THE CRYPTIC OCCIPITAL SPOT IN THE ACCIPITRIDAE (FALCONIFORMES) JOHN C. HAFNER AND MARK S. HAFNER AltSTRACT.--The heretoforeundescribed cryptic white occipitalspot is shownto be a conserva- tive familial trait of the accipitrid Falconiformes.Excluding the "naked-headed"vultures and the pale-headedforms, only two generaof the 55 accipitridssurveyed (Pithecophaga and Harpyopsis) were found to lack the triangular occipital spot. Differential spot trends occur in the New and Old World Accipitridaeand variation in the spotis detailed.The spotwhile functioningas an appease- ment signal also servesto inhibit an adversary behaviorally during agonisticencounters. An additional deflectivefunction is suggestedto constitutean antipredatory mechanism.Taxonomic and systematicaspects pertaining to the occipital spot are discussed.--Departmentof Biological Sciencesand The Museum, Texas Tech University, Lubbock, Texas 79409, and Moore Laboratory of Zoology, Occidental College,Los Angeles,California 90041. Present addressof both authors: Museum of Vertebrate Zoology, University of California, Berkeley, California 94720. Accepted 1 October 1975. SEVERALyears ago we noted a cryptic white spot in the capital tract of a captive Red-tailed Hawk (Buteojamaicensis). Although there have been ptilologicaistudies of many falconiform birds (Chandler 1914, Compton 1938), a searchof the literature uncoveredonly a few brief statementsregarding nape or occiputfeathers with white bases(e.g. Friedmann 1950: 167; Brown and Amadon 1968: 39). Our initial observa- tion prompted further investigation, culminating in this review of the occurrenceof a concealed white spot in raptorial birds and a discussion of its behavioral and taxonomic significance. We examined both living and museum specimensof 65 genera representingthe five families in the order Faiconiformes to assessthe occurrenceand general mor- phology of the spot. Becauseof similaritiesbetween the faiconiform and strigiform birds, we also examined representativesof several genera of owls for presenceof this character.'It wassoon evident that the crypticwhite occipitalspot occurs only in the Accipitridae. DESCRIPTION OF THE OCCIPITAL SPOT The typical accipitrid spot lies in the occipital region of the capital tract and is approximatelytriangular in shape(Fig. 1). The spotis composedof numerousbicol- ored contour feathers. The basal two-thirds of a spot feather is snow-white; the distal portionis often slightlydarker and longerthan surroundingcephaiic feathers. The terminal darkening of the feathers appears to accentuatethe whitenessof the basaiportion during cresterection and conceaisthe white when the cephalicfeathers are in the normal relaxed position (Fig. 2). A clear demarcation occursbetween the white basesof spot feathers and the darker basesof nonspotfeathers (Fig. 3). The microscopicstructure of the feathers comprisingthe spot showsno differentiation from that of the generalcephaiic contour feathers. The relative size and general ' Non-accipitrid species examined.--Cathartidae: Cathartes aura, and Sarcoramphuspapa; Pan- dionidae:Pandion haliaetus; Sagittariidae:Sagittarius serpentarius ; Falconidae:Daptrius americanus, Phalcoboenusmegalopterus, Polyborus plancus, Herpetotheres cachinnan, Micrastur semitorquatus,and Falco sparverius;Strigidae: Bubo virginianus,Speotyto cunicularia, Athene noctua,Asio fiammeus, and Otus asio; Tytonidae: Tyro alba. 293 The Auk 94: 293-303. April 1977 294 I•rNER ANDHAFNER [Auk,Vol. 94 Fig. 1. Cooper'sHawk (AMNH 470458)showing the occipitalspot. Distal one-third of all contour feathersin the coronal,occipital, and cervicalregions have beenexcised. morphologyof the white spotwas found to vary intergenerically(Table 1). The spot is presentand appearsidentical in both sexes.Age variation,when present,is de- tailed in Table 2. SURVEY OF GENERA Of the 64 recognizedgenera of accipitrids(Brown and Arnadon 1968), 55 were surveyed(Table 2). It was noted that New World accipitrids have generally bold TABLE 1 ANALYSIS OF THE SPOT INDEX IN 10 NEW WORLD ACCIPITRID GENERAI Species N Mean Range SE of Mean Ictinia misisippiensis 7 1.10 0.4--2.3 0.25 Rostrhamussociabilis plumbeus 7 2.39 0.7-3.5 0.47 Haliaeetus leucocephalus(juv.) 7 9.16 8.3-10.1 0.24 Circus cyaneushudsonius 20 2.48 1.0-3.6 0.15 Geranospizacaerulescens nigra 6 5.88 4.4-7.2 0.42 Accipiter cooperil 20 2.08 1.1-3.2 0.12 Buteogallus a. anthracinus 13 3.80 2.6-6.3 0.33 Parabuteo unicinctus harrisi 10 1.84 1.1-2.8 0.15 Buteo jamaicensis calurus 18 2.51 1.4-4.3 0.19 Aquila chrysaetoscanadensis 18 6.37 2.8-10.3 0.46 • Spotindex = areaof spoffinterorbitalwidth of head.Spot area computed as one-halflength times greatest width. This index re- latesspot area to headsize. A largermean value for spot index indicates a relatively larger occipital spot. April 1977] AccipitridOccipital Spot 2 95 Fig. 2. Red-tailedHawk showingthe terminaldarkening of the occipitalspot feathers. definitivespots while many Old Worldspecies exhibit poorly developed, possibly vestigialspots. Of the New Worldtaxa examined, all had occipitalspots except Leucopternisand the cosmopolitanElanus. These two generahave pale heads in adult plumage,perhaps explaining the absenceof the spot(see below). Four other genera,Elanoides, Busarellus, Harpia, and the cosmopolitanHaliaeetus have the spotin only the dark-headedjuvenile plumage; the spotis not apparenton the pale-colored head of the adults. Size and shapeof the occipitalspot variesconsiderably among Old World species.Notable deviationsfrom the typical spot are summarizedbelow. Sev- eral generapossess indistinct occipital spots (Heni½opernis, Lophoictinia, Cir- ½aetus,Terathopius, Dryotriorchis, Polyboroides, and Megatriorchis).Two other 1A 2A 3A 4A lB 2B 3B 4B Fig. 3. Contourfeathers of thecapital tract of fourgenera of accipitridbirds: A, featherfrom within theoccipital spot region; B, featherabout the periphery of theoccipital spot. 1, lcti•:ia misisippiotsis; 2, Circus½yaneus; 3, A½cipitercooperil; 4, Buteojarnaicensis. 296 HAFNERAND HAFNER [Auk, Vol. 94 TABLE 2 VARIATION IN THE OCCIPITAL SPOT OF ACCIPITRIDAE 1 Class 2 Species 1 2 3 4 5 6 Aviceda cuculloides 0 ..... Leptodon cayanensis N ..... Chondrohierax urcinatus N ..... Henicopernis longicauda - - 0 - - - Pernis celebensis 0 ..... Elanoides forficatus - N .... Machaerhamphus alcinus 0 ..... Gampsonyxswainsonii N ..... Elanus leucurus - - - C - - Chelictinia riocourii 0 ..... Rostrhamus sociabilis N ..... Harpagus bidentatus N ..... Ictinia misisippiensis N ..... Lophoictinia isura - - 0 - - - Hamirostra melanosternon 0 ..... Milvus migrans 0 ..... Haliastur indus - - - 0 - - Haliaeetus leucocephalus - C .... Gypohieraxangolensis - 0 .... Gypaetus barbatus - - - 0 - - Necrosyrtes monachus ..... 0 Gypsfulvus ..... 0 Aegypiusmonachus ..... 0 Circaetus gallicus - - 0 - - - Terathopius ecaudatus - - 0 - - - Spilornis cheela 0 ..... Dryotriorchis spectabillus - - 0 - - - Polyboroides typicus - - 0 - - - Geranospizacaerulescens N ..... Circus cyaneus C ..... Melierax metabates 0 ..... Megatriorchis doriae - - 0 - - - Erythortriorchis radiatus 0 ..... Accipiter cooperii C ..... Urotriorchis macrourus 0 ..... Butastur indicus 0 ..... Leucopternis albicollis - - - N - - Buteogallus anthracinus N ..... Heterospizias meridionalis N ..... Busarellusnigricollis - N .... Geranoaetus melanoleucus N ..... Parabuteo unicinctus N ..... Buteo jamaicensis C ..... Morphnus guianensis N ..... Harpia harpyja - N .... Harpyopsisnovaeguinae .... 0 - Pithecophagajefferyi .... 0 - Aquila chrysaetos C ..... Hieraaetus morphnoides 0 ..... Spizastur melanoleucus N ..... Lophaetusoccipitalis 0 ..... Spizaetus ornatus C ..... Stephanoaetuscoronatus 0 ..... Oroaetus isidor N ..... Polemaetus bellicosus 0 ..... Total per Class: 34 5 7 4 2 3 New World genera per Class: 14 3 0 1 0 0 PercentNew World generaper category: 77.7 16.7 0 5.6 0 0 Old World genera per Class: 15 1 7 2 2 3 PercentOld World generaper category: 50.0 3.3 23.3 6.7 6.7 10.0 • Symbols:O = Old Worldgenera (N = 30), N = New Worldgenera (N = 18),C = cosmopolitangenera (N = 7). 2 Class1, spot presentin both adult and juvenileplumages; 2, spot presentin only the juvenileplumage and adult plumageex- hibitinga pale head;3, indistinctspot present; 4, no spotseen in any plumage,but pale headpresent; 5, no spotseen; 6, nakedhead present. April 1977] AccipitridOccipital Spot 297 Fig. 4. The Ornate Hawk-Eagle (Spizaetusornatus), among others,shows the white occipitalspot each time the crest is erected. genera, Haliastur and Gypaetus, have no spot in any plumage, but these gen- era have pale-coloredheads in the adult plumage similar to that of the New World Leucopternis. Gypohierax has the spot in only the dark-headed juvenile plumage, the white head of the adult exhibiting no apparent spot. The three vul~ turine genera examined, Necrosyrtes,Gyps and Aegypius, being "naked-headed," lacked the occipitalspot. Lastly two Old World generaof accipitrids,Harpyopsis and Pithecophaga, lacked the spot. FUNCTIONAL SIGNIFICANCE Signalingorgans are not uncommonamong birds and a studentof avian ethology will rarely dismissas functionlesssuch a bold plumage marking as the accipitrid spot. As the spot is highly organized with extreme contrast of colors and can be displayedand concealedat will, we proposed,a priori, that the spot functionsas a 298 Hx•a aNI) HAFNER [Auk,Vol. 94 Fig. 5. FemaleCooper's Hawk displayingthe occipitalregion upon the male'sarrival at ttie nest. Sunlightmay enhancethe spotin this photograph.(Photograph courtesy of Helen Snyder.)