PRE-ROOSTING FLIGHT OF THE CLARK'S NUTCRACKER DIANA F. TOMBACK • Department of Biological Sciences, University of California at Santa Barbara, Santa Barbara, California 93106 USA ABSTI•CT.--Each summer at Tioga Pass, California, Clark's Nutcrackers participate in a nightlypre-roosting flight beginningin mid- to late July. Beforesunset, nutcrackers are activeon the sunlit, west-facingMt. Dana slope. As light intensity rapidly decreases,they fly in small groupsto the east-facingGaylor Ridge slopeto roost--a distanceof 2 km. Gaylor Ridge is the first slopein the study area to receivesunlight at sunrise.In 1974, 1975, and 1976, the number of pre-roostingflight participantspeaked at 30, 33, and 47, respectively,and dwindled to a few birds by the end of August. Irregular pre-roostingmovements of nutcrackerswere also observed in 1973 in the Minaret Summit area. Data on temperature and solar radiation gatheredat Tioga Passin summer 1974 suggesttwo explanationsfor this behavior: 1) nutcrackersmay derive some energeticbenefit by remainingin the warmest area at sunsetand by roostingin the first area to be warmed at sunrise,or 2) nutcrackersmay extendtheir activity time for foraging, feedingyoung, and socializingby being in the last sunlit area at sunsetand in the first sunlit area at sunrise. Observationsof the feedinghabits and pre- and post-roostingactivities of nutcrackerssuggest that light intensitymay be the primary factor triggeringpre-roosting flights. Received 13 April 1977, accepted 28 August 1977. IN the summersof 1974, 1975, and 1976, I observedan unusual behavior pattern of the Clark's Nutcracker (Nucifraga columbiana) at Tioga Pass, Inyo National Forest, Mono County, California. Each evening, a number of nutcrackerspartici- pated in a unidirectional pre-roostingflight--a mass movement acrossthe valley to a roostingarea. Ferdinand J. Castillo, Yosemite Park Ranger at Tioga Pass, ob- served this phenomenonfor several summersin successionprior to my observations. Such regular flights have not been reported elsewherefor the nutcracker, although I have observed occasionaland less structured pre-roosting movements of nutcrack- ers in the vicinity of Mammoth Mountain, Inyo National Forest, Madera County, California. Each evening before sunset,most of the nutcrackersin the Dana Meadows vicinity of Tioga Passwere found on the west-facingMr. Dana slope, the last terrain in the area to receive sunlight. When light intensity on the slope dropped rapidly between 1930 and 2000, nutcrackersin groupsof two to five birds left the Mr. Dana side of the pass.The groupsflew about 100 m above the valley, directly acrossTioga Pass-- a distance of 2 km--into the dense forest on the east-facing slope of Gaylor Ridge. Here I examine two factors that may be responsiblefor the occurrenceand fixed pattern of such pre-roostingflights. STUDY AREAS AND METHODS Minaret Summit.--Minaret Summit, elevation2,825 m, is 3 km northwestof Mammoth Mountain. On the summit and in the adjacentforest are mature standsof whitebark pine (Pinusalbicaulis). Other conifersin the area include lodgepolepine (Pinus contorta), red fir (Abiesmagnifica), and mountain hemlock (Tsugamertensiana). Approximately 1 km southeastof Minaret Summit is a ridge, 125 m in height, which is the last terrain northwestof Mammoth Mountain to receivesunlight at sunset.Part of the eastdope of the sameridge is the first area to receivesunlight at sunrise. • Presentaddress: Department of Zoology,Seaver Laboratory, PomonaCollege, Claremont, California 91711 USA. 554 The Auk 95: 554-562. July 1978 July 1978] NutcrackerPre-roosting Flight 555 Tioga Pass.--At Tioga Passdata were collectedin the Dana Meadowsarea, elevation3,030 m, on the west-facingslope of Mt. Dana, and on the east-facingslope of Gaylor Ridge up to about 3,150-m elevation.The two slopesface each other and are aligned on an east-westaxis in the study area. Both slopesare steep(approximately 40% grade) in the areas frequentedby nutcrackers. At the baseof Mt. Dana are mixed standsof whitebark pine, lodgepolepine, and red fir, interspersed with meadow. Higher on the slopein the area frequented by nutcrackersare small standsof whitebark pine scatteredamong meadowsand talus slopes.At the base of Gayior Ridge and up to about 3,120-m elevation is a denseforest of whitebark pine, lodgepolepine, and red fir. Above the forest is a steep, 70- m slopebearing scatteredwhitebark pine. Observations on the behavior patterns of nutcrackers prior to roosting, at roosting sites, and imme- diately after roosting, were made in the subalpine habitats of Minaret Summit and Tioga Pass in 1973 and 1974, respectively.Intermittently throughoutthe summersof 1974, 1975, and 1976, I censusedthe number of nutcrackersparticipating in the nightly flights at Tioga Passfrom a point adjacent to the Yosemite National Park entrance station. I recordeddaily temperaturesand solarradiation using meteorographs (WeatherMeasure model M701) and actinographs(WeatherMeasure model R401) with 7-day chart recorders. One of each instrument was set on the ground, the meteorographsin vented housings,at 3,150-m elevation on both the Gaylor Ridge and Mt. Dana slopes.The instrument• were placed just upslopeof the trees where nutcrackers usually perched or roosted. As a result, the recorded data approximated the conditions that the birds experiencedwhen perchedon treetopsat sunriseor sunset. RESULTS SUMMER TIMETABLE AND FOOD HABITS An evaluation of the factors responsiblefor the occurrenceof pre-roostingflights at Tioga Passand other pre- and post-roostingactivities must considerseveral as- pects of the biology of the nutcracker in the easternSierra Nevada, which I describe in detail elsewhere (Tomback 1978). In the south-central region, the majority of nutcrackers overwinter and nest below subalpine elevations. After young fledge, family groupsof nutcrackersmigrate to subalpinehabitat. By mid-July, the numbers of nutcrackersat subalpineelevations peak. Throughout the summer there are three principal sourcesof food for nutcrackers: 1) insectsand spiders, 2) storesof white- bark pine seed made by the birds the previous summer, and 3) seedsfrom unripe and partially ripe cones of the whitebark pine. The availability of the latter food source varies from year to year; in 1973 the cones were first harvested on 19 July, but in 1974 no seed was taken until 2 August. Many juvenile nutcrackers are fed by adults until late August when whitebark pine cones are ripe and seed storage begins. This occurred about 29 August in 1973 and 25 August in 1974. After white- bark pine seed has been harvested and stored, most nutcrackers descend to lower elevations in late September to harvest and store the seed of the Jeffrey (Pinus jeffreyi) and pition (P. monophylla) pines. PRE- AND POST-ROOSTING BEHAVIOR Minaret Summit.--In 1973 the whitebark pine in the forest on and adjacent to Minaret Summit had an abundant crop of cones, which attracted between 35 and 50 nutcrackers, mostly in family groups, into the area. On 31 July, 2, 9, and 13 August, I observed the pre-roostingbehavior of nutcrackers. Each evening until dark nutcrackers participated in two activities: 1) adults harvested whitebark pine seed and fed juveniles, which perched nearby and constantly gave hunger calls (vocalization terminologyfrom Mewaldt 1956); and 2) for periods of several minutes at a time, nutcrackersassumed treetop perchesand vocalized, most often using the regular call. 556 DX^N^F. TOMS^CK [Auk, Vol. 95 3s B 36 25 25 20 ..... f200 •o• 140 , July July August SeptcinDer Fig. 1. Variation in the numbers of nutcrackers participating in pre-roostingflights: A) vs. maximum temperature; and B) vs. light intensity. Recorded on both Gaylot Ridge and the Mt. Dana slope from July through September 1974. At sunseton 31 July and 13 August I observed group movementsof nutcrackers that were similar to the pre-roostingflights at Tioga Pass. Beginning at 1915, when light intensity began to drop rapidly, nutcrackers in groups of two to seven flew southeast onto the sunlit, west slope of the nearby ridge. Edited excerpts from my notes of 13 August illustrate the pre-roosting activities of the nutcrackers on the sunlit slope: 1920. Adult and two juveniles are perched on the top of a whitebark pine near the base of the west slope of the ridge. As the sun drops and the shadow travels up the slope, the three nutcrackers leave their perchesand fly upslope. The adult and juveniles land on top of another whitebark pine; the adult harvestsseed while juveniles beg. Both juveniles leave the tree and executewild flying maneuvers. 1925. Juveniles are fed by adult at the top of a red fir. 1935. (Sun drops below Minarets). Adult and juveniles move higher upslope onto the sunny, steepface. Other nutcrackersare already present.There is a constantexchange of the regular call and restlessflying from treetop to treetop by the nutcrackers.Groups of nutcrackersselect high, exposedperches, often with several birds in the same tree. Perchesare well above neighboring trees•at least 20 m high--and usually on red fir and mountain hemlock. Several nutcrackersuse the shrill call. 1945. Nutcrackers continue to move upslopeinto sunlight as shadow creepsupward. Light is fading quickly. A nutcracker is perched on a whitebark pine cone, harvesting seed. Some nut- crackershave reachedthe top of the west slope where they perch briefly, vocalizing, and then fly east over the top of the ridge and out of sight. A few nutcrackersremain perchedin treetopsbelow the summit, where they will