THE AUK A QUARTERLY JOURNAL OF ORNITHOLOGY VoL. 89 Ai, mL 1972 No. 2 SUBALULAR APTERIUM IN BIRDS WILLIAM G. GEORGE AND CLARK L. CASLER AN apterium, which evidently has remainedundescribed, exists under the alula of many avian speciesbut not by any meansin all groups. We first noted it in the Snakebird (Anhinga anhinga) in which the un- featheredarea, measuringabout 850 sq mm, is createdby an absenceof most upper median and other small upper wing covertsassociated with primaries1 (innermost) through6 (Figures 1A, lB). In an ordinary museumskin this apterium is apt to escapenotice, being shut away behind a dry, inflexible alula. Details of subalular plumage are difficult to see in skins for the same reason,and at first we soughtto limit our investigationto specimensin spirits and to freshly- collectedand living birds. But this kind of material proved available for only a few forms. Consequentlywe often resortedto dampeningthe wings of skins with a weak solutionof ethyl alcohol and examiningas much of the subalular area as could be glimpsedby slightly deflecting the alula. The present paper attempts to summarizeour still very incomplete knowledgeof the presenceand absenceof subalular apteria throughout the class. As used below, the term "apterium" refers to an absenceor size reduction of coverts which, as we interpret the evolutionary evi- dence, converted a once almost fully feathered region into a partly naked expanse. A wide variety of conditions intermediate between a well-marked apterium, as seen in Anhinga, and a feathered subalular region,as seenin most species,of courseexists. A showof skin may arise variously from loss of a few or many coverts,from size reductionof a few or many coverts,and from combinationsof both thesecircumstances. Hence the problem of defining the term "subalular apterium" in any strict fashionis complicatedfrom the start. Subalular pterylography varies within the class and remains to be worked out for many groups. In addition to the greater upper primary coverts,almost all specieswe have seen show some sign of at least one row of covertsinserted at the baseof the greaters--a row called "middle 245 The Auk, 89: 245-262. April 1972 246 GEORGEAND CASLER [Auk, Vol. 89 Figure 1. A, apterium in Anhinga i• fresh plumage. B, apterium in Anhinga •, worn plumage. upper primary coverts"by some authors,such as Humphrey and Clark (1961), and termed "medians"herein. One or two, and even three or more rows may be presentin somebirds: a row of "lessers,"and one or more rows of marginals,depending on the species. Table 1 showsthe numbersof rowspresent in certainorders according to our determinations. In renderingaccounts of the subalularapteria, difficultiesarise from not knowingtruly how many rows of marginalcoverts are presentand whethera given marginalrow in one speciesmay be a row of lesser covertsin another. Homologiespresumeally exist betweenthe lesserrow and one of the marginal rows in some birds, but not with reasonable certainty. As a result numerical or alphabeticalcodification of subalular feather follicles might produce misleadingaccounts if introduced as a basis for comparing the apteria of distantly related birds. We have neverthelesserected formulae below, but hasten to declare they are meant as referencepoints rather than homologictranslations. A detailed feather-by-feathercatolog of subalularplumage is beyond the scopeof this paper. Not that we could furnish such a treatment in any event, even if such a culmination were worth the effort, which it probably would not be becauseneither a functional nor phylogenetic explanationfor the loss of subalularplumage has been determined(see "Origin and Functionof the Apterium" below). Consequentlya complete descriptionof extant and missing plumes would be as apt to focus on April 1972] SubalularApterlure 247 TABLE 1 NUMBEROF Rows oF 8UBALULARUPPER WING COVERTSIN CERTAII•ORDERS OF BIRDS• Number of rows 4 3 2 1 Gaviiformes x Podicipediformes x Pelecaniformes x Ciconiiformes x (Ardea) x Anseriformes x (Branta, Cygnus) (?) x Falconiformes x x Gruiformes x Charadriiformes x Cuculiformes x Strigiformes x ( Bubo) (?) Piciformes x Passeriformes x (Corvus) x x (Myrmochanes, Gymnocichla) Not including the greater coverts. the trivial or superficialas on the significant. For example,it might be assumed--probablyrightly--that an apterium that automaticallybe- comesexposed to the environmentwhen the alula is extended,as proves to be the casein Anhinga(see below), is morelikely to have a functional role than an apterium that remains concealeddespite extensionof the alula, as appearsprobably the casein formicariids.But in view of the unknown function of the apterium, one can not be sure of the relative importanceof the lossand/or retentionof a •ven seriesof plumes. Pterylographic studies rarely have heeded the details of subalular plumage(Humphrey and Clark (1961) is one exception). Similarly, exactingobservations on the behaviorof speciesrarely have mentioned use of the alula. Anhingasseem always to flex the alula on assuming their famousspread-wing posture, yet despitemany accounts of snakebird behaviorand anatomy(i.e. Owre, 1967), no one seemsto have com- mentedon this phenomenon.The literaturecontains some fragments of informationon the nonaerodynamicemployment of the alula by a few birds,but without,as it happens,illuminating the apterium'ssignificance within any species. Our primary purposeis to call attention to an overlookedfeature exist- ing, for whateverreason, in the main organof flight of certainbirds. To 248 GEORCE^•D C^SnER [Auk, Vol. 89 achieveour objectivewe have adoptedprocedures calculated to indicate the speciesnow known to us to exhibit obvious loss and/or size re- duction of subalularplumage without, we trust, creatingan impression either that our surveyhas removeda needof examiningadditional forms in any stated taxon, or that our surveyhas discernedevery intermediate degreeof subalularplumage modification in any group. To date we are aware of no evidenceof an apterium involving the coverts for primaries 5 through 11 in over half the ordersof birds, for example,in anseriforms and charadriiforms. It nevertheless can not be assumed that some duck or shorebirdnot seen by us lacks an obviousapterium, much less an obscureone. In an early survey of Formicariidae,encompassing a few of the better known species,we noted no apteria. When Georgecarried the survey forward later to includean additional40 species,he found the structurein a considerablenumber of genera. No doubt further in- vestigationof certaingroups will yield similar results. RESULTS OF SURVEY Brief accountsof variation within groups accompanythe treatment of Anhingidae,Ramphastidae, and selectedpasseriforms of specialin- terest. Formulae adhere to the following: Neg. •-- no loss or size reduction of coverts was noted. A •- loss of some median coverts associatedwith primaries 4 through 9. All speciesappear to possessthe median covert for primary 10. Loss or severe size reduction of median coverts for primaries 1, 2, and 3 characterizes most groups. Typically the loss of median coverts for primaries 4 through 9 is accompaniedby size reduction of adjacent covertsand this is implied in formulae employingsymbol A. A • ---- severereduction of some median coverts associatedwith primaries 4 through 9. Such covertsare vestigial, or very nearly so. B • loss of some lesser coverts (as defined herein, the term "lesser" refers to a row of covertslying on the dorsal surface of the wing immediatelyin front of the median coverts and behind the marginal coverts at the leading edge of the wing). Many species evidently lack such a series; perhaps in these species some lesser plumes have moved forward to the leading edge, merging with or supplanting marginal coverts. In any case, formulae never incorporate the symbol B unless the existence of this series seems demonstrable. C ---- loss, severe size reduction or displacement of marginal coverts. C• -- apparentenlargement of somemarginal coverts. D ---- probable exposure of the apterium when the alula is extended. As one example of formulae emerging from this treatment, Anhinga is ABCD. Some formulae given below are amplified by comments. An examination of skins alone, as previously stated, yields less than an ex- haustive appraisal of the subalular traits of a species,particularly in the case of large birds, so we have indicated skin-derived information by an asterisk after the appropriate name. Nomenclature generally follows Peters' "Check-list of the birds of the world" (1931), with some exceptionsas noted. In most casesonly one or two specimensper specieswere examined, but as will appear from some accounts April 1972] Subalular Apterium 249 (see Anhinga) a single specimenordinarily is sufficient to establishwhether a species has lost a series of subalular plumes. To avoid any possible confusion between apteria and naked areas arising from molt, all information gleaned from wing- molting specimenshas been excluded. Struthioniformes*.Neg. Struthio camelus. Rheiformes*.Neg. Rhea americana,Pterocnemia pennata. Casuariiformes*.Neg. Casuariuscasuarius. (Dromiceiidae not examined.) Apterygiformes*.Neg. A pteryx australis. Sphenisciformes*.Neg. Megadyptes antipodes,Pygoscelis adeliae, Aptenodytesfosteri. Tinamiformes*.A•B. Eudromia elegans,Nothura darwinii. Gaviiformes.Neg. Gayla iraruer,G. stellata. Podicipediformes.Neg. Podiceps auritus, Aechmophorusoccidentalis*, Podilymbus podiceps. Procellariiformes. Neg. Diomedea immutabilis, D. nigripes*, Fulmarus glacialis*,