SHORT COMMUNICATIONS Diving Differencesbetween Western and Clark's Grebes GARY L. NUECHTERLEIN AND DEBORAH P. BUITRON Departmentof Zoology,North DakotaState University, Fargo, North Dakota58105 USA In the sixth edition of the A.O.U. check-list (1985), in Clark's Grebe and dull light-green in Western Clark'sGrebe (Aechmophorus clarkii) was designateda Grebes; Starer 1965, Starer and Nuechterlein 1985). separatespecies from the WesternGrebe (A. occiden- We then comparedthe relativefrequency of the two talis).Once considereda light colorphase of the West- speciesalong the eastshoreline (Hank's Marsh) and ern Grebe,Clark's Grebe is generallymore prevalent westshoreline (Eagle Ridge Marsh) of UpperKlamath in breeding populationssouth of 44øN(Starer 1965, Lake. Each marsh supported a large, mixed-species Feerer 1977). There is considerablelocal variation in nesting colony. Hybrid or intermediatebirds were the relative abundanceof the two species,and they excludedfrom our sample. are sympatricthroughout much of their range. In We observedthe diving behavior of Westernand areaswhere both are common,they frequently form Clark's grebeson the east shore of Upper Klamath mixed feeding flocks,but their Advertisingvacaliza- Lake, and at Lake Ewauna, a broadened section of the tians are distinct,and interbreeding is rare (Ratti 1979, KlamathRiver, which flowsinto Upper Klamath Lake Nuechterlein 1981). on the southeast end of the lake. To obtain infor- Westernand Clark'sgrebes are foot-propelled,fish- mationon diving behavior,we determineddepths at eating surfacedivers. Fish typically are capturedsin- both locationsby plumb lines, and then placed sty- gly in the bill and may be swallowed either under rofoambuoys in lines perpendicularto the shore to the water or on the surface. Nuechterlein (1981) and mark depth contoursat 1-m depth intervals. This al- Ratti (1985) showed that Clark's Grebestended to feed lowed us to record the water depth at which each farther from shore than Western Grebes at Upper observed dive occurred to the nearest meter. Klamath Lake, Klamath County, Oregon. Nuechter- We controlled for wave action by including only lein (1981) found also that Clark's Grebes were more observations made under low-wave conditions. Dive likely than WesternGrebes to engagein "springing typesform a continuum,but wereclassified into three dives"(Lawrence 1950),in which the diving bird ini- behavioral categories(see diagrams,Fig. 1): springing tially pulls its headback, then springsupward with dive(entire body of the bird emergesfrom waterprior the legsand leapsout of the water beforesubmerging to diving, breast and tarsusclearly visible), interme- (seediagram, Fig. 1).Although no dataon waterdepths diatedive (breast of the diving bird emergesfrom the were availablein the preliminary study,Nuechterlein water, but not the entire tarsus),and leveldive (neither (1981) speculatedthat birds engaging in springing breastnor tarsusemerges from the water). dives may attempt to dive more vertically and this In addition to water depth and dive type, we re- form of dive was related to the depth an individual corded dive times and the speciesand sex of birds bird reached.This would result in ecologicalsegre- that dived within the prescribed area. We distin- gation between the two species. guishedsex visually by bill size, which in malesof The major purposeof our study was to document both speciesis longer and deeper(Storer and Nuech- a possiblerelationship between the level and spring- terlein 1985). Birds which could not be reliably sexed ing divesand the depth of water at which they occur by this methodwere excluded.To avoidpossible bias- in thesetwo grebespecies. We then examinedwheth- esassociated with repeatedsampling of the samein- er thesetwo variableswere relatedto either the species dividual during extended dive bouts, observations or the sex of the bird or both. were extended over several weeks, and we included Although artificial impoundmentson many Na- only data from a maximumof 2 divesper individual tional Wildlife Refugessupport large breeding pop- observedon eachoccasion. We used one-wayANO- ulationsof both species,such areastend to be very VA testsand Newman-Keuls multiple-range tests to shallow and have irregular depth contours,which analyze dive times, and we used G-teststo compare may obscurerelationships that normally occur be- the relativefrequencies of differentdive types. tween depth and distancefrom shore(Ratti 1985).The A censusconducted on Upper Klamath on 1 May two study areasat Upper Klamath Lake provided a 1984showed highly significantdifferences in the dis- large population of each speciesthat feed within a tribution of feeding flocks of Western and Clark's naturallake basinwith water depthsthat vary from grebeson Upper KlamathLake. On the westshoreline at least 0-8 m. of Upper Klamath Lake, Clark's Grebeswere rare (7 Prior to the observations,we locatedmixed feeding of 103 birds), while on the eastshoreline they com- flocksof the two speciesduring censuscounts. With prisednearly 50%of the birds (152 of 305 birds, G = binocularsand a 15-60x spottingscope, we classified 71.6, df = 1, P < 0.001). birdsto speciesfrom bill color(bright orange-yellow At both Upper Klamathand Lake Ewauna,Western 467 468 ShortCommunications [Auk, Vol. 106 and Clark'sgrebes engaged in all three typesof dives, A. LAKE EWAUNA but at both locationsClark's Grebes were significantly more likely to engagein springingdives (Fig. 1, G = 60 71.1, df = 2, P < 0.001 for Lake Ewauna; G = 53.1, df = 2, P < 0.001 for Upper Klamath Lake). At Lake Ewauna,49% of the dives by Clark's Grebes(n = 79) 40 were springing dives comparedwith only 7% for $0 WesternGrebes (n = 113);at Upper KlamathLake the respectivepercentages were 47% (n = 78) and 6% (n 20 = 175). 10 In both speciesthere was a significantshift toward 0 relatively more frequent level dives at the Upper LEVEL INTERI•I. SPRINGING KlamathLake study area,which was shallowerthan TYPE OF DIVE Lake Ewauna (compareFig. 1A vs. Fig. lB; G = 7.7, df = 2, P < 0.05 for Western Grebes; G = 24.4, df = 2, P < 0.001 for Clark's Grebes). B. UPPER KLAMATH LAKE Dive-type frequencywas related significantlyto waterdepth for both speciesat both locations(Fig. 2, significancelevels in legend).In both species,spring- 100 ing diveswere morefrequent at greaterwater depths • 8O than level dives at that site. Clark's Grebes dived at the deepestwater-depth zone availableat a site, and • 6o were significantlymore likely to usespringing dives m 40 than were Western Grebes. (For Lake Ewauna at 6- z 7.9 m, Clark's Grebes used 62% springing dives vs. 2O 6% for Western Grebes, G = 36.2, df = 2, P < 0.001; for Upper Klamathat 4-5.9 m, Clark's Grebesused LEVEL INTERI•I. SPRINGING 61%springing dives vs. 15%for WesternGrebes, G = 35.4, df = 2, P < 0.001.) At Lake Ewauna,neither speciesdived at depths of <2 m. WesternGrebes most frequently fed at water depthsof 4-5.9 m (50.4%of 113 dives;Fig. 2A), and Fig. 1. Types of feeding dives of Western Grebes they usuallyused level or intermediatedives. Clark's (hatchedbars) and Clark's Grebes(open bars)at Lake Grebesat LakeEwauna were significantlymore likely Ewauna (A) and Upper Klamath Lake (B), Oregon. than Western Grebesto dive in deeper water (Fig. 2B; Springingdives (right illustration)differed from level G-test on frequency totals for the water depths: G = dives (left) in the extent to which the body emerged 13.7, df = 2, P < 0.01). from the water before submerging. At Upper Klamath Lake, water depths >6 m were not present,and Clark'sGrebes were not significantly more likely to dive in deeper water than were West- df = 3, P > 0.05, for Western Grebes; G = 5.3, df = ern Grebes (Fig. 2A; G-test on frequency totals for 3, P > 0.05, for Clark's Grebes). Although female water depths, G = 5.6, P > 0.05). At depths <2 m, Western Grebesalso used relatively more interme- level dives predominatedin both speciesand spring- diate and springing dives than males,springing dives ing dives were absententirely (Fig. 2, n = 13). were rarely used by either sex in this species(7% of At both sites, the mean dive times for springing female dives, 5% of male dives), and overall sex dif- divesby Clark'sGrebes were significantly longer than ferenceswere not significant(Table 1). for level dives(Fig. 3). This relationshipalso held for The most fundamental influence on dive type in WesternGrebes at Upper Klamath,although sample our study was water depth. Our data are consistent sizesfor timedspringing dives were small(n = 9). At with the hypothesisthat birds that engage in spring- Lake Ewauna sample sizes for springing dives by ing divesattempt to dive deeperin the water column. Western Grebes were even smaller (n = 5), and dif- Becauseshallow water makesit physicallyimpossible ferenceswere not significant. for a bird to dive deeply, our observationsover shal- Significantsex differences in dive type werefound low water provide the most direct test of this hy- only in Clark'sGrebes. Female Clark's Grebesused pothesis.Springing dives seldom were usedby either springingdives significantly more often than males speciesat depths <4 m (7% of 204 dives, speciescom- (59% of 59 female dives vs. 42% of 98 male dives; bined, Fig. 2) and were never seen at depths <2 m Table 1). This was true despite the fact that water- (n = 13). Springingdives alsowere almostnever ob- depthdistributions of divesby malesvs. females were servedduring 7 yr of studyingthe breedingecology not significantlydifferent for either species(G = 1.7, of Western Grebeson two shallow (2-m depth max- July 1989] ShortCommunications 469 A. WESTERN GREBE LAKE EWAUNA'x' 'x- UPPERKLAMATH LAKE ')(")(' BEPTH OF WATER DEPTH OF WATER 1 2-3.9 m • 0-1.9 m 4-5.9 m 2-3.9 m LEVEL DIVES 6-7.9 m INTERM. DIVES SPRINGING DIVES 4-5.9m 1'0 1•5 2•0 2'5 10 20 30 40 50 60 70 NUMBER OF DIVES NUMBER OF DIVES B.