HAVE THE EGGS OF THE ORANGE-BREASTED FALCON (FALCO DEIROLEUCUS) BEEN DESCRIBED? by DouglasA. BoyceJr.' School of Natural Resources HumboldtState University Arcata, California 95521 and Lloyd F. Kiff WesternFoundation of VertebrateZoology 1100 Glendon Avenue LosAngeles, California 90024 The only publisheddescription of Orange-breastedFalcon (Falcodeiroleucus) eggs knownto us is that of Coltart (1952),who presenteddetails on two setsof eggssaid to be of thisspecies collected for G. D. Smookerin Trinidad.One of thesesets, a clutchof 3 takenon 28 March 1937in the Aripo Savannah,is now in the collectionof the West- ernFoundation of VertebrateZoology (WFVZ cat. no. 15,728);the other,a setof 2 col- lectedon 21 April 1930 in the Coroni Marshes,is in the collectionof the Zoological Museum,University of Helsinki,Finland (ZMUF cat. no. 15,721). The authenticityof theseeggs has been questioned by ffrench(1973) because of their smallsize compared to the bodysize of the species.The 3 eggsin the WFVZ collection measure40.6 x 34.7, 41.8 x 35.6, and 39.9 x 34.6 mm, and the 2 eggsin the ZMUF set measure 43.0 x 35.0 and 42.2 x 34.7 mm. All of these measurements fall within the rangegiven for eggsof the AplomadoFalcon (Falco femoralis) by Bent (1938) and Brownand Amadon(1968) and are only slightlylarger than the extrememeasurements knownfor eggsof the muchsmaller Bat Falcon(Falco rufigularis) (Brown and Amadon op cit., Kiff unpubl.data). Eggs of severalfalcon species, including one of Smooker's Trinidadeggs, are shownin Figurei to illustratetheir comparativesizes. Heinroth(1922) first demonstratedthe fundamentalrelationship between egg weight andbody weight in birds,and this was further refined by Huxley(1923-1924). Amadon (1943),Lack (1968),and Rahnet al. (1975)have presentedevidence that suggeststhat eachgroup of relatedbirds (at leastto the subfamilylevel) has a characteristicpropor- tionalityconstant, which expresses the rate at whichegg size increases to that of body sizeincrease. It is possible,therefore, to deriveindividual regression equations to express the relationshipsbetween these parameters for particulargroups of birdsand to predict fairlyclosely body weight from egg weight, or vice-versa. We obtaineddata on eggsize and female body weight for 12 Falcospecies, represent- ing the entiresize range found in the genus(Table 1), and plottedthe log of female bodyweight against the log of egglength (L) X eggbreadth (B), a sizeindex that is highlycorrelated with actualegg weight(Anderson and Hickey 1970).The resulting regressionline (r2= 0.92) is shownin Figure2. The Orange-breastedFalcon data point lieswell awayfrom the regressionline. Basedon the meanweight of 605 g (n= 7) for 'Presentaddress: 4445 Old GravensteinHighway, Sebastopol, CA 95472 89 RaptorResearch 15(3):89-93 90 RAPTOR RESEARCH Vol. 15 No. 3 femaleOrange-breasted Falcons, the log LB for the species(assuming a samplesize of 20) shouldbe 3.26. When this figure is plottedagainst egg breadtha value of 37.5 mm for Orange-breastedFalcon egg breadthis predicted.When that figure is dividedinto the productof L X B, an estimateof 48.0 mm for Orange-breastedFalcon egg length is obtained.The 95% predictioninterval for thesevalues range from 44.0 to 52.1 mm (L) and33.8 to 40.0 mm (B); Smooker'ssets fall outsidethis rangein lengthand just inside in breadth. Table 1. Mean eggsize and femalebody weight of Falco species. Egg S-ize(ram) •½wt. Species Code (LengthX Breadth) Log L-B (g) Logwt. Source AmericanKestrel F. sparverius A 35.15 x 28.92 3.007 142.2 (13) 2.15 6 Red-lootedFalcon F. vespertinus B 35.34 x 29.59 3.038 182.2 (5) 2.26 3 EuropeanKestrel F. tinnunculus C 39.82x 31.37 3.097 230.4 (20) 2.36 3 Merlin F. columbarius D 40.21 X 30.84 3.094 212.0 (14) 2.33 2 BatFalcon F. ruJ•gularis E 40.80x 31.81 3.118 220.0 (13) 2.34 4 EuropeanHobby F. subbuteo F 41.82X 32.86 3.138 229.0 (10) 2.36 9 Eleonora'sFalcon F. eleonora• G 41.92 X 34.30 3.160 388.0 (11) 2.59 7 Little Falcon F. longipennis H 44.48 X 33.14 3.169 310.0 (4) 2.49 5 AplomadoFalcon F. jbmoralis I 44.26X 34.79 3.188 406.0 (8) 2.61 10 BrownHawk* F. berigora J 51.66 x 39.28 3.308 505-635 (4) 2.89 5 Prairie Falcon F. mexicanus K 52.01 X 40.18 3.320 863.0 (31) 2.94 8 PeregrineFalcon F.p. peregrinus L 51.27X 40.72 3.320 1010.0 (17) 3.00 5 Gyrfalcon F. rusticolus M 59.17 X 45.92 3.434 1470.0 (10) 3.17 i *We usedthe BrownHawk egg data only in Fig. 3 andEleonora's Falcon data only in Fig. 2. Codeletters are thoseused in Figs.2 and3. Eggsample size is 20 per species;sample sizes for femalebody weights are given in parentheses.Sources of data:i = Mattox(1970), 2 = Cramp(1980), 3 = Dementievand Gladkov (1954), 4 = Kiff (unpubl.data), 5 = Brownand Amadon(1968), 6 = Porterand Wiemeyer(1972), 7 = Walter (1979),8 = Enderson(1964), 9 = Glutzvon Blotzheim et al. (1971),10 = Hector(pers. comm.) In additionto the smallsize of the eggs,there are other reasonsto doubttheir au- thenticity.In a longpaper on the habitsof Trinidadand Tobagobirds which Smooker co-authored(Belcher and Smooker1934-1937), the only allusionto breedingin the Or- ange-breastedFalcon account is the tentativestatement, "we think it a resident,"de- spitethe fact that the 1930set had been taken years earlier. The Orange-breastedFal- con hasalways been regardedas exceedinglyrare in Trinidad,and there is as yet no additionalevidence that it breedsthere (ffrench1973). Both setsof eggswere takenfor Smookerby a native,apparently the sameone in eachinstance, and Smookermay never have seenthe nestingbirds personally.Several other instancesof misidentificationof eggsin the Belcher-SmookerTrinidad collection have been reported (ffrench 1973, Kiff in press),probably resulting from errors by nativehelpers, rather than dishonesty on the part of the collectors. Three falconspecies, the AplomadoFalcon, Bat Falcon,and AmericanKestrel (F. sparverius)are knownto nestin Trinidad.The eggsof the latter are somuch smaller thanthe onesin questionthat it cansafely be ruledout asa candidatefor layingthem. The Smookereggs are mostsimilar in sizeto thoseof AplomadoFalcons and plotting the measurementsof the questionableeggs against mean female Aplomado Falcon body weightyields a pointvery close to the oneseen for the species(Fig. 2). Bothnests, how- ever, were saidto be in cavities,one (WFVZ no. 15,728)being "30 feet up in a knot- hole in a ceibatree," whereasthe otherwas "in the hollow at the baseof a palm-branch Fall 1981 Boyceand Kiff -- FalconEggs 91 about40 feetup" (Coltart1952). According to DeanHector (pers. comm.), who has studiedthe species intensively, there is no recorded instance of AplomadoFalcons nest- ingin suchcavities. The nest descriptions are virtually identical to thosewhich Smooker gavefor setsof Bat Falconeggs collected for him in Trinidadand now in the WFVZ collection,but the eggswould be unusuallylarge for that species.We plottedegg breadthagainst egg length for twelve species of falconsand constructed a 90% predic- tioninterval for the genus. The set deposited in the WFVZ lies outside this interval (Fig. 3) andthe other set lies on the line. This suggests that these purported eggs may not evenbelong to the genus!Interestingly, the eggsare mostsimilar in colorand size to thoseof theYellow-headed Caracara (Milvago chimachima) (a genus related to Falco), butthat species israre in Trinidadand is not known to breedthere (ffrench 1973). We constructeda 95% confidence interval for a randomsample of Yellow-headedCaracara eggs(n = 20) andnoted that the Orange-breastedFalcon eggs lie on bothsides of the lowerlimit (42.59 x 34.56mm). Thus,it isimpossible to identify Smooker's alleged Orange-breasted Falcon eggs une- quivocally,but it isvirtually certain that they are not eggs of theOrange-breasted F•I- conand that the eggs of thatspecies await formal description. Acknowledgements We aregrate fid to DeanHector and Dave Craigie for commentingon the manuscript andto ClarkSumida for preparingFigure 1. Hector,G. Clark,and M. Kirvenprovided uswith unpublishedweight data. Juhani Terhivuo provided us with datafrom the Zoo- logicalMuseum of theUniversity of Helsinki.Julie Kiff assistedin measuringeggs. This notewas supported by theWestern Foundation of VertebrateZoology. Literature Cited Amadon,D. 1943.Bird weights and egg weights. Auk 60:221-234. Anderson,D. W., andJ. j. Hickey.1970. Oological data on egg and breeding character- istics of Brown Pelicans. Wilson Bull. 82:14-28. Belcher,C., andG. D. Smooker.1934-1937. Birds of the Colonyof Trinidadand To- bago(in sixparts). Ibis 13:572-595,et seq. Bent,A. C. 1938.Life historiesof NorthAmerican birds of prey. Pt. 2. Bull. U.S.Nat. Mus. 170. Brown,L. H., andD. Areadon.1968. Eagles, hawks and falcons of theworld. McGraw- Hill, New York.945 pp. Coltart,N. B. 1952.Eggs of theFalconinae. Oologists' Record 26:42-46. Cramp,S. (ed.).1980. Handbook of thebirds of Europe,the Middle East and North Af- rica.Vol. 2. OxfordUniv. Press, Oxford. 687 pp. Dementiev,G. P. andN. A. Gladkov.1954. Birds of the SovietUnion, Vol. 1. Moscow: StatePublishing House. Enderson,J. H. 1964.A studyof thePrairie Falcon in the CentralRocky Mountain Re- gion.Auk 81:331-352. ffrench,R. 1973.A guideto thebirds of Trinidadand Tobago.Livingston Publ. Co., Wynnewood,Pennsylvania. 470 pp. Glutzyon Blotzheim, U. N., K. M. Bauer,and E. Bezzel.1971. Handbuch der Vogel Mit- teleuropas.Vol. 4. AkademischeVerlagschaft, Frankfurt am Main. Heinroth,O. 1922.Die Beziehungenzwischen Vogelgewicht, Eigewicht, Gelegegewicht undBrutdauer. ]. f. Ornithol.70:172-285. 92 RAPTOR RESEARCH Vol. 15 No. 3 Huxley,J. S. 1923-1924.On the relationbetween egg weight and body weight in birds. ]. Linnaean Soc.,London 36:457-466. Kiff, L. F. In press.Notes on eggsof the Hook-billedKite Chondrohieraxuncinatus, in- cludingtwo overlookednesting records. Bull. of theBritish Ornithologists' Club. Lack, D.