192 SHORT COMMUNICATIONS

that Snow Buntingsmigrating through coastalareas in LDVENSKIOLD, H. L. 1964. Avifauna Svalbardensis. the subarcticwhere spring melt has occurred are not Norsk Polarinstitutt, Oslo. only granivores, but also herbivores. MATTSON,W. J. 1980. Herbivory in relation to plant nitrogen content. Annu. Rev. Ecol. Syst. 1:119- I thank Fred Cooke for permission to use the Snow 61. Bunting data from the La P&rouseBay Bird List (1980 MILLER, P. C., P. J. WEBBER,W. C. OECHEL,AND L. to 1984), and for commenting on the manuscript. I L. TIESZEN. 1980. Biophysicalprocesses and pri- also wish to thank Euan Dunn, Peter Ewins, Bob Jef- mary production, pp. 66-101. In J. Brown, P. C. feries, and Peter Kotanen for providing valuable com- Miller, L. L. Tieszen, and F. L. Bunnel [eds.], An ments, and Thomas Custer who reviewed the manu- arctic ecosystem: the coastal tundra ai Barrow, script. This work was carried out while the author was Alaska. U.S. IBP Svnth. Ser. 12. Dowden. Hutch- supported by a Natural Sciencesand EngineeringRe- inson and Ross, Pi. searchCouncil of Canada PostgraduateScholarship. NETHERSOLE-THOMPSON,D. 1966. The Snow Bunt- LITERATURE CITED ina. Oliver and Bovd. Edinburgh. PARMELEE,D. F. 1968: The Snow-Bunting,p. 1652- BAZELY,D. R. 1984. Responsesof salt marsh vege- 1675. In 0. L. Austin [ed.], Life historiesof North tation to grazingby LesserSnow Geese. M.Sc.thesis, American Cardinals, Grosbeaks, Buntings, To- Univ. of Toronto, Canada. whees,Finches, Sparrows and allies.U.S. Nat. Mus. CARGILL,S. M., AND R. L. JEFFERIES.1984. The ef- Bull. 237. fects of grazingby LesserSnow Geeseon the vege- POLUNIN,N. 1940. Botany of the Canadian Eastern tation of a sub-arcticsalt marsh. J. Appl. Ecol. 2 1: Arctic Part I: Pteridophyta and Spermatophyta. 669-686. National Museum of Canada Bulletin #92 in Bi- CUSTER,T. W., AND F. A. PITELJSA.1975. Correction ological Series #24, Ottawa. factors for digestion rates for prey taken by Snow PORTENKO,L. A. 1973. The Birds of the Chukotsk Buntings(Plectrophenax nivalis). Condor 77:210- Peninsulaand Wrangel Island. Vol. 2. Leningrad. 212. [Russian]. JE~RIES, R. L. 1977. The vegetationof salt marshes ROBBINS,C. T. 1983. Wildlife feeding and nutrition. at some coastal sites in Arctic North America. J. Academic Press, New York. Ecol. 65:661-672. WATSON,A. 1957. Birds in the Cumberland Penin- KOTANEN,P., ANDR. L. JEFFERIE. In press. The leaf sula, Baffin Island. Can. Field-Nat. 71:87-109. and shoot demography of grazed and ungrazed plants of Carex subspathacea.J. Ecol.

The Condor 89: 192-194 0 The Cooper Ornithological Society 1987

THE POSSIBLE SIGNIFICANCE OF LARGE EYES IN THE RED-LEGGED IUTTIWAKE’

ROBERT W. STORER Museum of Zoology and Department of Biology, The Universityof Michigan, Ann Arbor, MI 48109

Key words: Arctic gulls; kittiwakes:vision; eye size; ing in the sclerotic ring of the former specieshas not, adaptation; scleroticrings. to my knowledge, been reported in the literature. This difference can be seen by comparing skulls of The Red-legged Kittiwake (Rissa brevirostris)differs the two species(Fig. l), but is more conspicuouswhen conspicuouslyin leg color and bill length from its con- comparing the sclerotic rings (Fig. 2). The mean di- gener,the Black-leggedKittiwake (R. tridactyla).These ameters of the openings within the sclerotic rings are well known charactersserve as the basesfor the species’ 11.8 mm and 10.0 mm in the Red-lemed and Black- English and scientific names, respectively. In con- legged kittiwakes, respectively. (For‘;iistribution of trast, a third difference, the much larger eye and open- measurements,see Fig. 3.) This differenceis even great- er proportionally becausethe former speciesis smaller than the latter. (Mean humerus length of 6 Red-legged Kittiwakes is 8 1.3 mm as opposedto 88.6 for 10 Black- ’ ’ Received 21 April 1986. Final acceptance29 Sep- leggedKittiwakes-sexes equally represented.) tember 1986. Large eyes are consideredan adaptation for noctur- SHORT COMMUNICATIONS 193

FIGURE 1. Skulls of migratory gulls (left column, top to bottom: Bonaparte’s Gull, Ring-billed Gull, Black- leggedKittiwake) and of gulls wintering in the Arctic (right column, top to bottom: Ross’ Gull, Ivory Gull, Red- leggedKittiwake). nal or crepuscularvision (Walls 1942); yet I can find relatively large orbits of the two Arctic gulls can be no evidence that the Red-legged Kittiwake is crepus- seen. This is corroborated by the larger size of the cular or nocturnal like the Swallow-tailed Gull (Creu- openingsin the scleroticrings of these species(Fig. 3). gncsfurcatus)of the Galapagos(Snow and Snow 1968). As in the caseof the kittiwakes, this difference is pro- Furthermore, Gabrielson and Lincoln (1959) remark portionallygreaterbecause the Arctic-wintering species that “it is a common sight to see mixed companies of are smaller than the migratory ones with which they the two species[of kittiwakes] working on schoolsof are compared.(Mean humeruslengths: Ross ’ Gull, 59.8 small fish or on concentrationsof other marine life.” The two speciesare broadly sympatric only during the breeding seasonwhen days are long and extending the feeding period may not be important. On the other hand, the major wintering ground of the Red-legged Kittiwake is within the Bering Sea (Gabrielson and Lincoln 1959). Here, nights and twilight periods are long, and I suggestthat extending the foraging period through greater visual sensitivity during these periods could bestowa strongselective advantage on birds with larger eyes. To test this hypothesis,skulls and sclerotic rings of two other gulls that winter in the Arctic, Ross’ Gull (Rhodostethiu rosea) and the Ivory Gull (Pugo&ilu &mea) were compared with those-of migratory-gulls of similar size, Bonaparte’s Gull (Laru.sphiludefphiu) and the Ring-billed Gull (L. deluwurensis),respective- FIGURE 2. Sclerotic rings of the Red-legged Kitti- ly. Skulls of these are shown in Figure I, in which the wake (left) and the Black-leggedKittiwake. 194 SHORT COMMUNICATIONS

El R. rasea El P. eburnea 133 R. brevkastris L. philadelphia !ZZ L. delawarensis EZ R. tridactyla I

a.60-a.04 12.40-12.60 12.40-12.60

0.30-0.44 ll.ao-(2.00 il.ao-12.00

P 'E 0.00-0.14 ll.20-11.40 11.20-11.40 8 d 7.70-7.04 10.60-10.00 10.60-10.00 b B uI 7.40-7.54 10.00-10.20

7.10-7.24 9.40-8.60 9.40-9.60 6.00-6.94 0.00-9.00 a.ao-g~oo1 0 1 2 3 0 1 2 3 0 1 2 3 4 Frequency FIGURE 3. Distribution of measurements(in mm) of diameters of the opening in the sclerotic ring of six speciesof gulls. In each graph, the measurementsfor the Arctic-wintering speciesare greater than those of the somewhat larger specieswintering to the south. (“Frequency” is the number of specimens.)

[n = 81; Bonaparte’s Gull, 66.4 [n = lo]; Ivory Gull, of specimensin the Museum of Vertebrate Zoology, 9 1.2 [n = 61;and Ring-billed Gull, 99.1 mm [n = lo].) and William R. Dawson, Steven M. Goodman, Mi- From these data, it is evident that the three gulls chael D. Kern, Storm L. Olson, Robert B. Payne, and wintering in the Arctic have relatively larger eyes than Richard 0. Prum for offering helpful suggestionson those wintering farther south. A detailed study of the the manuscript. morphology of the eyes of these speciesand compar- isons with the eyesof gulls wintering at lower latitudes LITERATURE CITED should prove interesting. GABRIELSON,I. N., AND F. C. LINCOLN. 1959. The I thank Declan Troy of LGL Ltd. for providing spec- birds of Alaska. Stackpole Co., Harrisburg, PA, imens, David Bay for preparingthe photographs,Phil- and Wildlife Management Inst., Washington, DC. ip Myers for preparingthe graphs,Allan J. Baker, Henri SNOW,B. K., AND D. W. SNOW. 1968. Behavior of Ouellet, and Richard L. Zusi for making available spec- the Swallow-tailed Gull of the Galapagos.Condor imens in the Royal Ontario Museum, the National 701252-264. Museum of Natural Sciences,Ottawa, and National WALLS, G. L. 1942. The vertebrate eye. Cranbrook Museum of Natural History available to me, Douglas Inst. Sci. Bull. 19. Bell and Ned K. Johnsonfor providing measurements