Aging Catharus Thrushes by Rectrix Shape

j. Field Ornithol., 60(2):230-240

AGING CATHARUS THRUSHES BY RECTRIX SHAPE

BEVERLY COLLIER AND GEORGE E. WALLACE Long Point Bird Observatory Box 760 Port Rowan, Ontario NOE 7MO, Canada

Abstract.--Rectrix shapewas examinedas a methodof agingthe four speciesof migratory North American Catharusthrushes. The tip angle of pointed and rounded rectriceswas found to differ significantlyfor eachspecies. The observationthat pointedrectrices indicate hatching/second-yearbirds whereas rounded rectrices indicate after-hatching/after-second- year birds correspondedto other acceptedaging criteria. Rectrix shape allowed greater precisionin aging than keys in current use.

UTILIZACI•N DE LA FORMA DE LAS RECTRICES PARA DETERMINAR LA EDAD EN ZORZALES DEL G•;NERO CATHARUS Rcsumcn.--La forma dc las rcctriccsfuc utilizada comom•todo para dctcrminar la cdad dc cuatrocspccics dc zorzalcsCatharus spp.) Nortc Americanos.Sc cncontr6quc difcrlan significativamcntccn cadacspccic cl •tnguloapical dc las rcctriccspuntiagudas y rcdondas. Rcctriccspuntiagudas corrcspondicron a nacidosdcl afio/avcsdc segundoafio micntrasquc rcctriccsrcdondcadas a avescn dondcno habla rcmancntcdcl plumajcjuvcnil. La forma dc las rcctriccspermite dctcrminar con mayor prccisi6nla cdad de estasaves, quc otros m&odos cn uso.

In Europe, rectrix shape is usedwidely as a characterto distinguish immature passerinesfrom adults in the hand (Svensson1984). In North America, rectrix shape has proven useful for aging a variety of species (Fairfield and Shirkoff 1980, Meigs et al. 1983, Samson 1974, Yunick 1983) and aging passerinesby rectrix shapeand other plumagecharac- teristicshas been advocatedrecently in Pyle et al.'s "IdentificationGuide to North American Passerines"(1987). If plumage characteristicsare to be useful, quantificationof the methodsis required to establishtheir reliability. Our aim is to quantify the rectrix shapemethod of agingNorth American Catharus. Currently acceptedmethods of agingNorth AmericanCatharus thrushes include noting the presenceor absenceof greater secondarycoverts and/or tertials retained from juvenal plumage and the degree of skull pneumatization (USFWS and CWS 1978). Using these criteria, birds may be aged as hatching-year(HY) or second-year(SY) by the presence of retainedjuvenal covertsor an incompletelypneumatized skull. Alter- natively, birds may be aged as after-hatching-year(AHY) when there are no remnants of juvenal plumage and the skull is completelypneumatized or (in fall only) has small pinhole "windows." Thus, SY in fall and ASY in spring are not consideredacceptable age codes. Personal observations,and those of Pyle et al. (1987), indicate that rectrix shapeis usefulin agingCatharus thrushes. Juvenal rectrices appear more pointed than thoseof subsequentplumages. The first prebasicmolt

230 Vol.60, No. 2 AgingThrushes byRectrix Shape [23 1 of Catharusthrushes is partial and all flight feathersare retained until the secondprebasic molt (Dwight 1900). Hence, HY/SY birds should have pointed rectrices;AHY/ASY birds rounded ones. The following studyquantifies variability in rectrix shapein Hermit Thrush (Catharus guttatus),Veery (C. fuscescens),Swainson's Thrush (C. ustulatus),and Gray-cheekedThrush (C. rainlinus).We investigatethe reliability and usefulnessof rectrix shapeas a field techniquefor agingthese species by comparingrectrix shapeto agesassigned by acceptedplumage and skull criteria.

METHODS The four speciesof Catharusthrushes were studiedat Long Point Bird Observatory,Long Point, Ontario during springand fall migrationsfrom 29 Mar. 1986 to 23 May 1987. Thrushes were capturedin mist nets at three field stations located along the 30 km point. All stations lie at approximately42ø35'N and between80o03 ' and 80ø25'W.Results from the three stationswere examinedcollectively. We recordedthe following information from a sampleof eachspecies: rectrix shape,by visual inspection,as "pointed" or "rounded";the presenceor absenceof greatersecondary coverts retained from juvenal plumage; and the degreeof skull pneumatizationusing Baird's (1964) technique. Rectrix shapewas determinedby evaluatingthe anglesat which the distal edgesof the inner and outer websmet the rachisat the tip of the feather. On rounded rectricesthe distal edgesare more nearly per- pendicularto the rachis,giving both webs, particularly the outer,a squared appearance. On pointed rectricesthe distal edgesmeet the rachis at a much shallowerangle, creating a tapered appearance(Fig. 1). In order not to bias observers'opinions of rectrix shape,shape was judged before skulling and before examination of the wing coverts.Reconsideration of rectrix shape was not permitted. On somebirds, many juvenal coverts had been retained and it was difficult to ignore their presence.However, in thesecases, every effort was made to judge rectrix shapeobjectively. In the courseof the study, we encountereda small number of rectrices which defied classificationas pointed or rounded. These were termed "intermediate." A typical pattern of Catharuspneumatization and our rating system are shownin Figure 2. Small, horizontal,elliptical unpneumatized patches ("windows") within the occipital triangle were ignored, as these are presentin all individualsregardless of age (pers.obs.). Windows in nearly pneumatizedskulls (="3") were measuredwith calipersto the nearest 0.1 mm alongtheir longestaxes. We trained ten field biologists,in addition to ourselves, to collect the above data. After recordingthe aboveinformation, either a left or right fifth rectrix was collectedfrom a sampleof individualsof each of the four species. On any individual, rectricesone through six differ among themselvesin basic shape. Therefore, it was necessaryto standardizethe rectrix col- lected.Feathers were then measuredusing an adaptationof the method 232] B. Collierand G. E. Wallace J.Field Ornithol. Spring 1989 '1

FIGURE1. Rectrix tips of a Catharusthrush. A. The tip angle0 was the anglebetween the lines ab and bc where x = y = 2 mm. B. A pointedfifth rectrix typical of an immaturein first basicplumage. C. A roundedfifth rectrixtypical of an adult in basic plumage. employedby Meigs et al. (1983) (Fig. 1). Rectriceswere placedflat on a 1 mm grid with the rachisoriented along one grid line. The locations where the outer and inner feather edgescrossed a grid line 2 mm to each sideof the tip and parallel to the rachiswere plotted.The tip angle0 was measureddirectly with a protractorto the nearestdegree. Spring Gray-cheekedThrush rectrix sampleswere augmentedby examining specimens(rz = 31) from the Royal OntarioMuseum, Toronto. Rectrices were measuredin the samemanner, but were not removedfrom the birds. Resultswere analyzedwith standardstatistical methods (Steel and Torrie 1980). Birds with intermediate rectriceswere not included in analysesthat involvedrectrix shapeas one of the variables. Agingnomenclature follows that usedin the Bird BandingManual (USFWS and CWS 1978). For the purposesof this study,we make the followingdefinitions. A bird of knownage is a bird agedas SY in spring or as HY in fall by the presenceof an unpneumatizedskull (exceptas notedbelow) and/or retainedjuvenal greater secondary coverts or a bird agedas AHY in fall by a completelypneumatized skull and an absence Vol.60, No. 2 AgingThrushes by Rectrix Shape [233

0 T 1 2 3 4

FIGURE2. Typical pneumatization pattern of a Catharusthrush. Skulls were rated from unpneumatized("0") to completelypneumatized ("4"). T = trace pneumatization.The elliptical "windows" in the occipitaltriangle are presentinto adulthoodand, exceptto note their formation between "T" and "1," these windows were not used in the skull evaluation. Pneumatized areas are stippled. of juvenal coverts.We stressthat, by this definition,an AHY bird may be two, three or more yearsold, and thus scarcelyof known age. However, we use the term throughout the paper, and readers should be aware of our usage.Alternatively, a bird of presumedage is a bird agedby rectrix shape alone as SY or ASY in spring when the skull is completelypneumatized and juvenal covertsare absentor a bird aged as SY in fall by a nearly pneumatizedskull (="3," before 1 Oct.) or by pointed rectrices alone when the skull is completelypneumatized (before 1 Oct.). Spring migrationcovered the period 1 Mar.-30 Jun. and fall migrationthe period 1 Jul.-30 Nov.

RESULTS Table 1 divides the number of birds examined of each speciesinto categoriesbased on the three variables used in this study: rectrix shape, skull pneumatizationand juvenal covertretention. Within each season, the percentageof birds with intermediate rectrix shape never exceeded 4.5% in any species. Mean rectrix tip anglesare shown in Figure 3. Rounded and pointed formedtwo distinctclasses of rectrix shape,as definedby their tip angles, in both spring and fall for each of the four species.Within each season and species,the mean tip angle of pointed and roundedrectrices differed significantly (Table 2). Differences in rectrix shape, which could be discernedsubjectively in the field, were verified objectivelyin the lab. Thus, rectriceswhich observersjudged to be pointed were measurably pointedand thosejudged to be roundedwere measurablyrounded. Rel- ative differencesin tip angle betweenpointed and roundedrectrices were consistentfrom fall to spring in each species,and indicatedconsistency in the measurement method. Worn feathers, in which the distal 1-2 mm had abraded away, always producedlarger tip anglesthan comparable fresh feathersof the same basic shape (pointed or rounded), becauseof the way in which the tip angle 0 was measured.This was exemplified by the generallyincreased tip angle in spring rectricescompared to fall 234] B. Collierand G. E. Wallace J.Field Ornithol. Spring 1989 Vol.60, No. 2 AgingThrushes by Rectrix Shape [235

•=SPRING

O=FALL 120

11o 15+t511•11 9 lOO- ,4+

z 90-

80-

H S V G SPECIES

FIGURE3. A comparisonof the mean rectrix tip anglesof rounded(R) and pointed (P) rectricesfor Hermit Thrush (H), Swainson'sThrush (S), Veery (V) and Gray-cheeked Thrush (G). Bars represent+ one standarderror. Sample sizesare given besideeach mean. ones. Feather wear was presumably responsiblefor the only significant increase(t = 2.29, df = 24, P < 0.05) in mean tip angle from fall to spring (Swainson'sThrush pointedrectrices). In fall, pointed rectriceswere correlatedwith the presenceof an incompletelypneumatized ("0"-"2") skull (x2 values: Hermit Thrush = 62.61; Swainson'sThrush = 145.42; Veery = 24.03; Gray-cheekedThrush = 57.29, df = 1 for all, P < 0.005 for all) and retainedjuvenal coverts (x 2 values:Hermit Thrush = 59.87; Swainson'sThrush = 28.46; Veery = 19.30; Gray-cheekedThrush = 45.23, df = 1 for all, P < 0.005 for all, Table I). Similarly, rounded rectriceswere correlatedwith the pres- 236] B. Collierand G. E. Wallace J.Field Ornithol. Spring 1989

T^BLE 2. Comparisonof mean tip angles between the two classesof rectrix shape in Hermit Thrush, Swainsoh'sThrush, Veery, and Gray-cheekedThrush. Spring(S) and fall (F) resultsare treatedseparately. See Figure 3 for mean valuesand samplesizes.

Species Season df t Hermit Thrush S 30 7.97** F 14 5.20** Swainson's Thrush S 23 3.61 * F 21 6.01'* Veery S 11 6.85** F 19 4.86** Gray-cheekedThrush S 32 7.45'* F 22 4.47**

* = P < 0.01, ** = P < 0.001.

enceof a completelypneumatized skull and an absenceof juvenal coverts. Depending on the species,96.7% (n = 60)-100% (n = 25) of all birds with skull ratings "0"-"2," and 97.7% (n = 43)-100% (n = 64) of all birds with juvenal covertshad pointed rectrices.Thus, the reliability of rectrix shape as an aging criterion was strongly supportedby other acceptedaging criteria. All samplesof springbirds were madeup of just two skull classes,"3" and "4." In spring, pointed rectriceswere correlatedwith the presence of retainedjuvenal coverts(X 2 values:Hermit Thrush = 227.81; Swain- son'sThrush -- 48.45; Veery = 42.87; Gray-cheekedThrush = 7.08, df = 1 for all, P < 0.005 for all exceptGray-cheeked Thrush, P < 0.01). Depending on the species,71% (n = 7)-100% (n = 28) of all birds with juvenal covertshad pointed rectrices.The lowest confidencerate was associatedwith Gray-cheeked Thrush, probably becauseof the small sample size involved.Without this species,confidence rates ranged from 90.9% (n -- 22)-100%. In Swainson'sThrush and Veery, 86.2% (n = 29) and 87.5% (n -- 16), respectively,of all birds with "3" skullshad pointed rectrices.In these two species,pointed rectriceswere correlated with the presenceof an incompletelypneumatized skull (X2 values: Swain- son'sThrush = 38.42; Veery = 23.37, df = 1 for both, P < 0.005 for both). The sample size of Hermit Thrushes with "3" skulls (n = 7) and the overall sample size of Gray-cheekedThrush were both too small to demonstratea relationship between rectrix shape and degree of skull pneumatization.Nonetheless, the useof rectrix shapeas an aging method was again stronglysupported by currently acceptedcriteria. Incidenceof completejuvenal covertreplacement and adventitiousrectrix loss are shown in Table 3. Values in the spring rectrix loss and spring covertreplacement columns were underestimatedby an unknown amount becauseSY birds that had completelypneumatized skulls, no juvenal covertsand rounded rectrices, as a result of adventitiousloss, would have been excluded from both columns. Rates of adventitious rectrix Vol.60, No. 2 AgingThrushes by RectrixShape [237

TABLE3. Comparisonof completejuvenal greatersecondary covert replacement and adventitiousrectrix lossbetween fall (known HY) and spring(known and presumedSY) Hermit Thrush, Swainson'sThrush, Veery and Gray-cheekedThrush. Values given are percentagesof birds with no retainedjuvenal coverts(covert replacement) or with roundedrectrices (rectrix loss).Sample sizes are givenin parentheses.

Covert replacement Rectrix loss Species Fall Spring z Fall Spring z Hermit Thrush 7.1 (70) 14.1 (185) 1.51' 2.9 (69) 7.0 (158) 0.11 Swainson'sThrush 32.3 (158) 50.0 (60) 2.42** 2.6 (152) 8.8 (45) 1.60' Veery 4.2 (25) 17.9 (28) 1.57' 0.0 (25) 16.7 (24) 2.13'* Gray-cheeked Thrush 14.3 (63) 12.5 (8) 0.14 3.2 (62) 28.7 (7) 3.07**

Binomial test, * = P < 0.06, ** = P < 0.05. lossin fall amongall four specieswere comparable,and indicateda rate of rectrix losscommon to all species.Since no one specieswould be expectedto havea greaterincidence of rectrixloss, these results supported our banders'abilities to detectrounded rectrices on birdsdisplaying ac- ceptedHY characteristics.Thus, in spring,the larger variation in rates of rectrix lossamong the four specieswas probablya result of small samplesizes, especially in Gray-cheekedThrush, rather than inaccurate determinationof rectrix shape.The increasein extentof covertreplacement from fall to springin Veery, Hermit Thrush, and, particularly, Swainson'sThrush may indicatethat somebirds of thesespecies complete their covertmolt in the periodbetween fall and springmigrations. This would increasethe numberof birdsin springthat did not displayknown SY characteristics. In spring, the percentageof known (excludingbirds aged by skull alone) and presumedSY birds with completelypneumatized skulls were: Hermit Thrush--96.2% (r• = 184); Swainson'sThrush--49.1% (r• = 55); Veery--42.3% (r• = 26); and Gray-cheekedThrush--50% (r• = 8) (see Table 1). Amongknown and presumedSY birds with pointedrectrices, degreeof skull pneumatization(ratings "3" or "4") was not correlated with the presenceor absenceof juvenal covertsfor eachof Hermit Thrush (X2 = 0.16, df = 1, P > 0.50), Swainson'sThrush (X2 = 1.07, df = 1, P > 0.25) and Veery (X2 = 0.53, df = 1, P > 0.25). The samplesize of Gray-cheekedThrush was too small to permit statisticalanalysis. Lack of correlationbetween these two charactersmeans that in any sampleof SY birds there will always be a percentageof birds that have both "4" skullsand nojuvenal coverts. These birds could be accuratelyaged as SY only by rectrix shape.Thus, if springcovert replacement (from Table 2) and the aboveskull pneumatizationpercentages among known and pre- sumedSY birds are combined(i.e., multiplied as probabilities)within eachspecies, the percentageof SY birdswhich would, on average,display no agingcriteria exceptpointed rectrices are as follows:Hermit Thrush-- 238] B. Collier and G. E. Wallace J. Field Ornithol. Spring 1989 13.6%; Swainson'sThrush--24.6%; Veery--7.6%; and Gray-cheeked Thrush--6.3% (assumingno correlationfor that species).Rates of pneumatizationand extentof covertreplacement in SY birdsmay vary among subspeciesor populationsof thesefour species.However, the non-cor- relativerelationship between pneumatization and covertreplacement would probably hold for each species.Thus, for any group of SY birds, some percentageof birds would be agedless precisely as AHY if rectrix shape was not usedas an aging method. In fall, skull ratings were almost exclusivelydivided into just two classes:"4" and "0"-"2" (Table 1). Amongall species,only 16 birdshad "3" skulls. Careful considerationof rectrix shape and date of capture allowedus to age 14 of these16 birdswith reasonablecertainty. Rounded rectricesindicated birds at leastone year old which had alreadyundergone a prebasicmolt. Pointedrectrices, depending on the capturedate, indicated either rapidly pneumatizingHY birds or SY birds which had not yet completedtheir secondprebasic molt. In two birds we could not differ- entiate betweenthese last two categoriesand agedthe birds as unknown. Thus, rectrix shapeallowed fall birds to be aged with greater precision than current Bird Banding Manual keys (USFWS and CWS 1978) or Pyle et al. (1987), which do not permit agingof SY birdsbeyond June/ July. In addition,the presenceof pointedrectrices after the prebasicmolt, combined with a "3" skull, indicated an HY bird, even in the absenceof retained juvenal coverts(Leberman and Clench, as cited in the Bird Banding Manual [USFWS and CWS 1978]). The average "window" size of a "3" skull in fall was 2.0 mm (n = 10) and no window was less than 1.0 mm.

DISCUSSION We have demonstrated a measurable difference between the rectrix shape of different age classesin the four speciesof migratory North American Catharusthrushes. Furthermore, rectrix shape as an aging method was useful in the field. With experience,correct determination of rectrix shapecould be made by visual inspectionalone; precise mea- surementof the featherwas not necessary. As with any plumagetechnique, experience is requiredin order to gain accuracyin the method.Rectrix shapemay be a difficult methodto learn becausethe tip angle of a pointed (or rounded) rectrix varies somewhat from speciesto species.However, our banderswere able to distinguish between pointed and rounded rectriceson all speciesafter seeingap- proximatelytwelve rectricesof each shapefrom any of the four species. As well, the presenceof additionalaging characteristics in HY and many SY birds will help inexperiencedbanders. The bander can age Catharus with greaterprecision using rectrix shape than by usingpresently accepted keys. This methodwill be most helpful in spring and summerwhen a significantproportion of SY birds can be agedas SY that would otherwise be aged less precisely(i.e., as AHY). In fall, this methodprovides a criterionfor agingmost birds with "3" skullsmore precisely. Vol.60, No. 2 AgirzgThrushes byRectrix Shape [239

In summary, all spring birds with pointed rectricesmay be reliably aged as SY. The low rate of adventitiousrectrix losswhich occursin all specieswill result in a few known SY birds with rounded rectrices. Therefore, all spring birds with rounded rectricesshould be checkedfor other SY characteristicsbefore they are aged as ASY. Since, at most, about 25% of SY birds must be aged by rectrix shape alone, and the overallrate of springrectrix lossis about 10%, only a very small percentage of SY birds would be agedincorrectly as ASY if rectrix shapeis usedas an aging method. In fall, birds with pointed rectricesmay be reliably agedas SY beforethe prebasicmolt or HY after the prebasicmolt. Birds with rounded rectricesmay be reliably aged as AHY. Early fall (i.e., before 1 Oct.) birds with rounded rectrices and "3" skulls are almost certainly SY birds which have completedtheir secondprebasic molt. However, since skull pneumatization may take one or more years to complete,these birds may be older than one year (USFWS and CWS 1978). Additional data on the rate of skull pneumatizationin Catharus is neededto assessthe reliability this agedesignation. Caveats with regard to adventitiousrectrix lossalso apply in fall. Generally, we advocatethe useof multiple characters--skullpneumatization, presence or absenceof juvenal coverts,and rectrix shape--to arrive at an accurateage assessment. Some birds have intermediate rectriceswhich cannot reliably be as- signedto any specificage class.However, in our study, approximately 95% of all rectricescould be classifiedas either roundedor pointed.This levelof discriminationshould apply to all four Catharusspecies, regardless of subspecificstatus (Pyle et al. 1987). With experiencebanders can discriminateaccurately between rectrix shapeclasses without measurement; i.e., rectrix shapeis a practicalin-hand criterionfor agingCatharus thrushes.

ACKNOWLEDGMENTS

We extendour thanksto the helpful staff of the Department of Ornithologyat the Royal Ontario Museum, who allowed us accessto their collections.We thank J. J. Hubert, D. J. T. Hussell, A. L. A. Middleton and M. K. McNicholl for their conscientiouscritiques of variousdrafts of this manuscript.Most of all, we thank all the volunteerfield biologists at the Long Point Bird Observatorywho helped gather the informationpresented in this paper.

LITERATURE CITED

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