Plumage Variation in Young Razorbills and Murres

J. Field Ornithol., 56(3):246-250

PLUMAGE VARIATION IN YOUNG RAZORBILLS AND MURRES

By T. R. BIRKHEAD AND D. N. NETTLESHIP

Variation in the head,chin, and throat plumageof youngThick-billed Murres (Uria lomvia)and Razorbills(Alca torda)has been reported by Gastonand Nettleship(1981) and Hudson(1984), respectively.These authorsfound that young,just prior to leavingthe colony,were of one of two plumagetypes, having: (1) a dark head,cheeks, chin, and throat, referred to as"summer" plumage because it resemblesthe adult summer plumage,or (2) whitecheeks, chin, and throat, similarto the adultwinter plumageand referred to as "winter" plumage.We presentdata on the occurrenceof thisand other plumagevariation among Razorbills, Com- mon Murres (U. aalge),and Thick-billed Murres.

METHODS Data on all Razorbills and Common Murres, and some for Thick- billed Murres were obtained from the Gannet Clusters (53ø56'N, 56ø32'W), Labrador in 1983. Data for Thick-billed Murres were also obtainedfrom coloniesat Coburg Island(75ø48'N, 79ø29'W) in 1979; Cape Hay, Bylot Island (73ø45'N,80ø23'W) in 1979; and East Digges Island, Hudson Strait (62ø34'N, 77ø45'W) in 1981. Data on the plumagesof youngRazorbills were obtainedduring visits to breeding sitesmade every secondday to obtain weight and wing length;additional data came from chickscaught as they left the colony. We examinedthe plumageof CommonMurre chicksby catchingthem asthey left the colony,or by observingundisturbed chicks on breeding ledgesat distancesof 15-30 m usinga 25 x telescope.Most of these chickswere on studyplots examined regularly (Type 1 procedure;de- scribedin Birkheadand Nettleship 1980) and were of known age. The plumageof Thick-billed Murre chickswas examined in the sameway at Cape Hay, but not at Coburg Island (seeBirkhead and Nettleship 1981) and DiggesIsland where data were obtainedfrom chickswhich died accidentallyas they left the colony. We divided chicksinto 3 categories,"summer," "winter," and "intermediate."Among Razorbills "intermediate" was similar to the "summer" type, but with somewhite featherson the chin and throat. The "summer" and "winter" plumagesof Razorbills are shown in Fig. 1. We noted another aspectof plumagevariation in youngRazorbills. In the alternate(but not basic)plumage of adult Razorbills,a white line runs from the anterior of the eye to the baseof the upper mandibleon each side of the head. In some chicks this feature was distinct, while in others the white lines were absent. We scored this white line as: 0 (absent),1 (trace),2 (poorly-definedor incomplete),and 3 (well-defined)

246 VoL 56, No. B Razorbill and Murre Variation [247

FIGURE1. Plumagevariation in Razorbillchicks. The upperand lowerphotographs show the same chicks viewed from the side and below. The chick A is "winter," and chick B is "summer,"from the GannetClusters, August 1983. Note alsothe differencein the developmentof the white line: chickA is category3 (full white line), whilechick B is category2 (seetext, and Fig. 2).

(Figs. 1 and 2). There wasno changein the white-linescore as chicks got older. Although the presenceor absenceof a white line is also a summer-winterplumage difference, both of the "winter" chickswhich we examined had well-definedwhite lines (Fig. 1). This indicatesthat summerand winter typesof both plumagecharacteristics can occuron the same chick.

RESULTS Razorbill.--Of 134 chicks examined, 128 (95.5%) were classedas "summer," 4 (3.0%) as "intermediate," and 2 (1.5%) as "winter." At SkomerIsland, Wales,Hudson (1984) found that 49% of 51 chickswere in "summer" plumageand 51% in "winter" plumage,significantly different from our results (X• = 68.5, 1 df, P < .001). Approximatelyequal proportionsof Razorbill chickswere assigned to eachof the white-linecategories, and theseproportions did not differ amongdifferent islandsat the Gannet Clusters(X• = 5.53, 13 df NS). There wereno statisticaldifferences in hatchingdates or fledgingweights among chickswith "summer," "intermediate," or "winter" plumage, 248] T. R. Birkheadand D. N. Nettleship j. FieldOrnithol. Summer 1985

F•Ct•RE2. ¾ariation in the developmentoœ the white line in Razorbill chicks at the Oannet Clusters,Auõust 1983. Chick A is cateõory3 (full white line), chick B is category0 (no whiteline). or amongthe 4 white-linecategories. P. J. Hudson(pers. comm.) also noted variationin the distinctivenessof the white line amongRazorbill chicksat Skomer Island, but did not quantify it. CommonMurre.--Before consideringthe plumage variation among CommonMurre chicks,we will outline the differencesin plumagewhich exist between the young of the two murre species.These were most pronouncedamong chicks 1-5 daysold and thoseold enoughto leave the colony(16-25 days).The dorsaldown of youngThick-billed Murre chicksvaried through silver gray, to brown, to dark charcoalgray, in mostcases with brown brindling on the longer down. In contrast,that down on youngCommon Murre chickswas usually medium gray with no trace of brown brindling. They alsoretain more down featherson their head--at leastuntil they leavethe colony,than Thick-billed Murre chicks(Fig. 3). Unlike Thick-billedMurres, Common Murre chicksaged 14-1-days have a dark gray "mask," white throat, cheeks,and line run- ning up behind the eye. This line also distinguishesadult Common

Fmu•tE3. Differencesin the headand throat plumageof a typicalCommon Murre chick (A) and a summer-plumageThick-billed Murre chick (B). Both chickswere photo- graphedthe day prior to fledgingat the Gannet Clusters,Labrador, in August 1983. Vol.56, •o. • Razorbilland Murre Variation [249

F•Ct•RE4. Variationin the headand neckplumage of CommonMurre chicksaged 21- 25 days.Drawn from photographsof chickscaught as they left the GannetClusters colony,Labrador, August 1983. A, the mostfrequent type, note dark collar and white behindthe eye;B & C are two rare types--B, note reducedcollar and white area behindthe eye,and C, note lackof whitebehind the eye.

Murres from adult Thick-billed Murres when in the basic (i.e., winter) plumage(Salomonsen 1944). Variation in the head and throat plumage of CommonMurre chicksaffects only the amountof white on the cheeks and behind the eye (Fig. 4). It is lessstriking than the summer/winter typesfound in the Razorbill and Thick-billed Murre (seebelow), and thereforewe did not attemptto quantifyit. Thick-billed Murre.--Of 94 Thick-billed Murre chicks at Gannet Clus- ters that were classifiedaccording to plumage type, 64% were "summer," 25% "intermediate," and 11% "winter." The proportion of these 3 plumagetypes respectively was similar at all colonies:Digges Island-- 59%, 24%, and 18% (n -- 22); CoburgIsland--70%, 20%, and 10% (n -- 69); Cape Hay--64%, 27%, and 9% (n = 58), (x2 -- 2.6, 6 df NS). We recordedweight and wing lengthof chicksas they left the colony at Coburg Island and Cape Hay (Birkhead and Nettleship 1981), but there were no statisticaldifferences in these parametersfor chicksof different plumagetypes at the two colonies.Similarly, Gaston and Net- tleship(1981) found no differencesin any growthparameters or hatch- ing datesfor "summer" or "winter" chicksat Prince Leopold Island. Our resultssuggest that geographicvariation in the summer-winter plumage types among Thick-billed Murre chicksin eastern Canada is rather small. In contrast, the differencesin the proportion of these plumage types among Razorbillsfrom Labrador and Skomer Island, Wales,is striking.Further surveyswill showwhether any geographical pattern to this variationexists. The bridled-normalplumage polymor- phism in (adult) Common Murres showsa clear geographicpattern (Southern1938). Variation in the plumageof the youngof other alcid specieshas been reported (e.g., Wehle 1982), but any functionalsig- nificancehas yet to be demonstrated.

SUMMARY Variation in the head and throat plumageof young Razorbillsand Common and Thick-billed murres is described.The proportions of 3 plumagetypes among Thick-billed Murre chicksat 4 coloniesin eastern 250] T. R. Birkheadand D. N. Nettleship J.Field Ornithol. Summer 1985

Canada did not differ. The proportion of these sameplumage types among Razorbills,however, differed markedlybetween Labrador and Wales.

ACKNOWLEDGMENTS We are grateful to those individualswho helped with field work or providedlogistic support. Thanks are due to G. Calderwood,W. N. Carson,K. Clarkson,R. D. Elliot, E. Greene, S. D. Johnson,B. Lyon, and D. Reid for assistancein the field, and to A.J. Gastonfor providing data for Digges Island. We also thank Bill and Millie Elson, Richard Morris (Petro-Canada,Cartwright), and Sid Elson. A. Macfarlane pro- videdthe photographsfor Fig. 4. We thank A.J. Gaston,R. Sluys,and P. Hope Jones for helpful commentson the MS. This research was fundedby the SeabirdResearch Unit, CanadianWildlife Service(CWS) and is associatedwith the program "Studieson northern seabirds," SeabirdResearch Unit, CWS, Environ. Canada(Report No. 175).

LITERATURE CITED

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