Identification of the Common Tern and the Arctic Tern*

IDENTIFICATION OF THE COMMON TERN AND THE ARCTIC TERN*

Jean-PierreVande Weghe

[The CommonTern Sterna hirundo and the Arctic Tern Sterna paradisaeapresent a very difficult problemof identification. Observers in California do not have at hand adequateinformation on the sub- ject. We asked Mr. Vande Weghefor permissionto publishin this journal a translationof the part of his paper "La Sterne pierregarin Sterns hirundo et la Sterne arctique Sterna paradisaea. Identifi- cation ef Passageen Belgique"(Aves 3: 1-5, 1966) that pertainsto field identification(the restof the papertreats the statusin Belgium). Nine speciesof terns are regularlyseen in California. They are convenientlyclassified in five groups.(1) The largeterns - Caspian Tern Hydroprogne caspia, Royal Tern Thalssseusmaximus, and ElegantTern Thalssseuselegans are easilyseparated from the others by their large size and their red or orangebill color;identification of the individualspecies presents no particularlydifficult problem. (2) The fairly large,thick black-billedGull-billed Tern Gelochelidon niloticawhich is confinedto the SaltonSea area. (3) The darkBlack Tern Chlidoniasniger which is alwayseasily identified by its plumage color. (4) The very smallLeast Tern Sternaalbifrons which is easily identifiedby its size. (5) The mediumsized terns - Forster'sTern Sterna forsteri, Common Tern Sterns hirundo, and Arctic Tern Sternaparadisaea. In this groupForster's Tern is reasonablyeasy to separatefrom the other two, though the identification is delicate and requiresgreat care. Thoughit is quite beyondthe scopeof this article to discussit in detail, we suggestthe following guidelines: (1) A bird showingdark shoulderpatches is a youngCommon/ Arctic Tern. (2) A bird without shoulderpatches, and showingno contrast betweenback and rump in flight (generalwhiteness) is an adult Forster's Tern. (3) Both adult Common/Arcticand youngForster's lack shoul- der patchesand display a contrastbetween white rump and darkerback; young Forster'sare bestdistinguished by the brownishedgings on the back as well as the black tips to the outer three or four tail feathers.

* First publishedin the Belgianjournal Aves.

Calif. Birds 1:33-36 33 IDENTIFICATION OF COMMON AND ARCTIC TERNS (4) In every plumageexcept breedingadult, Forster'sTern has a diagnostichead pattern; a blackear patch,limited to the auriculars,and curvingdownwards toward the rear. The head patch of the Common Tern (when it has a white forehead)curves upwards to includethe nape.

In Californiathe CommonTern is a regular springand fall migrantalong the coastbut appearsto havea limitednumber of regularstopping points where it is common. It alsooccurs inland duringthe fall, especiallyat the SaltonSea. Thereare a few winter recordsfor coastalSouthern California. Recordsalso span the entire summerseason, but there is no evidenceof breeding. Duringthe springthe CommonTern appearsto be confinedto the coastand offshorewaters. Springmigrants first appearin coastal SouthernCalifornia during the last week of April and aremost numer- ousduring the first two weeksof May. Smallnumbers are still found until the end of May, and stragglershave beenrecorded during June. Fall migrantsappear early, and the first are usuallyin Calif- orniaby the first weekof July. Numbersreach a peakduring Septem- ber and early October, and fair numberspersist in coastalSouthern California until early November. A few regularly stay in coastal Southern California until late December, and individualshave remained throughthe winteron SanDiego Bay. Duringthe fall CommonTerns occur inland in limited numbers as well as being fairly numerous along the coast,and they have been recordedfrom suchlocalities as Lake Almanor, Lake Tahoe(regular), Mendota, Victorville, the Salton Sea (regularflocks of up to 100), and alongthe ColoradoRiver from Needles to Yuma. Along the coastthe CommonTern appearsto prefer the larger bays,and it alsoappears to be more numerousin SouthernCalifornia. The Arctic Tern is an offshore migrant during the fall; it is highlypelagic, seldom, if ever,occurring near the shore. Thereare un- verifiedreports of smallflocks during May far off the coastof South- ern California,and there is onespecimen, from Laguna Beach, which cannot be found. During the fall it is fairly numerousoff the coast from August until October, with recordsfrom July 27 to October 14. We are grateful to Mr. Vande Weghefor permissionto translate his paper and MM. van Groenendaeland Dobrski for allowingus to reproducethe photographswhich illustrated Mr. VandeWeghe's paper. We alsowish to thank the editorsof Avesfor their cooperation.- Eds.]

34 IDENTIFICATION OF COMMON AND ARCTIC TERNS Four speciesof Ternscan be observedannually in Belgium.The SandwichTern $terna sandvicensisand the LeastTern $terna albifrons do not presentany difficulty of identification,and with somepractice they can be easilyrecognized even at a greatdistance. A very different situationprevails, unfortunately, for our two red-billedspecies: the CommonTern Sterna hirundo and the Arctic Tern Sterna paradisaea.Even the experiencedobserver with a wide knowledgeof seabirdsis often unableto make an exactidentification. The reason for this is that very few ornithologistsknow the true diagnosticcharacters and can evaluate them correctly in the field. Moreover,these birds are often seenat a great distance,briefly, or in poor light. The detailed descriptionsfound in standardguides and hand- books are generallyof very little value for the field observer. At most, one finds the charactersthat separatethe two specieson the ground. Thus we learn that the bill of the Arctic Tern is shorter and all red, that its tail is longer,and that its legsare shorter. It is, however, very difficult to judge the lengthof t-helegs on an isolatedbird in the field sinceit dependslargely on the way in whichthe bird hasarranged its breast and belly feathers. Moreover,I have never seenan Arctic Tern on the groundin Belgium. Therefore it is mostly in flight that the two speciesmust be separated. In this casethree groupsof characterscan be used. First, it appearsthat the silhouetteof the Arctic Tern is slightly,though clearly,different from that of the CommonTern. The ArcticTern has the bill and the head, thus the wholepart of the body situatedin front of the wings,much shorterthan the part situatedbehind the wings, which includesthe rump and tail; the ratio is 1 to 2.5 or 3 in the Arctic Tern comparedto 1 to 1.5 or 2 in the CommonTern (see photographs1 and 2). In addition, if the bird is closeenough, one can see that the bill itself is much shorter, which givesthe head a morerounded appearance (see photographs 1, 3, and 5). The secondimportant character is the transparencyof the wings. This comesfrom the fact that the secondaries(especially the external secondaries)and the primariesare much whiter and lesspowdered with light gray in the Arctic Tern than in the CommonTern. Thus, there is a wide transparenttriangle visible on the.underwingswhen the observeris lookingup at the bird, and a longwhitish triangle contrast- ing with the gray covertsvisible on the upper-wings(see photographs 3 and 4). The rectrices,especially the externalrectrices, are also 35 IDENTIFICATION OF COMMON AND ARCTIC TERNS much whiter in the Arctic Tern, henceits generalappearance is much whiter than that of the CommonTern (see photographs3 and 4). With wear and molt this difference is most marked during the fall. The third character is the voice. As far as I am concerned this in itself can be diagnostic.The Arctic Tern hasa completeseries of calls different from thoseof the CommonTern, but mostare heardonly on the breedinggrounds and are thereforeof little value to an observerin Belgium. One, however,is important and is heardregularly during migration; this a fine "keeh... keeh... keeh...", soft, short, clear, and often repeateda few times. The correspondingcall of the Com- mon Tern is a longer,lower-pitched, more nasal"krre" whichincludes rather different tones. To know thesecalls it is necessaryto hear them a few times,but oncelearned they arenot easilyforgotten. Many observerstry to identify the Arctic Tern by the all red bill. This characterhas value only in the springand on the breeding grounds. Indeed, in the summerand fall, beginningas early as July, the bills of both speciesbecome increasingly black. It is only in the spring,and then only when the bird can be seenat very closerange in excellentlight, that the absenseof a black tip to the bill canbe a help in confirmingidentification. This point, however,is never conclusive by itself sincesome Common Terns lack the blackbill tip. In general,the charactersthat I havedescribed are not very clear- cut, and one will not identify a passingArctic Tern in a flock of Common Terns on the first try. On the contrary, only after seeing severalbirds, and after learninghow to evaluatethe variouscharacters accordingto the distance,the light, the sky, the colorand stateof the ocean,the mannerof flight of the birds,and the angleof obser- vation,will one succeedin makinga positiveidentification. It mustbe admitted that, evenfor the best observer,some birds must go uniden- tified. In these casesit is best to put the bird down as "Sterna species"or to use the Britishterm "CommicTern" derivedfrom the words Common and Arctic. 149 $tropstraat,Ghent, Belgium