Behavioral Evidence on Skimmers' Evolutionary Relationships

170 GeneralNotes [Auk, Vol. 93 ^ B Fig. 1. Alizarin-stainedstapes of A, Phileplttacastanea and B, Neodrepanls sp. Both are from AmericanMuseum of Natural Historyalcoholic specimens Nos. 2230and 2231,respectively. Both approximatelyX 50. of the New World Furnariidae(including woodhewers), Formicariidae, Cono- pophagidae,Rhinocryptidae, Cotingidac, Pipridae, Tyrannidae, and Phytotomidae, with the Old Worm Eurylaimidaeand Pittidac. I have recentlybeen able to dis- sectthe stapesfrom akoholic spedmens of Pitilepittacastanea and Neodrepanlssp. and havediscovered that they both possessthe derivedsuboscine type of stapes (Fig. 1), thusillustrating their affinity to the othergroups of subnscines. This work was supportedby a grant from the Universityof North Carolina ResearchCouncil Specimensof Philepittaand Neodrepanisin the collectionof the AmericanMuseum of Natural History were lent throughthe kindnessof WesleyLanyon.---ALAN FED•JCCIA, Department of Zoology,University of North Carolina,Chapel Hill, NorthCarolina 27514. Accepted 6 Dec.74. Behavioralevidence on sldnmaers'evolutionary relationships.--The sub- orderLari (Charadriiformes)is composed of the familiesStercorarildac (skuas andjaegers), Rynchopidae (skimmers), and Laridae (gulls and terns). It is gener- ally acceptedthat withinthis suborder the gullsand ternsare moreclosely re- latedto oneanother than either is to theskua• (A.O.U. 19S?), but theevolutionary positionof the skimmerswithin the suhorderis lesscertain. Schnell(1970a, 1970b) concluded that skimmersare moreclosely related to terns than to gulls or skua•, basedon a pheneticstudy of skeletaland external characters,as did Zusi (1962,1971) on the basisof a comparativestudy of anatomicaland behavioral feeding adaptations. We feelthat manyof the anatomical similaritiesbetween skimmers and ternsmay be the resultof convergencecaused by similaritiesin flightand feedingbehavior. Mayr (1969)suggested that any set of featuresarising after the invasionof a new food nichewill evolvevery rapidlyand thereforeare poorcharacters for ba•inghypotheses of evolutionary relationship. January1976] GeneralNotes 171 On the basis of comparativestudy of courtshipand agonisticdisplays, Wolk (1959: 69; see also Moynihan 1959) suggestedthat skimmersare "derivedfrom an ancestralLarus-type gull independentlyof the terns (Sterna)." Further study and reinterpretationof the behavioralevidence leads us to believe that the divergenceof the skimmersfrom their parentalevolutionary line took place at least as early as the divergenceof the gulls and terns and perhapssignificantly earlier. This belief is largely in agreementwith the conclusionsof Hudson et al. (1969), who placed the skimmersin the family Rynchopidaeand the gulls, terns, and skuasin the family Laridae on the basis of a detailed study of wing and leg muscles. Also Zusi has indicated (pers. comm.) that his published works and currentunpublished opinions based on functionalanatomy are not in conflict with this interpretation. Our study was conductedduring the springand summerof 1971and 1973 on an unnamed island in Bogue Sound 0.5 miles south of Morehead City, Carteret County, North Carolina. We worked in a colony of about 20 pairs of Black Skimmers(Rynchops niger) and in a larger mixed colony of skimmers,Common Terns (Sterna hirundo), and Gull-billed Terns (Gelochelidonnilotica). Visual displayswere photographedon super-8movie film, and vocalizationswere recorded with an Uher 400.0Report L tape recorderand M514 microphoneat a tape speed of 71/2 i.p.s. These recordingswere spectrographicallyanalyzed on a Kay Ele- metrics Corporation Sona-Graph Model 7029 A using the wide band filter. The more common displays of the Black Skimmer are describedbelow, and their taxonomicimplications are discussed.For more completedescriptions, see Wolk (1959). Upright oblique.--In this posture,the body and tail are horizontal,the neck is extendedupward almost vertically, and the bill is pointedup at an anglebetween 0 ø and 45ø above the horizontal. The wings are slightly abducted. During this display, the bird may vocalize(see bark below), and the bill may be open or closed. The upright oblique is one of the most commonpostures seen during aggressive encounters.This display is almost certainly homologouswith the upright posture of most larids (see Tinbergen1959, for descriptionsof gull displays),but it is considerablysimpler in form. Almost all gulls and the noddy term have two forms of the upright posture,a bill-down form most often followed by attack and a bill-up form most often followed by escape. These have been termed the aggressiveupright and the intimidated or anxiety upright, respectively.The black-capped terns lack the upright postures, but elements of these appear in the terns' erect display, and this lack seemsto be due to loss of some components and exaggerationof others (Moynihan 1962). Skimmers,in contrast,have a simple undifferentiatedupright, and the sameis true of skuas(Moynihan 1962, Anderson 1971; but see Perdeck 1960, 1963). Low oblique.--In this posture, the head and neck are lowered so that the entire body is almost parallel to the ground, with the bill pointing slightly below and the tail slightly above the line of the body. As in the upright oblique,the wings are held quite closeto the body, and frequentlya short bark (seebelow) is uttered. This is also a commonposture in aggressiveencounters, but it is seen more often after an attack whereas the upright oblique occursmore often before an attack. The commonly occurringsequence of vocal upright oblique followed by vocal low oblique is very similar to the commonlarid oblique-cum-longcall display. It seemsvery likely that thesetwo sequencesare homologousand thus that the skimmer low oblique is homologouswith the larid oblique posture. The oblique 172 GeneralNotes [Auk, Vol. 93 0.1 sec. 7- • Fig. 1. $pectrogramsof eight selectedbarks of the Black Skimmershowing variation in duration and in form. is also presentin skuas(Perdeck 1960, Moynihan 1962, Anderson1971) and in most gulls, the noddy terns, and the Inca Tern (Larosternainca) (Moynihan 1962). It is not presentand thus has presumablybeen lost in the black-capped terns. Bark.--This commonnote of the skimmervaries in form from a short bark, whichis a soft but sharp"out," to a longbark, whichis a longercawing sound. Wolk (1959) seemsto suggestthat the bark occursin two discreteforms, the long bark and the short bark; but our observationssuggest that the duration of this note may form a continuum(Fig. 1). The bark occursduring aggressive encounters and duringcourtship, the longforms being more commonly delivered during flight and the short formsbeing more commonlyuttered on the ground.The bark is probablyhomologous with the commonlarid long call. As indicatedabove, it accompaniesthe skiminer'shomologue of the larid oblique-cure-longcall display. In its simplicityit is more similarto the long call of the skuasand gulls than to the more complexcalls of the terns. Bill high.--In this posture,the head and tail are pointedvertically upwards, givingthe skimmera U-shapedappearance. It is usuallyperformed in a sitting position,and oftenthe bird will scrapesand backwards with its fect. The wingsare droopedand abductedslightly. This postureis involvedin pair formationand maintenanceof the pair bond. It doesnot seemto be homologouswith any display of the other members of the Lari. Draw.--This display is a simple movement of the bill down and under the sitting bird and then back to a nearly horizontalposition. It occursin the same situationsas the bill high and in highintensity territorial encounters. Thesetwo postures,the bill high and the draw, often occurtogether; and the situationsin whichthey occur are those in whichchoking is performedin gulls.We secno justificationfor hypothesizinga hornologybetween them as Wolk (1959) does,because the formsof the displaysare completelydissimilar. Choking is a bill-downdisplay performed standing and with noneof the scrapingor drawing movementsof the bill high and draw. On the other hand, the bill high and draw are similarto the kickingand drawingmovements of functionalnest building in the skimmer, and we suspectthe displayshave been ritualized from this source. January 1976] General Notes 173 Choking in gulls may be derived from nest-building behavior and it is possible that evolutionarychanges in chokinghave paralleled changesin nest-buildingmove- ments. The exaggeratedmovements of choking, bill high, and draw are presumably ritualized and emancipatedfrom their original motivations, and there would seem to be no adaptive value in the displaysevolving to resemblenesting behavior; indeed, further divergencefrom nest building seemsmore likely than convergence.It thus appears that skimmers do not have the choking posture, nor do skuas, whereas most gulls and the noddy terns do. It is not present and has presumablybeen lost in the black-capped terns. We feel that this is an important conclusion,because Wolk seemsto considerthe hypothesizedhornology between the skimmer bill high and draw and the gull choking to be strong evidence for a close skimmer-gull relationship. Food begging.--In the skimmer begging consistsof simple pecking by the female at the bill of the male, her head held lower than his. This display occurs during pair formation. Food beggingis a very commondisplay, occurring in every family of the suborder, but it is very simple in form in the skimmers. There is no call or head tossingmovement, either or both of which are associatedwith food begging

Load more