may leave the egg to attack a human being; I have never . seen a Laysan do this. Even juvenile Black-footedsdisturbed at night ;or interruptedin their dancinglash out with the bill. Young Laysans walk or run away. It is impossibleto ascribespecific isolating functions to general temperament,but the fundamental differencesjust described underlie much of the behavior. Posture and gait.--Differences are at once apparent to the human observer,and they must be to the birds. The Laysan holds the head high on a nearly vertical neck when standingor walking (Figure 3, top); the breast is higher than the abdomen. Black-footedAlbatrosses walk with the neck partly extendedforward and the head kept low; the body is nearly horizontal, and the gait is straddling or waddling (Figure 3, bottom). The standingBlackfoot often holds the head up in the Laysan position. Males of both speciesapparently recognize the interspecific differences even in distant females and do not entice those of the o•ther species. Sounds.--Although one may use the same syllables to describe the same vocalizationsof the two species,and the same types of vocaliza- tionsare produced,the soundsare differentin pitch, intonation,intensity, and duration. The speciescan always be identified by an experienced $00 HAgv• I. •_SH•t [Auk, Vol. 89 April 1972] Laysan and Black-Jooted Albatrosses 391

Figure 4. The bill-touch greeting of Laysan Albatrosses. listener. As Rice and Kenyon (1926b: 524) note, "The calls of the Blackfoot have a decided nasal quality and are louder, lower pitched, and hoarserthan thoseof the Laysan." The double call is the basic syllabification of both species;it is the primary sound from which other calls are derived. In the Laysan it is "eh eh," in the Blackfoot "haw haw." It is the greetingbetween mates, betweenyoung and adults, and on occasionbetween strange individuals. It is also the most used vocalization of the male on territory and at- temptingto attract a female, mate or otherwise,and of both sexesin communicatingwith the unhatched offspring. In the latter behavior, the male or female parent bends toward the egg and with the bill near the egg, double calls. If this is indeed a communicationwith the ad- vanced embryo, as I suggest,it is probable that before hatching the chick learns the basic vocalization of the species,as well as the in- dividuality of the parent's voice, before the chick can be visually or otherwiseaware of other species.If so, it is a powerful factor acting against mixed pairing later. This fundamentaldifference in the double call makesit difficult, if not impossible,I believe, for males to attract femalesof the other speciesor to greet them properly if they do approach. One other attraction call, the "whinny," is an ululating high-pitched 392 HARWy I. FZSH• [Auk, Vol. 89

Figure $. The rushing, tiptoe approach in the dance of the Black-footed Alba- tross. cry similar to the whinny of a horse. It is made frequently by male Laysans on station, seldom by Blackfoots. The "sky-moo" indicated earlier is perhapsa displacementvocalization, but, when uttered by a male on his territory after being ignored by a passingfemale, it sometimes causes the female to return. The interspecificdifference may prevent femalesfrom respondingto a male of the other species. Courtshipdance.--Attention is focusedonly on certain aspectsthat appear to play somepart in preventingsuccessful dancing between members of different species. The commentsapply only to courtship behavior in full action, not the often atypical and abortive displays of immatnres or of males not successfulin holding a partner for at least 10 minutes. Initial greeting.--Touching of the bills is an almost universal behavior when two Laysan Albatrossesfirst come near each other. It is always a precursorand a repeatedpart of the courtshipdance. It is less general in the Blackfoot. A Laysan male on territory and approached by a female extendshis head and bill on arched neck to meet her bill, which is somewhatlower and on a horizontallyheld neck. The touch of April 1972] Laysan and Black-looted Albatrosses 393

Figure 6. The posture in the Laysan dance in which the male (on left) de- ternfineshis height dominance over the female. bills is gentle and usually soundless;sometimes the curved tip of the upper ramus touches the inside surface of the floor of the partner's mouth (Figure 4). When the bill touch occursin Black-footsit usually comeslater, followingmutual head raising,and is often a rough,hard side-to-sideslap that sometimescan be heard30 feet away. Two field observations show how effective the difference is as a barrier to any interspecificsocial behavior. On 15 December1968 I watched a "female Blackfootmaking advancesto a male Laysan on his territory. She extendedneck, head low, and tried to bill-touch; male stood his groundbut avoidedcontact by turning head to side; female came closer and tried again, uttering Blackfoot doublecall; male backed up; female persisted,advancing and 'growling'; male 'uneasy,' turned and started to walk away; female followed. Every few feet male stoppedto stand his ground but was pursuedby female walking with head low and extended." The female Blackfootfollowed the male Laysan on a circuitous route until, some40 feet from his territory, she was distractedby a male Blackfoot. After 2 or 3 minutesthe Laysan slowlyreturned to his territorial station, turning severaltimes to watch the female. On 9 February 1969 a Blackfootof unknownsex rapidly approached a young territorial Laysan and attemptedto greet it. The male Laysan 394 I•wY I. FXSX•ER [Auk, Vol. 89

Fibre 7. Dancing Black-footed Albatrosses simuRan•usly lift both win• to d•play the dorsal wing surface. stoppedall enticementactivity when the Blackfoot came within a couple of yards and recoiled a step or two when the Blackfoot extended its bill. Spacing, speed, and posture of dancers:--The dance of the Black- footed Albatrossis speedier,more intense,more vigorous. The birds approach each other rapidly in the dance sequences,sometimes running toward each other and touchingbreast to breastß Their heads are held high, and the birds mince forward on tiptoes (Figure 5). Laysans also hold the head high and rise on tiptoes, but only when posing statue- like, not when approachingeach other, and their breasts are usually a foot apart (Figure 6). Figures 5 and 6 show that the Blackfoot uses only the distal phalanx on each toe in the tiptoe position; the Laysan usesthe penultimatephalanx as well. As male Laysansrequire that their matesbe shorterand have shorter bills (Figure 6 and Fisher, MS), as this sexual differenceis less than the increasein height producedby standing on tiptoes, and as a female Blackfoot on tiptoes is taller than most male Laysans, I believe male Laysans in most instanceswould cease courting and attack Blackfoot females. At least they do attack prospectiveLaysan mates that presenta April 1972] Laysan and Black-looted Albatrosses 395

Figure 8. Dancing Laysan Albatrosseslift only one wing in the wing-lift-bill- tuckand the partnersoften ta•e turnsin the action;they also alternately lift the right and left wings. highertip of the bill. Female 'Laysans might be attracted bythe intensi- fied and impressiveheight of a Blackfootmale on tiptoes,but the latter's forward rush and vigorouscontacts probably repel them. Wing-lift:--Blackfootsof both sexeselevate both wingssimultaneously (Figure7). Laysansraise but•one wing and alternate right and left (Figure 8). When the wing or wings come up and flare out laterally, the dancingBlackfoot or Laysan puts its bill near and below the baseof the elevatedwing; this is the bill-tuck. The bird is rigid, strained,and holds the posture several secondsas the bill nibbles on the surface of the feathers. This is probably a derivative of the scapular action described by Richdale (1950: 18). Two dancing Blackfoots wing-lift together (Figure 7), but bill-tuck on oppositesides of the body so that the birds are bill-tuckingin the samedirection. They do not alwaysbill-tuck after a wing-lift as Laysansdo, and the latter do not always wing-lift at the 396 HARVEYI. FXSHZa [Auk, Vol. 89

Figure 9. Lateral view of bill and head coloration. Top: Laysan Albatross. Bottom: Black-looted Albatross. April 1972] Laysan and Black-looted Albatrosses 397

Figure 10. The female Laysan (on right) stares at the posing male just •fore attempting to touch hg breast. same time. The membersof the Laysan pair characteristicallywing-lift and bill-tuck alternately (Figure 8). Colors:--The bill, head,and breastappear to be the areaswhere color differenceswould be of most importance,although one cannot dismiss the difference in color of the flared wing or the general impressionof whitenessin the Laysan and of blacknessin the Blackfoot. The bill playssuch a noticeablerole in the initial bill-touch,and later in bill clapperingand buzzingin the dance,that its rosy-yellowhorn color in the Laysan and its blacknessin the Blackfoot must be important (Figure 9). The bill is usuallydirected toward the other bird or upward and, mademore visible by the motionsof clappering,buzzing, and bowing,it becomesa centerof attention. The breast is the largest and most obviouspart of the male's body, at the level of the female's head. The size and curved fullness of the breast in both sexesof both speciesare accentuatedin three ways during the dance: 1) the breast is thrust forward and up by tilting the rear of the body downward;2) the neck is drawn posteriorlyover the back 398 HARWY I. Fxs•r• [Auk, Vol. 89

Figure ll. Female albatrosses touching the breasts of quiescent male Laysan (top) •nd posing male Blackfoot (bottom).

(Figure 10); and 3) as the birds bow low, especially the male, the breast is alternately visible and hidden. Consequently,the femalesof both speciesconcentrate a great deal of their attention on the male's breast as the two face each other (Figure 11). The female Laysan stares at it while the male posesin the sky-call and bill-tuck, then tries to touch it with her bill (Figure 10). The female Blackfoot may touch the male's breast as a substitute for the initial mutual bill-touch. Further, the snowy Laysan breast and the brown-black Blackfoot breast are probably the most impressiveobjects in the lives of newly hatchedchicks, and consequentlyin post-hatchingimprinting. It is not surprising,then, that the breast shouldplay a critical role in adult behavior. The female shows this influence as just described; the male April 1972] Laysan and Black-lootedAlbatrosses 399

Figure 12. Black-footedAlbatrosses bill-buzzing in the courtshipdance.

Laysan later exhibits his focus on the breast by attempting to touch the female's in precopulatorybehavior (Fisher, 1971). It would seem, therefore, that the solid white versus solid brown-black color on the breast may constitute one of the fundamental barriers to interspecific socializing. The role of head coloration may be similar, but the head is presented frontally to the partner (Figure 8) and its area is relatively small. The white at the base of the bill and behind the eye of the Blackfoot and the pastel gray in the suborbitaland loral areason the Laysan are most obviousin lateral view (Figure 9), which is primarily but only briefly displayed to the partner in the clapper behavior describedbelow. Rapid bill clapper:--This sound, producedby a rapid opening and forceful closingof the mandibles,is like two pieces of seasonedwood striking together. The Laysan usesit more frequently. Rapid repetition of the soundin the dancemakes it into a buzz--softer and more irregular in the Laysan. The major interspecific differenceslie not so much in the sounditself as in the position and posture of the birds making it. Black-looted Albatrossesbuzz simultaneouslyand repeatedly alongside the partner's head, first on one side and then the other, and almost touching (Figure 12). As they buzz, both birds lift both wings very slightly. Laysanssometimes buzz alongsidethe partner'shead, but usually 400 HARVEYI. F•S•tER [Auk, Vol. 89 the buzz comesas the head is thrown forward, downward,and sidewise away from the vicinity of the partner'sheaa. Seldomdo they buzz simul- taneously;the male buzzesmore often than the female; and neitherwing is flared as in the wing-lift, althoughone wing may be twitchedslightly upward for a moment. Simultaneousaction by both members:--The greater frequencyof the samebehavior at the sametime by Blackfootsis worthy of further empha- sis. They wing-lift,buzz, sky-call,tiptoe, and approacheach other at the sametime, and do all thesethings with greaterspeed and vigor. Laysan Albatrossestend to alternate specific acts between the partners. To humaneyes, it appearssimpler for the Blackfootpair to performsimul- taneous acts than to remember the cues for alternate or one-member performancesas found in the Laysan Albatross. This suggeststhat the Laysan has a more intricately developedcourtship dance. One further feature of albatrossbehavior may be a final or corrective barrier to the formationof interspecificpairs. In the Laysan Albatross (Fisher, MS) and perhapsin someof the southernalbatrosses (Rich- dale, 1950; Tickell, 1968), there is a long prenestingperiod of associa- tion betwee• the membersof newly formed pairs. Seldom does a pair nest in the first breeding seasonafter formation of the pair; the oc- casionalexceptions are pairs in which at least one member is pairing for a secondtime, having lost the previous mate by death. This "engagement"period in the Laysan is at least 1 year and sometimes2. As most pairs are formed in the spring months, the "engagement"period generallyextends from March or April of one year to November of the subsequentyear. The associatingbirds are usually and frequently together on the male's territory, testing or firming the pair bond for several months in each of two breeding seasonsbefore they attempt nesting. They may be togetherat sea during the remainingmonths of this period,but we do not know. In any event, this long period of courtship activity provides time for any hasty misalliances (certainly inter- specificones) to be broken by realappropriatebehavior on the part of either member.

ACKNOWLEDGI•ENTS I want to thank •oscph Beatty, Clark C•slcr, Gcrritt KIock, Howard Stains and George Watson for reading the manuscript and making valuable suggestions.

SUMMARY The three speciesof North Pacific albatrossesrepresent two lines of emigration from the Southern Hemisphere--the Short-tailed Albatross and the Blackfoot-Laysan lines. The Black-looted and Laysan albatrossesare similar in blood proteins, complexity of courtship, sexual April 1972] Laysan and Black4ooted Albatrosses 401 participationin courtshipand incubation,and patterns and chronology of behavior,including breeding. But, comparedto the Laysan Albatross, the Black-footedAlbatross is more primitive in habitat, behavior,and color, and more representativeof the hypothesizedprogenitor of the Laysan-Blackfootline. Once this Blackfoot-typeprogenitor establisheditself on North Pacific atolls, it was isolated from significantgenetic ingress from the south by the warm equatorialwaters. The growingpopulations on small islets were eventually forced to occupy additional islands. The new populations evolved rapidly on islands in different stagesof vegetative succes- sion, the changebeing aided by small numbers,spatial isolationof breeding colonies,and the strong tendencyof young to breed on the island of their own natality and of adults to maintain life long territories. One group, the Black-footed,was or became a breeder on exposed,sandy islands; the Laysan group occupieda new niche on more densely vege- tated islands. Population density caused the occupation of still more islands and the use of marginal habitat. The Blackfootsand Laysans began to breed on the same islands--Blackfoots on the barren beaches and Laysans in the brushy interiors. Increasing numbers caused the coloniesto coalescealong their edges,and the constant reshapingof the sandy shoresby wind, waves and water currentssometimes forced the Black-footedcolonies inland into the Laysan colonies.But by this time the two formswere morphologicallydistinct and behaviorallysepa- rated. Little evidence of hybridization exists at the present time. It is too rare to be of taxonomicsignificance, and, as the hybrids appear to be unsuccessfulin gaining mates, hybridization is of no continuing bio- logical significance. The two speciesare now reproductivelyisolated, not as originally by geography,habitat or chronology,but by their in- ability to enter interspecificbehavior. The most important barriers to the formationof mixed pairs includedifferences in postureand gait, temperament, and courtship behavior. In the latter, it is postulated that initial greeting ceremony,sounds, wing-lifts, bill-tucks, posture, color of bill and breast, and complexity of dance pattern include the more probableisolating differences. One factor that is impossibleto describe is the almost overpoweringvigor and intensity displayed by dancing Black-footed Albatrosses. The long period of associationbetween members of a new pair before copulationoccurs not only increasesthe probability of an eventually firm pair bond, but providesan opportunityfor correctionof interspecific alliances.It is, thus, a basic and final part of the isolatingmechanism. 402 HARVEYI. FISHER [Auk, Vol. 89

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Department of Zoologyand Schoolof Medicine, SouthernIllinois Uni- versity, Carbondale,Illinois 62901. Accepted13 April 1971.