October 1996] ShortCommunications and Commentaries 955 ZUSERSI-IULER,E. A. 1971. Observaciones sobre las Received•6 August 1994, accepted 9 May 1996. aves de la Provincia de Buenos Aires. Hornero AssociateEditor: J. S. Marks 11:98-112. The Auk 113(4):955-957, 1996 Interbreedingof a TricoloredHeron and a SnowyEgret in SouthDakota WILLIAM A. M•cS, ' DAVID E. NAUGLE,• REX R. JOH•qSO•q,• AN• K• F. HIGGINS 2 1Departmentof Wildlife and Fisheries Sciences, South Dakota State University, Brookings,South Dakota 57007, USA; and 2NationalBiological Service, South Dakota Cooperative Fish and Wildlife Research Unit, SouthDakota State University, Brookings, South Dakota 57007, USA Reportsof interbreedingamong herons are uncom- servingthe TricoloredHeron perchedon the rim of mon and includea Little BlueHeron (Egrettacaerulea) a nestbowl. This nestcontained four light-bluisheggs and a Cattle Egret(Bubulcus ibis) in California(Bailey and was constructed of Russian olive branches about et al. 1989),a Little Blue Heron and a Snowy Egret 30 cm abovethe water. On 2 July, a Snowy Egret was (E. thula) in Florida (Sprunt 1954), and a Little Blue incubatingthe remainingtwo eggsin the marked Heron and a Tricolored Heron (E. tricolor)in Arizona nest, which also contained two nestlings. While we (Phillipset al. 1964).In addition,a possiblehybrid were observingthe incubatingSnowy Egret,a Tri- betweena SnowyEgret and a TricoloredHeron was colored Heron landed near the nest. Subsequently, photographedin Floridain 1960 (Dickermanand we observeda "nest relief ceremony,"in which the Parkes1968). In this paper, we report interbreeding TricoloredHeron and the SnowyEgret raised their betweena TricoloredHeron and a Snowy Egret. To headplumes and began bill-nibbling and vocalizing our knowledge,this hybrid combinationhas not been (seeRodgers 1977). Following the nestrelief cere- reportedpreviously. mony,the TricoloredHeron settled on the nestand Tricolored Herons and Snowy Egrets occur sym- incubatedthe eggs. patricallyalong the Atlanticcoast from New Yorkto All four eggshad hatched by 6 July.During that SouthAmerica, and alongthe Pacificcoast from Mex- visitwe photographedand recorded a nestrelief cer- ico to Peru (Hancock and Elliott 1978). Tricolored emonyon standard1.25-cm VHS tape. The Snowy Heronsrarely nest inland as far northas South Dakota Egretfed the chicks regurgitated food. The Tricolored (Schmidt 1979,Skadsen 1986), whereas Snowy Egrets Heron also fed the chicksin the absenceof the Snowy havebeen locally common breeders in easternSouth Egret.On 1 August,photographs were taken of the Dakotasince at leastthe early 1980s(South Dakota fourjuveniles (which were in thelate branching stage Ornithologists'Union 1991). of development)at the nest site (VIREO accession On 23 June 1995, we observeda Tricolored Heron batchV06/24/001-005; Academyof Natural Sciences, in a mixed-speciesheronry in Brown County, South Philadelphia).Their plumage and soft-part colors were Dakota(45ø40'N, 98ø05'W). This observationwas only unlike thosetypical of juvenileSnowy Egrets or Tri- the fifth record of a Tricolored Heron in South Dakota coloredHerons (McVaugh 1972, 1975). Their heads (SouthDakota Ornithologists'Union 1991).The her- weremarked with a gray-browncrown that extended onry was in a flooded, 5-ha stand of dead Russian downthe napeof the neckand graded to a slategray olive (Elaeagnusangustifolia) trees. Extensive flooding on the backand wings(see Fig. 1). The sidesof the in 1993 and 1994 had increasedthe available aquatic neckand breast varied among individuals from slate habitatand probablycontributed to the establishment grayto palerufous. All juvenileswere white on the of the heronry.Nesting species included Cattle Egrets, head and undersidesof the neck and body. Their Great Egrets(Ardea alba), Snowy Egrets,Little Blue irideswere yellow, and their bills wereblack above Herons, and Black-crownedNight-Herons (Nyctico- anddark orange below, grading to black distally. The rax nycticorax).About 1,200 pairs of herons (mostly legswere yellow-green proximally and posteriorly, CattleEgrets) nested in the heronryin 1994(Peterson gradingto darkon thedistal anterior surface. There 1995),and about 5,950 pairs (95% Cattle Egrets) nested was no brownish-red color on the primary or sec- there in 1995(Naugle unpubl. data). ondarycoverts or on the sidesof the neckor breast, On 30 June 1995, we marked a nest site after ob- aswould be typicalof juvenileTricolored Herons. 956 ShortCommunications andCommentaries [Auk, Vol. 113 FIG. 1. JuvenileTricolored Heron x Snowy Egret,Brown County, SouthDakota, 1 August 1995. Successfulinterbreeding, although rare in natural Departmentof Game, Fish and Parks,South Dakota settings,occurs when interspecificisolating mecha- StateUniversity, U.S. Fish and Wildlife Service(Re- nismsbreak down (Mayr 1963).Isolating mechanisms search Work Order No. 54), and the Wildlife Man- maybe morphological,ecological, or behavioral(Dusi agementInstitute. We thank D. Hinrichs for granting 1968). Promiscuousbehavior also may give rise to accessto the heronry, and J. Rodgers,Jr., E. Dowd interbreedingin largeheronties (Hancock and Elliott Stukel, F. B. Gill, G. R- Graves, and R. S. Palmer for 1978).Palmer (1962) reported several observations of reviews of the manuscript. promiscuityin Snowy Egrets.Birds locatedoutside their normalrange and isolatedfrom conspecificsmay acceptrelated speciesas matesto form mixed-species pairs (Mayr 1963).Factors that may have promoted BmLEY, S. F., R- A. EmCKSON,AND D. G. Y•m. 1989. the hybrid pairing we observedinclude the largecon- Middle PacificCoast region. AmericanBirds 43: centrationof egretsand heronsin closeassociation, 1362-1366. and the presenceof a singleTricolored Heron among DIc•, R- W., AND K. C. PARKre. 1968. Notes a low number of Snowy Egrets. on the plumagesand genericstatus of the Little Acknowledgments.--Staffof Sand Lake National Blue Heron. Auk 85:437-440. Wildlife Refugeprovided financialassistance and lo- Du$I, J. L. 1968. The competitionbetween Cattle gisticalsupport. Funding for this projectwas provid- Egretsand Little Blue Herons. AlabamaBirdlife ed by FederalAid to WildlifeRestoration (W-107-R, 16:4-6. JobNo. 8), SouthDakota Cooperative Fish and Wild- HANCOCK,J., AND H. ELLIOTT. 1978. The herons of life ResearchUnit in cooperationwith SouthDakota the world. Harper and Row, New York. October1996] ShortCommunications andCommentaries 957 MAYR,E. 1963. Animal speciesand evolution.Har- RODGERS,JR., J. A. 1977. Breeding displays of the vard University Press, Cambridge, Massachu- Louisiana Heron. Wilson Bulletin 89:266-285. setts. SCHMIDT,R. 1979. First nestingrecord of a Louisiana McVAUGH,W., JR. 1972. The developmentof four Heron in North Dakota. Prairie Naturalist 11:93- North Americanherons. Living Bird 11:155-172. 95. McVAUGH,W., JR. 1975. The developmentof four SKADSEN, M. 1986. First South Dakota Tricolored North Americanherons. Living Bird 14:163-183. Heron nest. South Dakota Bird Notes 38:95-96. PALMER,R. S. 1962. Handbook of North American SOUTH DAKOTA ORNITHOLOGISTS' UNION. 1991. The birds,vol. 1, loonsthrough flamingos.Yale Uni- birds of South Dakota, 2nd ed. Northern State versity Press,New Haven, Connecticut. University Press,Aberdeen, South Dakota. PETERSON,R.A. 1995. The South Dakota breeding SPRUNT,A., JR. 1954. A hybrid between the Little bird atlas.Northern StateUniversity Press,Ab- Blue Heron and the Snowy Egret. Auk 71:314- erdeen, South Dakota. 315. PHILLIPS,A., J. MARSHALL,AND G. MONSON. 1964. The birdsof Arizona.University of ArizonaPress, Received18 December1995, accepted5 June1996. Tucson. AssociateEditor: A. J. Baker The Auk 113(4):957-960, 1996 Skewed Sex Ratios in Cooper'sHawk Offspring ROBERTN. ROSENFIELD,TMJOHN BIELEFELDT, 2 AND SusAN M. VOS3 •Departmentof Biology,University of Wisconsin,Stevens Point, Wisconsin 54481, USA; 2ParkPlanning, Racine County Public Works, Sturtevant, Wisconsin 53177, USA;and 33500348th Avenue, Burlington, Wisconsin 53105, USA The selective forces influencing avian sex ratios determined(see Newton and Marquiss1979, Rosen- have received much recent attention (e.g. Bednarz field et al. 1985). We also calculatedsex ratio among and Hayden 1991,MacWhirter 1994,Weatherhead and fledglings(_> 25 daysof age)when, followingbanding Montgomerie1995, Leroux and Bretagnolle1996). Go- and sexing of nestlings (and some subsequentmor- waty (1993)and Weatherhead and Montgomerie (1995) tality), we revisitednests opportunistically for other noted the paucityof information on sexratios in avian purposes(Rosenfield and Bielefeldt 1993a, b). For offspringand called for more empirical data. Here, temporalanalyses of offspringsex ratios, time of clutch we provide new data for Cooper's Hawk (Accipiter completion was determined by back-dating from cooperii)based on 16 yearsof study in Wisconsin. nestling age, assuming a 34-day incubation period Rosenfield et al. (1985) found that sex ratios did not (Rosenfield and Bielefeldt 1993a). differ significantly from unity (53.5% males, 46.5% Becausewe found no significantdifferences in sex females;X 2 test, P > 0.25) in 71 broodsof Cooper's ratios among years at fertilization (X2 = 13.87, df = Hawks in Wisconsin from 1980 to 1983. Further re- 15, P = 0.54) or the nestlingstage (X 2 = 16.07,df = searchthrough 1995 has increasedthis samplesub- 15, P = 0.38), we pooled data acrossyears. We ex- stantially and changed our earlier conclusion. Based amined sex ratio at fertilization in 554 eggs at 130 on morphometricdifferences that are apparent at