SHORT COMMUNICATIONS 503

Condor 85:503 % The Cooper Ornithological Soclety 1983

DETERMINING SEX RATIOS caught birds following his plow with the use of a long- handled fish net, reaching out from the tractor seat and FROM COLLECTED catching gulls as they flew close by. The birds that were SPECIMENS shot were also feeding on invertebratesin the same fields, although only six were shot directly behind the plow. The three methods yielded different sex ratios (x2 = JOANNA BURGER 23.2, df = 2, P < 0.001). Gulls collectedby shootingwere mostly males (83%) those netted were mostly females (79%), and thosenest-trapped had an equalsex ratio (Table Ornithologists have recently shown considerableinterest 1). Characteristically, once a bird was shot, other gulls in determining the sex ratios of birds, particularly on the approached,circled over it, and were themselves shot. If breeding grounds. Field counts are possible only for sex- this behavior is related to the greater aggressivenessof ually dimorphic species.Investigators working on mono- males,it might explaintheir greatervulnerability. Or, males morphic specieshave therefore turned to the data accom- may simply have remained in the area longer. Females panying museum specimens. The method of collecting may have been lesswary, hence more likely to come closer these specimensis usuallyunknown, and may bias the sex to the plow and so be netted. Since birds were collected ratio, as I will show. by both methods in the same field, it is apparent that one In 1971 I collecteda number of Franklin’s Gulls (Lams or both samplesgave biased estimatesof sex ratios (Table pipixcan) in the vicinity of AgassizNational Wildlife Ref- 1). uge,Marshall County, Minnesota. The gullswere collected I assumethat the sexratio of Franklin’s Gulls was nearly during May and June at or within 8 km of a breeding equal in the area becauseno birds were loafing near the colonv of 10.000-l 5.000 nairs of aulls (see Burger.Anim. colony (Franklin’s Gulls loaf near their nests),and no males Beha;. 22:521-567,‘1974). Birds-were‘collected by three on territories lacked females. There were no areas where methods; nest-trapping (Burger, Bird-Banding 42: 123- birds loafed within 40 km of the refuge. I had enlisted the 124, 197l), shooting, and netting birds that were feeding aid of local farmers in reporting any gull flocks. Although behind a plow. I used all three methods only to increase I received many reports of foraging gulls, there were no my sample of specimens. I nest-trapped between 09:OO reports of any sizeable loafing flocks. Furthermore, this and 1l:OOthrouahout the incubation neriod. Shootingand was the only breeding colony of Franklin’s Gulls in Min- netting were done on the same days and in the same two nesota;consequently, there was no influx of gulls into the fields where 200-500 gulls were feeding. A local farmer areaaround the Agassizcolony during the breedingseason. My results indicate that the method of collecting can influence the sex ratio of the specimensobtained. Thus, TABLE 1. Sex ratios of Franklin’s Gulls collected by the sex ratio of this sample may be a misleadingindicator different methods. Shown are the number of each sex col- of the population’s sex ratio. The dangerin usingmuseum lected by each method. specimensto gaugesex ratios is greaterif the exact method of collectingis unknown, if the direction of the biaseshave Males Females Total not been examined for that species,and if sample sizes are small. Shot 29 6 35 Nest-trapped 22 20 42 Netted behind plow 6 22 28 Department of Biological Sciences, Rutgers University, New Total 57 48 105 Brunswick, New Jersey 08903. Received 23 December 1982. Final acceptance20 May 1983.

Condor 85:503-504 (ir The Cooper Ornithological Society 1983

DIVING DEPTHS OF THE GENT00 deeply than the more pelagic species.We report here on our measurementsof the diving depths of Gentoo Pen- PENGUIN (PYGOSCELIS PAPUA) euins at Marion Island. ” We conductedour studyduring November and Decem- ber 1981. Twenty-five G&too Penguinswere fitted with NIGEL J. ADAMS depth gaugesattached to leather harnessesand released.

AND Harnessedbirds were recapturedupon their return to the island to spend the night ashore or to feed chicks. CHRISTOPHER R. BROWN The harnessesconsisted of a contoured collar (nlaced above the flippers) joined to a thoracic band below the flippers by four narrow leather straps. The depth gauge At Marion Island in the southwestern Indian Ocean was fitted to the thoracic band and lay flush with the (46”52’S, 37”5l ’E), the Gentoo Penguin(Pygoscelispupua) penguin’s back. The harness is described more fully in breeds sympatrically with Macaroni (Eudyptes chrysolo- Wilson and Bain (in press).The depth gaugeswere a cap- phus), Rockhopper (E. chrysocome) and King (Apteno- illary type, coated on the inside with a water-solublepow- dytes patagonicus) penguins. Duration of foraging trips der. Water entering the open end of the capillary tube and chick feedingrates suggest that, in contrastto the other compressedthe volume of air trapped in the tube, and species,the Gentoo Penguinis primarily an inshore feeder compression varied with depth as described by Boyle’s (Croxall and Prince 1980a, Williams 1980, 1981). Conse- Law. Becausethe relationshipis non-linear, however, shal- quently, Gentoo Penguinsmay need to dive for food less lower depths were recorded more precisely than deeper 504 SHORT COMMUNICATIONS depths. The maximum depth attained by the diving pen- order to validate the biological significanceof maximum guin was indicated by the boundary between the dissolved diving depths. Such studies will require more expensive and undissolvedpowder. A similar device has been used and sophisticateddevices, such as the autoradiographic to measure diving depths in seals (Kooyman 1965) and devices of Wilson and Bain (in press) or the electronic Emperor Penguins(Aptenodytes forsteri; Kooyman et al. recordersof Kooyman et al. (1982). 1971). We thank Pieter van Litsenborgh for assistancein the Penguinswere recapturedone to eight days after release. field and Rory Wilson for kindly allowing us to use his Nineteen of the recaptured birds showed depth traces. unpublisheddata on JackassPenguins. Scientific research Four others had not been to sea in the interval between at Marion Island is carried out under the auspicesof the releaseand recapture,and two others lost their harnesses. South African Scientific Committee for Antarctic Re- Maximum diving depths recorded ranged from 3.5 m to search. Financial and logistical support of the Antarctic more than 70.0 m, the median depth being 9.0 m. The Division of the South African Department of Transnort diving depths were non-randomly distributed (x’ = 22.3, is gratefully acknowledged. P < 0.005, n = 19) with 16 dives reaching less than 20.0 m; two more attained 40.0 m, and only one dive exceeded LITERATURE CITED 70 m. The maximum diving depths attained by the pen- guins were not significantly correlated with the time in- CONROY, J. W. H., AND E. L. TWELVES. 1972. Diving terval between release and recapture (r = -0.291, P > depths of the Gentoo Penguin (Pygoscelis pupua) and 0.1). Blue-eyed Shag (Phalacrocorax atriceps) from the Penguinsswim at three levels in the water: on the sur- South Orkney Islands. Bull. Br. Antarct. Surv. 30: face, at the travelling depth (two meters or less in the 106-108. JackassPenguin [Spheniscus demersus]; R. Wilson, pers. CROXALL,J. P., AND P. A. PRINCE. 1980a. Food, feeding comm.), and at the foraging depth, the latter being the ecologyand ecologicalsegregation of seabirdsat South most variable and, generally, the deepest. Their ability to Georgia. Biol. J. Linn. Sot. 14:103-l 3 1. dive to great depths is well established.A depth of 130 m CROXALL,J.P., AND P. A. PRINCE. 1980b. The food of has been recorded for the JackassPenguin (R. Wilson, Gentoo Penguins Pygoscelis Papua and Macaroni pers. comm.), more than 240 m for the King Penguin,and Penguins Eudyptes chrysolophus at South Georgia. 265 m for the Emperor Penguin (Kooyman et al. 1971). Ibis 122:245-253. Circumstantial evidence indicates that Gentoo Penguins KOOYMAN,G. L. 1965. Techniques used in measuring may dive to 100 m (Conroy and Twelves 1972). Dive diving capacitiesin Weddell Seals.Polar Rec. 12:39l- duration for Emperor Penguins may exceed 18 min but 394. the longest dive recorded for Gentoo Penguins,observed KOOYMAN,G. L. 1975. Behavior and physiologyof div- feedina at shallow deaths, was 2 min (Koovman 1975). ing, p. 115-l 38. In B. Stonehouse[ed.], The biology The mean dive duration of the JackassPenguin, slightly of penguins,Macmillan, London. smaller than the Gentoo, was 2.5 min (R. Wilson, pers. KOOYMAN,G. L., C. M. DRABEK,R. ELSNER,AND W. B. comm.). Assuming that an appreciable proportion of a CAMBELL. 1971. Diving behaviour of the Emperor dive is spenthunting and pursuingprey, the time available Penguin Aptenodytes forsteri. Auk 88:775-795. for descentand ascentis limited. The dive duration of the KOOYMAN,G. L., R. W. DAVIS, J. P. CROXALL,AND W. Gentoo Penguin suggeststhat the modal depths of 3.5- B. CAMBELL. 1982. Diving depths and energy re- 20.0 m recorded in our study probably representthe nor- quirements of King Penguins. Science2 17:726-727. mal foraging depths, dives to 40.0 m or more being un- VOLKMAN,N. J., P. PRESLER,AND W. TRIVELPIECE.1980. usual. This is supported by the finding that birds recap- Diets of pygoscelidpenguins of King George Island, tured up to eight days after release still had not dived Antarctica. Condor 82:373-378. deeper than 20.0 m, despite the opportunity to make sev- WILLIAMS,A. J. 1980. Aspects of the breeding biology eral foraging trips and, consequently,many dives. of the Gentoo Penguin Pygoscelis papua. Gerfaut 70: Both major kinds of prey of Gentoo Penguins,known 283-295. from other localities, fish and krill, occur within the re- WILLIAMS,A. J. 1981. Factors affectingtime of breeding corded diving range of these birds at Marion Island. The of Gentoo Penguins Pygoscelis Papua at Marion Is- size classesof notothenid fish taken at South Georgia are land, p. 451-459. In J. Cooper [ed.], Proceedingsof known to frequent offshore kelp beds that are less than the Symposium on Birds of the Sea and Shore. 1979. 30 m deco (Croxall and Prince 1980b). while lame con- African Seabird Group, Cape Town. centrations of krill are known to occur at depths-of less WILSON,R. P., AND C. A. R. BAIN. In press.An inexpen- than 100 m, even during the day (Volkman et al. 1980). sive depth gaugefor penguins.J. Wildl. Manage. The depth gauge/solublepowder technique is inexpen- sive and the harnessesare easy to manufacture. On the Percy Fitzpatrick Institute of African Ornithology, Uni- other hand, the technique is limited becausethe device versity of Cape Town, Rondebosch 7700, South Africa. can measureonly the maximum depth attained during any Received 22 November 1982. Final acceptance20 May one, or a series, of foraging trips. More detailed infor- 1983. mation on diving depths and dive profiles is required in