566 AR*nUR,Feeding Habits of the Black Skimmer. [Auk

THE FEEDING HABITS OF THE BLACK SKIMMER.

BY STANLEY CLISBY ARTItUR.

Plates XX and XXI.

To thosewho have studiedthe sea birds no speciesis of more interest than the Black Skimmer (Rynchopsnigra) principally becauseof the unique formation of its bill, in which the maxilla is not only capableof excessivevertical movementbut is a great deal shorter than the lower mandible. With such a difference in the length of the two parts of the bill it is quite beyondcontradic- tion that this inch and a half of excessivegrowth denotes some highly specializedfunction. Just what is this function? ThomasPennant, who in 1781 gavethe bird its presentcommon name, wrote: "I call it Skimmer from the manner of its collecting its food with the lower mandible as it flies along the surface of the water. "• This British naturalistwas probablyamong the first to give voice in ornithologicalliterature to the commonly acceptedver- sionthat the Black Skimmerso secures its food. As Mark Catesby had already figured and describedthe bird (which he called a "Cut-water") it is quite possiblethat Pennantattributed func- tions of feedingfrom hearsay evidenceand not from personal observations. CharlesDarwin saidof a flockof "Scissor-beaks"he saw•flying over a lake in Argentina: "I saw severalof thesebirds, generally in small flocks,flying rapidly backwardsand forwardsdose to the surfaceof the lake. They kept their billswide open,and the lower mandible half buried in the water. Thus skimming the .surface,they ploughedit in their course. . and dexterously managewith their projectinglower mandible to ploughup small fish, which are securedby the upper and shorterhalf of their scissor-like bills. "•

Gen. of Birds, p. 52. Voyage of [he Beagle, Ch. VII. Vol. XXXVIII]1921 j ARTU•ZR,Feeding Habits of the Black Skimmer 567

Skimmers,according to Audubon: "spendthe whole night on the wing, searchingdiligently for food. I have seena few of these birds glide in this manner in searchof prey over a longsalt-marsh bayou or inlet,following the wholeof its sinuosi- ties, now and then lower themselvesto the water, passtheir bill along the surfaceand, on seizinga prawn or small fish, instantly rise,munch and swallowit on the wing."• AlexanderWilson goes rather into detail in regard to the feedinghabits of this bird which he callsa "Shearwater" and his observationsare most interesting: "The Shearwateris formed for skimming,while on the wing,the surfaceof the seafor its food, whichconsists of smallfish, shrimps, young fry, etc., whoseusual haunts are near the shore and towards the surface. That thelower mandible,when dipt into and cleavingthe water, might not retard the bird's way, it is thinned and sharpenedlike the blade of a knife; the upper mandiblebeing at suchtimes elevatedabove the water is curtailedin its length as beingless necessary, but taperinggradually to a point, that, on shutting,it may offerless opposition. To prevent inconveniencefrom the rushingof the water, the mouth is confinedto the mere openingof the gullet which, indeed, preventsmastication taking place there; but the stomach,or gizzard,to which this businessis solelyallotted is of uncommonhardness, strength and museularity,far surpassingin theserespects any other water bird with which I am acquainted. "Whoever has attentively examinedthis curiousapparatus and observedthe possessor,with his ample wings,long bendingneck and lower mandible,occasionally dipt into and ploughingthe surfaceand the facility with which he procureshis food, can- not but consider it a mere playful amusement,when com- pared with the dashingimmersions of the Tern, the Gull, or the Fish-hawk, who, to the superficialobserver, appear so superiorly accommodated. On examiningthe stomachsof several of these,shot at the time, they containednumbers of small fish usually called silver-sides."•

Ornith. Biog. IV, pp. 204-206. Am. Ornith., Vol 7; pp. 90-91. 568 ARTHUR,Feeding Habits of theBlack Skimmer. [oct.Auk

The Rev. J. G. Wood, widely read becauseof the lavishnessof the woodcuts in his works, in his text on the Skimmer quotes Rene P. Lesson,(noted for a number of his ornithologicalblun- ders)when describing the bird'sfeeding habits: "It [theSkimmer] does not, however, trust solely to the wide seas for its food for accordingto Lesson,who wasan eyewitnessto the scene,the bird feedson bivalves,adroitly insertingits beak into their shellsas they lie openand then bangingthe shellagainst a rockor stoneso as to breakthe l•ingeand exposethe inhabitantwhich is immedi- ately scoopedup and swallowed."t The careful Couesin describingthe Skimmer'sfeeding habits is not so positiveas someother writersas to the functionof the elongatedbill, but doeshelp in furtheringLessoh's preposterous observationthat the bird pries open oystersand other bivalves with its thin and weak bill. "They seemto feed," he says,"as they skim low over the water, with the forepartsinclined downward, the under mandibleprob- ably grazingor cuttingthe surface;but they are saidto usetheir odd bill to pry open weak bivalYemollusks. "• The popularbird literatureof today differsbut little in describ- ing the feedinghabits of the Skimmer. SaysChapman: "Skim- mers are unique both in the form of the bill and in their manner of feeding. Openingthe mouth, the blade-likelower mandible is dropped just beneath the surfaceof the water; then, flying rapidly, they may be said to literally 'plow the main' in search of their food of small aquaticanimals. "s Reed,in his popularwork, elalms: "They fly in compactflocks, in long sweepsover the water, feedingby droppingtheir long, thin mandiblesbeneath the surfaceand gatheringin everything edible that comesin their path."4 Hornaday has this to say of the Skimmer's feeding habits: "When seekingfood the Skimmer looksfor calm water, and then, with most dexterousand well-balancedflight, it slowlywings its

l•at. Hist. Birds, p. 753. Key to N. A. Birds, p. 1019. Handbook Birds N. E. America, p. 172. Bird Guide, Water Birds, p. 56. Vol. XXXVIII]1921 j •RT•UR,_ Feeding Habits of lhe Black Skimmer. 5•9 way closedown to the surface, so low that the lower mandible is actually held in the water while the bird is in full flight. Any small object that happensto lie on the surface is shot into the mouth, throughwhat is really a very smallopening. "• F. H. Knowlton, in his 'Birds of the World,' says the Skim~ mer's peculiar bill is an obviousadaptation for securingfood: "They do not dive for their food as do the Terns, nor do they pick it up while swimming,for they rarely sit upon the water; but they obtain it by skimmingrapidly over the surfacewith the lower mandibledipped into and cleavingthe surface,thus scoop- ing it up much after the mannerof whales. The thinningof the lower mandibleis clearlyfor the purposeof reducingthe friction in passingthrough the water, while the upper one is shortened and has becomemovable to keepit out of the way. The Indian Skimmer(R. alblcollis)according to Mr. Bla.nford, is usuallyseen on broad and smoothrivers down to the tideway, not in torrents nor, so far as known, on the sea. They are usuallyfound in the morning and evening flying, often in scattered flocks, rather slowlyclose to the water, now and then dippingtheir bills in the stream. They occasionallycatch fish, but he doubts if it is their generalcustom, and assertsthat the useof the bill is still unknown, a statementin direct conflictwith that of his distinguishedcoun- tryman." (Darwin quoted also as above.)• What I have to here advance,as a contribution to the feeding habits of a bird that I have closelyand carefully observedand studiedfor the past sevenyears, is in the nature of a respectful contradiction to the statements quoted above. My observa- tions fall into two classes,viz: 1. I have never seen a Skimmer secure its fish food while skim- ming the waterswith its under mandibleimmersed, as set forth by Pennant and those who have written of this bird since his time. 2. I have seen the Black Skimmer secure its food and collect the fish in an entirely different manner from the generally at- tributed mode.

Am. •N•'•. His'•., VoL III, p. 257. Birds of the World, p. 394. 570 ARTHI,R,Feeding Habits of the Black Skimmer. [oct.Auk

Louisiana offersexceptional opportunities for the study of the gulls, terns, Skimmers and other waterbirds. Up to 1915, from what I had gleanedfrom text booksand generalbird litera- ture,I heldthe beliefthat the Skimmerprocured its foodas previous- ly describedbut whenexploring the Louisianabird breedingislands with TheodoreRoosevelt a remark byhlm set me studyingthis par- ticular specieswith more than the cursoryinterest I had pre- viously bestowedupon it. As a result of •this study, after the observanceof thousandsof Skimmers, at rest and in flight; in summer, when the necessity of feedingyoung called for incessantfishing upon the part of the adults, and in the fall when the birds were either fishingin pairs or on solitary excursions,I am forcedto doubt that the Skimmer securesits sustenancein the orthodoxmanner, and am prepared to provethat it procuresits food,or at leasta part of it, by an entirely different method. Every trip of inspectionthat brought me in contact with the Skimmer subsequentto my discussionwith Col. Rooseveltas to the reasonfor the unequallengths of this bird's bill, I kept dose watch on every flying and "skimming" memberof this family seen. From the deck of the conservationpatrol boats,on which I mademost of my trips over the Louisianawaters, aided by the magnified vision binocularsgave me, I have had exceptional opportunitiesfor studying the Skimmer. On not one occasion out of the thousands of birds noted did ! once behold one secure, or make any movementof the bills that would suggestthat it had so securedfood, while in flight. The years 1915, 1916, 1917, 1918 were without resultsthat would tend to prove the bird se- curedits food in any othermanner than the methodset forth by most writers. In 1918,while makinga surveyof the foodhabits of the Brown Pelican for the United States Food Administration's benefit, I was workingfrom a blind on East Tambalier Island to secure motion and other photographsof a flock of Brown and White Pelicans on a shell reef. While waiting for the White Pelicans to join the Brown,! noted a commotionto the left of my blind-- a wild bird chorusmade by Black Skimmersand a few Laughing Vol. XXX¾III]1921 JAUTn•SU, Feeding Habits ofthe Black Skimmer. 571

Gulls. A schoolof small fish had been chasedinto a shallowflat, evidently by larger fish, and the birds were soonbusy securing food. A wheelingflock of Skimmers"skimmed" the surfaceof the lagoon,and then settledinto the water, whichwas scarcelymore than three inchesdeep, and immediatelycommenced to pick up fish while the birds were in a standingposition. As fast as a Skimmer would secure what fish it needed in this manner, it would fly off, without doubt to its nestlings,and otherswould comesplashing into the water, and, standingerect, would secure the fishwith straightdownward motions of the mandibles. While a numberof arrivingbirds would "skim" the surfaceof the shal- low flat beforealighting at no time was there any ocular evi- dencesof fish beingpicked up and, owingto my closeand con- cealed position, had the birds so securedfish that were most plentiful I would have certainlyseen such an act. The photographsherewith submittedwere made at the time and the birds can be seen in the different attitudes assumed while pickingfish out of the water. Owingto the positionof my blind and the tidal flat it was necessaryto make the photographicex- posuresagainst the sun which accountsfor the slighthaziness of the picturesbut they are sufficientlydistinct to showthe variety of attitudesassumed while the birdswere actuallyfishing. This was the first observationmade of the feedinghabits of the Skim- mers. SubsequentlyI made others, altho I was never near enoughto distinctly photographthe Skimmersin the act of so securingtheir food, but the methods afterwards ol:servedwere absolutelyidentical with thosefirst seen. The habit of skimmingwith the mandiblesimmersed is most noticeablewith Skimmerswhen their nestinggrounds are invaded. Alarmedat one'sappearance, the birdstake to the air and circle aboutwith their characteristicyap, yap, yap, yap, yapping. When they passover the water certain individualsof the flock will skim closeto the water, "cutting the surface"with their bills. At this time they are in a highly excitedand alarmed state and are not "intent on gatheringup everythingedible that comesin their path!" Indeed, they are most apprehensiveas to what man is 572 FeedingHabits of the Black Skimmer. [Oct.Auk goingto do to their eggsand young. Why, then, if this skimming the waters with the mandibles is a highly specializedfeeding function, should they at this time adopt their so-calledfishing mode Of flight? Accordingto Win. T. Blanford'sobservations of the Indian Skimmer this bird'only "occasionallycatches fish" while skimming and he doubtsif it is their generalcustom to so securefood, and assertsthat the actual use of the peculiar bill is still unknown. I cannot but feel that this is equally true of Rynchopsnigra. In the caseof the actual capturingof prey that I have observedI note that the Skimmer always fishesin very shallowwater and while in a standingposition. The act of seizingprey is done by straight downward motions of the head and neck, something akin to the choppingmovement of an axe. The fish are seized and held at right anglesto the bird's bill before swallowing,and the movementsnecessary to securethe fish are rather slow and deliberate. When seized and held the Skimmers do not show any marked ability to "deftly swallowtheir prey," as a matter of fact, the act of swallowingis rather laboredand accompanied by a great deal of shakingof the mandiblesto place the fish in the proper "head foremost" position to insure unobstructed passageto the crop. "Skimming" birds joining the feeding birds perform the "cut water" trick with bothmandibles closed tight before alighting and commencingfeeding in the manner just set forth. I thereforecount it more than merely strange that with the opportunitiesthat I have had to observe the act, I have yet to see a Skimmer secureits fish food by the skimmingmethod set down in ornithologicalliterature by veteran and abler ob- servers. l When carrying fish to their young the parent birds carry the whole fish crosswise in their mandibles. This I have observed on hundreds of occasions. Examinations of the food show that the Skimmers, at least in Louisiana, feed exclusivelyon small fish, and of a great variety of species. I have found they feed upon,

• Cf. GeneralNotes beyond. Ed. Tm• A•, Vo•.. XXXVITI. PI•TE XX.

BLACKSKIMMERS (RYNCHOPS NIGRA). }•ird ha center clearhagwater with one mandible, not feeding. 2. Feeding on F•sh on a Tidal Flat. Vol. XXXVIII]1921 J ARTHUR,Feeding Habits of lhe Black Skimmer. 573 the following speciesarranged in the order of their numerical importance: Cynoscionnotbus, Atherina sp. (Silversides,note Wilson'sobservation), Mugil cephalus,Scomberomorus maculatus, Pomatomussaltatrix, Cynoscionnebulosus, Sciaenops ocellatus, Trachinotuscatolinus, Carangus hippos, and othersunidentified. The majority of thesefish were secured on the breedinggrounds beingthe food carriedto feed the young. The largestfish fed to the young that I was able to securewas 73 min. in length, but fish this size are only fed to young with feathers. The Black Skimmershave two waysof feedingtheir young. The downy young are fed by regurgitation,the food being droppedon the groundby the parent bird, but so avid are the little onesfor food that they pick at the parentbird's bill as the fishis beingdropped, they then pick it up as a tiny chick would take up moistened bread. When the nestlingscommence to showfeathers they are fed entire fish. The adultsfly in from their fishinggrounds with the food carriedcross-wise in the mandiblesand it is givendirect to the youngbird which invariably securesit head-first. If, by chance,the fish is first droppedto the groundthe nestlingwill secureit by turning its head and .bill side-wise. This is not a diffleulttask and at this time theredoes not seemto be any phys- ical reasonfor so doingas at this periodthe differencein length betweenthe upper and lower mandiblesis slight but, to me, indi- catesa precocitythat will proveuseful in the adult stage. When Skimmersare first hatchedthe bills are of equallength and it is not until the youngare someweeks old that the curiousformation of the lower mandiblebegins to showby slightlyexceeding the upper one in growth. Attentive observationof the youngin variousstages was barren of resultsin a searchfor a reasonfor the unusualand unequal growth of the Skimmer's mandibles. The answer lies wholly with the adult bird. Audubon'sassertion that Skimmers"spend the wholenight on the wing, searchingdiligently for food" is not borne out by my observations. While a few of the birds are heard at night I would not say that they are nocturnalbut that they are, to a marked 574 ARTHUR,Feeding Habits of the Black Skimmer. [oc•.Auk degree,crepuscular. During the reproductionperiod their fish- ing habits must be diurnal, especiallyif young are to be fed. The insistenceof most writers that the skimming is done "with the under mandible immersed" does not agree with my observations. The skimming and "plowing the main" is usually done with the mandiblesdosed save for the times they are openedto permit the cry or "bark." This I can show by photographsof the birds in the act of cleavingthe surfaceof the water. In my collectionof photographsI have but one showing the bird cutting the water with its lower mandible only and this bird was "yapping" loudly when so flying over the surface. My conclusionsare that the Black Skimmerdoes not secureits food in the waygenerally set forth and that the functionof the uniqueinequality of the mandiblesis as yet unknown. 60]•3 Perrier St., New Orleans, La.

EXPLANATION OF PLATES.

Plate XX, fig. 1. Black Skimmer cleaving the water with its lower mandible only. This bird was excited and apprehensiveover my near- nessto its nest of eggsand was not fishing. It was emitting its charac- teristic "yap, yap, yap" while circling me and when it would passthe tidal fiat would lower itself to the surface of the water and cut it with its bill. Plate XX, fig. 2. Skimmersfeeding on a schoolof fish in a tidal fiat. The different attitudes adoptedwhile securingfish can be seenby close inspection. The two birds in the upper left can be observedin the act of catchingtheir prey. One is making straight downwardplunges of its mandibleswhile the other is belly deepin the water having a tusslewith the bastard weakfish it has caught (the school consistedof Cynoscion notbus). One Skimmer can be observedin the act of alightingin the water to fish in preferenceto securingfood by "skimming" which, in this case, would have been an easy matter if this were its mode of fishing. Plate XXI, fig.1. The same flock fishing in the same fiat. Note one bird joining the othersby skimmingover the water with both mandibles closed. After alighting it fishedas did the others--standingin the water. Plate XXI, fig. 2. Skimmers in flight over the water and cleaving it. Note that the two lower birds are cutting the water with the mandibles closed. T•E Av•, Vor. XXXVIII. PIA?EXXI.

BLACKSKIMMERS (RYNcHoI's NmRA). 1. Fishing on a Tidal Flat. 2. In Flight over the Water.