Aging and Sexing Black-Headed Grosbeaks in Alternate Plumage

j. Field Ornithol., 58(3):311-317

AGING AND SEXING BLACK-HEADED GROSBEAKS IN ALTERNATE PLUMAGE

GEOFFREY F•. HILL • Departmentof Biology Universityof New Mexico Albuquerque,New Mexico 87737 USA

Abstract.--Becausemale Black-headedGrosbeaks (Pheucticus melanocephalus) display delayed plumagematuration specialcare must be taken in distinguishingamong the age-sex classesof this species.I presenta methodfor scoringthe plumagebrightness of Black-headed Grosbeaksand a schemeby which plumage characteristicscan be used to distinguishage and sex classes.Males in first alternate plumagecan be separatedfrom femalesprimarily by the presenceof black featheringon the face or chin, black rectrices,or black upper wing covertsand from malesin definitivealternate plumage principally by the presenceof brown primaries.I was not able to determinethe age of femalesin alternateplumage, but by using featherwear I was able to distinguishadult female from juvenile grosbeaks.Finally, males in first basicplumage are generally brighter than femalesin first basic plumage, so most juvenilescan be sexedby plumagecharacteristics.

EDAD Y SEXO DE PHEUCTICUS MELANOCEPHALUS EN PLUMAGE DE TRANSICION Sinopsis.--Debidoa que la maduraci6ndel plumaje en los machosde Pheucticusmelano- cephaluses lenta debe tenersemucho cuidado para poder distinguir entre las diferentes etapasde edad-sexode esta especie.Presento un m•todo para clasificarla brillantez del plumajede estasaves y un esquemamediame el cual las caracterlsticasdel plumaje pueden serutilizados para distinguirla edady el sexo.Los machos en su primer plumajede transici6n pueden ser distinguidosde las hembras por la presenciade plumas negras en la cara o barbilia, rectricesnegras o cobijas superioresdel ala negras. Los machosen plumaje de transici6ndefinitivo se diferenciande las hembraspor tener las primarias de color pardo. No pude determinar la edad de hembrasen plumaje transicional.Sin embargo,utilizando el desgastede las plumaspude diferenciarentre hembrasadultas y juveniles.En avesen su primerplumaje bfisico, note que generalmentelos machos son mas brillantes que las hembras. E1 sexode la mayoria de losjuveniles se puededeterminar por caracteristicasparticulares del plumaje.

In severalof the conspicuouslysexually dichromatic passerine species that breedin North America malesdelay attaining a definitivealternate plumagefor one year, spendingtheir first springand summerin a generally dull, but highly variable, first alternate plumage (Rohwer et al. 1980). In someof thesespecies the male first alternateplumage closely resemblesthe male definitivealternate plumage; in othersit morenearly resemblesthe plumageof females;and in some,such as the Black-headed Grosbeak(Pheucticus melarzocephalus), it varies between these extremes. Thus, correctlydetermining the age of malesof thesespecies presents a challenge,as well as an opportunityfor distinguishingseveral age classes

• Currentaddress: Museum of Zoologyand Departmentof Biology,University of Michigan,Ann Arbor, M•chigan d8709 USA.

311 312] G.E. Hill j. FieldOrnithol. Summer 1987 of males. In this paper I presenta schemefor age and sex determination of Black-headedGrosbeaks in alternate plumage in the spring and summer. Guides currently available for aging and sexingBlack-headed Gros- beaks (e.g., North American Bird Banding Manual 1984, Oberholser 1974, Sheppardand Collins 1971) are either inaccurateor incomplete and can lead to errors in aging and sexingthis species.

METHODS This paper is basedon three seasons(1984-1986) of summer (May- August) fieldwork in central New Mexico during which I capturedover 500 Black-headedGrosbeaks of all age-sexclasses. Each bird was color bandedand its plumagecoloration and pattern recorded(see Appendix). Basedon plumagecharacteristics, I assigned each bird to an age-sexclass at the time of capture. For brevity I will follow the terminologyof the U.S. Fish and Wildlife ServiceBird Banding Laboratory by referring to malesin first alternateplumage as SY (secondyear), malesin definitive alternate plumage as ASY (after secondyear), females in at least their first alternate plumage as AHY (after hatch year), and birds still in juvenal/first basicplumage as HY (hatchyear). I testedthe validity of my aging and sexingtechnique by recapturing markedindividuals in subsequentyears and by collectinga seriesof female and SY male grosbeaks.Twenty-nine birds that I recordedas females and 11 birds that I recordedas SY males at the time they were initially bandedwere recapturedthe following year. All "females"retained their female plumage (none matured into an ASY male) and all "SY males" acquired a definitive alternate plumage. This confirmed my ability to distinguishbetween female and SY male grosbeaksand also indicated that males delay attaining a definitive alternate plumage for only one spring. I recapturedone male that had been banded as an HY bird in the previousyear. It returnedin a typical first alternateplumage (i.e., not a definitive alternate plumage). In addition, I held 13 males in captivity for their first winter. Although someof these males attained an adult- male-likeplumage, none acquired a completedefinitive alternate plumage. (The two brightestmales each attained plumage scores[see Appendix] of 44, two points higher than the brightest SY males that I handled in the wild, but they still retained all juvenal primaries and somejuvenal secondaries.)These resultssupport the assumptionthat yearling males do not molt from a first basic into a definitive alternate plumage and, thus, that males in a definitive alternate plumage are at least two years old (ASY). As an independenttest of my key, I collected25 birdsbetween 13 May and 14 June that were either femalesor SY males.Each bird was assigned an age-sexclass before it was dissectedand its sex positivelydetermined. In all casesI was able to assignthe correctsex to a bird by examining its plumage. vol.58, •o. • Agingand Sexing Black-headed Grosbeaks [313

AGING AND SEXING

BecauseBlack-headed Grosbeaks are sexuallydichromatic, ASY males can be readily distinguishedfrom females.ASY males have a jet black head including the chin, flashy black and white wings and tail, and a brilliant rufousbreast, rump, nape, and flanks. In contrast,females have a brown head with buffy to white (occasionallylemon yellow) crown and superciliarystripes and a pale chin. Their wings are brown and buff and their breast, nape, rump, and flanks are generally dull rufous to buff with a variable amount of streaking. The mostdifficult age-sexclass to distinguishis SY malesbecause they display a variable plumagethat at one extremeis much like the plumage of a female and at the other extreme much like that of an ASY male. Dull plumagedSY malescan be distinguishedfrom femaleson the basis of four plumagecharacteristics: (1) black featheringon the face or head (particularlybetween the crownand superciliarystripes), (2) blackfeath- ering on the chin (evenvery spottyblack), (3) black upper wing coverts, or (4) black rectriceswith bold white terminal spotson the underside.In a sample of 106 SY male grosbeaksI found that 96.2% have someblack featheringon their face or head, 87.7% had someblack featheringon their chin, 67.0% had black upper wing coverts,and 63.2% had one or more adult (black) rectrices. All SY males that I examined had at least one of these features, so the absenceof all of these characteristics indicates that the bird is a female. Many femaleshave a relatively bright rufous breast,nape, and/or rump with little or no streakingwhile someSY males show little rufous pigmentationand heavy streakingof the same regions.Contrary to the Bird Banding Manual thesetraits cannotbe used to separatefemales and yearling males. The presenceof a brood patch indicatesthat a bird is a female, but care must be taken in what is called a broodpatch. Both male and female Black-headedGrosbeaks sit on eggs(Weston 1947, Ritchison 1983) and both ASY and SY male grosbeakshave a large path of bare skin on their belly and breastduring the incubationperiod. Only in femalesdoes the patch becomehighly vascularizedand puffy, and only sucha puffy brood patch shouldbe usedto identify females. Bright SY males can be distinguishedfrom ASY males on the basisof their brown and indistinctly spotted(as opposedto black and boldly spotted)primaries and secondariesand, secondarily,by the presenceof brown and indistinctly spottedrectrices. In a sample of 106 SY males, all hadjuvenal primaries,93.7% had completelyjuvenal secondaries,and 91.5% had one or morejuvenal rectrices. I was unableto agefemales by plumage.I attemptedto distinguishSY and ASY femaleson the basisof featherwear, assumingthat ASY females had undergonea prebasicmolt of their primaries and secondariesand that SY femaleshad not (Oberholser 1974), but I coulddetect no consistent differences even between marked birds that I knew to be SY and ASY. Oberholser(1974) suggestedthat malesin a secondnuptial plumage 314] G.E. Hill J.Field Ormthol. Summer 1987

10.

z •;• FEMALE :3 5 .

lb i5 2'0 2'5 PLUMAGE SCORE

FIGURE1. Distribution of plumagescores of juvenile Black-headedGrosbeaks of known sex.

differ from males in third and subsequentnuptial plumagesby having a more extensivecrown stripe, duller rufous breast and flanks, lessblack on the chin, and browner wings and tail. My observationsindicate that this is not the case.Considerable intra-male plumagevariation existsin thesecharacters, but after the acquisitionof the secondnuptial plumage, the variation is not age related. The plumage scoresof the secondand third nuptial plumagesdid not changein the one male for which I was able to make such a comparison.Moreover, the plumage scoresof 10 malesknown to be in a secondalternate plumage had the samemedian value (46) and a similar range (42-47) as the plumagescores of 42 males known to be in at least a third alternate plumage (range = 39-48; P > 0.15; Mann-Whitney U-test). No ASY male lost or gained more than 3 points on its plumage scorebetween years, and there was no tendency for ASY males to either increaseor decreasein plumage scoresbetween years (P > 0.14; sign test). After 1 June it becomesimportant to distinguishHY individualsfrom AHY females.These two age-sexclasses of grosbeaksoverlap completely in plumagecolor and pattern, soaging characteristicsother than plumage pigmentationmust be employed.Examination of the skull for pneuma- tization (North AmericanBird BandingManual 1984) is the mostreliable way to distinguishAHY female from HY Black-headedGrosbeaks, but thick skin about the cranium can make skull aging difficult. Fortunately, there are other characteristics that can be used at least for the months of June, July, and August. The presenceof a vascularizedbrood patch indicatesthat a bird is an AHY female. Conversely,juvenal feathering on the flanks identifiesHY birds. Young grosbeaksmolt the majority of their juvenal plumage soon after fiedging, but frequently retain fluffy juvenal feathersalong their flanks at least until 1 August. A third dis- tinguishing feature is the degree of feather wear of the remiges and rectrices.By mid to late summer when HY birds begin to appear the remigesand rectricesof AHY femalesare very worn while thoseof HY birds are fresh with untatterededges. Vol.58, No. 3 Agingand Sexing Black-headed Grosbeaks [315

o u.I 5*

5 10 1•,

PLUMAGE SCORE

FIOURE2. Distribution of plumage scoresof juvenile Black-headedGrosbeaks netted at Cedar Crest, New Mexico.

The banding manual proposesthat eye color be used to separateHY and AHY birds. I found that eye color differs very subtly betweenHY and AHY grosbeaks,the eyesof young birds having a slightlygrayer cast than the deep chocolatebrown of adults. Becausethe difference is so slight, I recommendthat eye color be usedonly when other aging tech- niques are not feasible and after one has examined several known HY and AHY grosbeaks. Finally, I found that it is often possibleto distinguishthe sex of HY grosbeaksusing plumage characteristics.Relative to females, male HY grosbeaksgenerally display brighter rufous pigmentation of the breast, flanks, nape, and crown stripe and less streaking on the breast. In a sample of 39 HY Black-headedGrosbeaks of known sex, I found very little overlap in the plumage scores(see Appendix for methodology)of the two sexes(Fig. 1, P < 0.001; t-test). Moreover, for the 98 HY birds that I banded and released, I found a highly bimodal distribution of plumagescores (Fig. 2) suggestingsexual dichromatism.Because the two sexesoverlap slightly in appearance and the indexing methodologyis complex and requires practice, I have not attempted to include a means for distinguishingmale and female HY grosbeaksin the key. Researchers who need to determine the sex of HY grosbeaksshould use the indexing method outlined in the Appendix to scorea sample of HY grosbeaksof known sex and then compareunknown birds to this standard.

PROPOSED REVISED KEY FOR AGING AND SEXING BLACK-HEADED GROSBEAKS

1A Black featheringbetween crown and superciliarystripes, or black chin feathers,or black outer rectriceswith bold white terminal spots,or black upper wing coverts ...... Male (see 2) lB Area betweencrown and superciliarystripes gray/brown, chin buff to white, rectrices brown with indistinct terminal spots, upper wing covertsbrown with no contrastwith remiges ...... Female/HY (see5) 316] G.E. Hill J.Field Ornithol. Summer 1987

2A Most primaries or secondariesbrown with indistinct spots,or somerectrices brown with indistinct terminal spots ...... SY 2B Primaries and secondariesblack with bold white markings and rectricesblack with bold white terminal spots ...... ASY 3A January throughMay ...... AHY-Female 3B June through August ...... see4 3C Septemberthrough December ...... see5 4A Flight feathersfaded, worn, frayedor flight feathermolt or vascularizedbrood patch ...... AHY-Female 4B Flight feathersbright, freshand unworn with no evidenceof molt or vascularized broodpatch ...... HY-U 5A Skull incompletelypneumatized ...... HY-U 5B Skull completelypneumatized ...... U-U

ACKNOWLEDGMENTS

All bandingand collecting of Black-headedGrosbeaks reported in thispaper was conducted with permissionof the USFWS (permit #21661 and #680917) and the New Mexico Departmentof Game and Fish (permit #1707). I thank William Howe, J. David Ligon, Kenneth Parkes, Sievert Rohwer, and Edward H. Burtt Jr. for reading an earlier version of this manuscriptand making many helpful suggestions.My fieldwork was supportedby an E. Alexander BergstromAward from the Associationof Field Ornithologists,a Paul A. Stewart Award from the Wilson Ornithological Society,a Frank M. Chapman Research Grant from the American Museum of Natural History, a grant-in-aid of researchfrom SigmaXi, The ScientificResearch Society, and a researchgrant from the WesternBird BandingAssociation and the Graduate StudentAssociation and the Department of Biology at the University of New Mexico.

LITERATURE CITED

OBERHOLSER,H. C. 1974. The bird life of Texas. Vol. II. Univ. of Texas Press,Austin. RITCHISON,G. 1983. Breeding biology of the Black-headedGrosbeak in northern Utah. West. Birds 14:159-167. ROHWER,S. A., S. D. FRETWELL,AND D. M. NILES. 1980. Delayed maturation in passerine plumagesand the deceptiveacquisition of resources.Am. Nat. 115:400-437. SHE??^RD,J., ^NOC. COLLINS. 1971. Bandingworksheet for westernbirds. Supplement to West. Bird Bander. U.S. FISH AND WILDLIFE SERVICE AND CANADIAN WILDLIFE SERVICE. 1984. North Amer- ican bird bandingmanual. Department Manuals and Reports Division, Ottawa, On- tario. WESTON,J. G., JR. 1947. Breedingbiology of the Black-headedGrosbeak. Condor 49: 54-73. ReceivedSept. 10 1986; accepted8 Dec. 1986.

APPENDIX I quantifiedthe relativeplumage brightness of Black-headedGrosbeaks by scoring16 featheredregions on a scalefrom 0 (dullest/mostfeminine) to 5 (brightest/mostmasculine). The sixteen scoreswere then added togethergiving an overallbrightness index for that bird. Below are listed the sixteen charactersthat I used in this index. To devisea four point scalefor characterssuch as the rufous of the breast, flanks, nape, and rump, one must first handle enoughbirds to get a feel for the extremes and then assigncategories accordingly. Vol.5a, •4o. • Agingand Sexing Black-headed Grosbeaks [317

1) Eye stripecolor: no stripeby default scores3 in color.Otherwise, 3, bright rufous;2, dull rufous; 1, bright buff; 0, dull buff. 2) Crown stripe color: samescale as eye stripe color. 3) Head color: 3, black; 0, brown (intermediate scoresnot used). 4) Chin color: 3, black; 0, buff or white (intermediate scoresnot used). 5) Breastcolor: 3, bright rufous;2, dull rufous;1, bright buff; 0, dull buff. 6) Breaststreaking: 3, no streaking;2, faint streaking;1, distinctbut not bold streaking;0, bold streaking. 7) Flank color: same scale as breast color. 8) Flank streaking:same scaleas breast streaking. 9) Belly: 3, a large patch of yellow feathers •2 cm x 1 cm; 2, a patchof yellow feathers(2 cm x 1 cm and containingmore than five feathers;1, five or fewer yellow feathers;0, no yellow feathers. 10) Nape color: same scaleas breastcolor. 11) Nape streaking:same scaleas breaststreaking. 12) Rump color: samescale as breast color. 13) Rump streaking:same scale as breaststreaking. 14) Upper tail: 3, more than six black tail feathers;2, three to six black tail feathers; 1, one or two black tail feathers; 0, all tail feathers brown. 15) Under tail: 3, all tail feathersblack with bold white terminal spots; 2, sometail feathersbrown and indistinctlyspotted but both outer tail feathersblack with bold terminal spots;1, only one outer tail feather black with a bold terminal spot;0, both outer tail feathers brown and indistinctly spotted. 16) Upperwing: 3, primaries, secondariesand covertsall black with boldwhite spots;2, somebut not all primariesor secondariesblack with bold white spots;1, only the covertsblack, contrastingwith brown primaries and secondaries;0, primaries, secondaries,and covertsbrown with indistinct spots.