NOTES

FOOD STORAGE BY ACORN WOODPECKERS AT THE SANTA ROSA PLATEAU PRESERVE, , FLOYD E. HAYES, Department of Natural Sciences,Section of Biology, Eoma Linda University,Loma Linda, California92350 WILLIAM S. BAKER, Department of Biology, La Sierra University,Riverside, California 92515 EARL W. LATHROP, Departmentof Natural Sciences,Section of Biology,Loma Linda University,Loma Linda, California92350

The Acorn Woodpecker(•V•elanerpes formi½ivorus) is well knownfor its habitof storing food, especiallyacorns, in "granaries"(e.g., Ritter 1938, Bent 1940, MacRobertsand MacRoberts1976, Koenigand Mumme 1987). However,the food itemsstored and storagesites used may be expectedto vary geographically,depend- ing on the availabilityof food itemsand potentialstorage sites. This note reportson food itemsstored and storagesites used by Acorn Woodpeckersat the SantaRosa PlateauPreserve, Riverside County, California. The Santa Rosa PlateauPreserve was establishedby the Nature Conservancyin 1984 to protectvernal pools on three lava-cappedmesas of the SantaRosa Plateau, a distincttopographic unit of rollinghills covered with grassland,oak woodland,and chaparrallocated at the southeasternend of the Santa Ana Mountains(Lathrop and Thorne 1978, 1985). Two species of oaks, the Engelmann oak (Quercus engelmannii) and coastlive oak (Q. agrifolia), dominatethe preserve'soak wood- land community(Lathrop and Zuill 1984, Lathrop and Wong 1986). The Acorn Woodpeckeris a conspicuousresident of thesewoodlands, occurring in smallfamily groups. In November1984, on 27 March 1985, and on 22 August1991, we searchedfor acorns,presumably stored by Acorn Woodpeckers,in porphyriticbasalt boulders in or near oak woodlandscattered along an approximately0.5-km ridge at the edge of Mesa de Colorado.On 23 March 1990, we inventoriedfood itemsstored by Acorn Woodpeckersin woodenfence posts, a rockwall, and granitic boulde•'s at Rancho Santa Rosa, where the surroundingtrees included introduced blue gum (Eucalyptus globulus), pecan (Carya illinoensis), olive (Olea europaea), and fan palm (•Vashingtoniafilifera) in addition to the native oaks. On 22 August 1991, we searchedfor additionalfood items storedin graniticboulders 100 m west of the wooden fence posts. During other visits we estimated the proportion of Q. enaelmannii and Q. agrifolia acorns stored in Acorn Woodpecker granariesin native oak woodland 1 km or more from the Rancho Santa Rosa site. In November 1984, Baker foundapproximately 20 smallacorns stored in numer- ousholes (0.3-1 cmwide and 5-10 cmdeep) on approximately1 m 2 of thesurface of a boulderof porphyriticbasalt (Figure 1 ) alonga ridgeof Mesade Colorado.On 27 March 1985, only three acorns remainedinside the boulder,and none was presenton 22 August1991. Severalnearby boulders possessed holes suitable for the storageof acorns,but only one more acorn was found on 27 March 1985. The acornswere all from Q. agrifolia. They were tightlyinserted with the cap end out, WesternBirds 23:165-169, 1992 165 NOTES the usualmethod of storageby Acorn Woodpeckers(MacRoberts and MacRoberts 1976). T. Griggs(pers. comm.) photographedacorns in the same boulderin early 1984; a colorslide is on file at the headquartersof the Santa RosaPlateau Preserve (G. Bell pers. comm.). On 23 March 1990, we found that 66.6% of the food items stored by Acorn Woodpeckersin holes drilled into wooden fence posts at Rancho Santa Ana consistedof acorns(apparently 100% from Q. agrifolia), while 26.1% were pecans and 7.3% were olives (n -- 2,474 items sampled; Figure 2). Four pecans were wedgedinto the endsof a metalpipe on top of a gate.Along a 65-m sectionof a rock wallroughly 1 m talland 0.5 m wide(beside the fence posts), we found117 acornsof Q. agrifolia tightlyinserted into cracksor holesbetween basaltic rocks or on rock surfaces:no pecansor oliveswere found. We also foundfive Q. agrifolia acorns wedgedinto cracksin threegranitic boulders within 25 m of the rockwall. Most of the acorns stored in rock creviceswere tightly inserted with the cap end out. indicatingthat they were storedby Acorn Woodpeckers.Small numbersof Q. agrifoliaacorns were alsostored in the woodensides of a barn, in the trunksof severallarge fan palmsat the ranch,and betweenthe woodenwindow frames and plasterwall of an old building.Eucalyptus globulus seeds and Q. agrifolia acorns

Figure1. Porphyriticbasalt boulder where acornswere foundin smallholes during 1984 and 1985 but not when photographedin 1991. Photo by Floyd E. Hayes

166 NOTES

were foundloose in the largercrevices along the rockwall, but becausethey were not tightlywedged in, we believethey were depositedby rodentsrather than woodpeck- ers. On 22 August 1991, we found 30 Q. agrifolia acornswedged into narrow crackson six graniticboulders 100 m west of the fence posts;one overhanging verticalcrack contained 14 acorns (Figure3). In other Acorn Woodpeckergranaries in oak woodlandaway from Rancho Santa Rosa(> 1 km), we foundthat 90-95% of the acornswere from Q. agrifolia and only 5-10% were from Q. engelrnannit. Obtaining exact percentageswas difficult becausethe acornswere presentin largequantities and usually had to be pulledout to be identified. The acorns were stored in the trunks and lower limbs of dead or live oaksof both species.No pecansor oliveswere found. Acorn Woodpeckersusually store acorns in trunksand largebranches of trees.but other locationssuch as telephonepoles, fence posts.eaves of buildings,pine cones, and clay roof tilesalso suffice (MacRoberts and MacRoberts1976, Jehl 1979). This appearsto be the first report of Acorn Woodpeckersstoring acorns in rocksand metal objects. Crevicesof almost any size or shape may be sufficientto release storagebehavior in Acorn Woodpeckers,especially if storagespace is limited(Jehl 1979, Koenig and Mumme 1987). Our finding acorns in the basaltic boulder irregularlysuggests that acornsare more likelyto be storedin rocksduring years with surplusacorn crops.

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1 i !

Figure2. Pecansand a singleolive (center)stored in holesdrilled into a fence post. Photo by Earl V•. Lathtop 167 NOTES

Bent (1940:216) reportedthe storageof pecansby Acorn Woodpeckers.How- ever,there appear to be no previousreports of olivesin the dietof AcornWoodpeck- ers. The olive trees at Rancho Santa Rosa have since been cut down. Acorn Woodpeckersfeed on a varietyof plant and animalfood itemsin additionto acorns (Ritter 1938, Bent 1940, MacRobertsand MacRoberts1976, Koenig and Mumme 1987). Our observationssuggest that they readily exploit novel items of food,such as pecansand olives,when available. We thank W. D. Koenigand M. H. MacRobertsfor reviewingthe manuscript.We alsothank T. Griggsand G. Bell of The NatureConservancy for permissionto study at the Santa Rosa PlateauPreserve and, along with N. Backstrand,for pertinent information.This studywas fundedin part by grant IHRMP-86/2 to Lathropfrom the Universityof Californiaat Berkeley.

Figure3. Overhangingvertical crack in a graniticboulder (left) and closeup(right) showingacorns. Photo by Floyd E. Hayes

168 NOTES

LITERATURE CITED

Bent, A. C. 1940. Life historiesof North Americanwoodpeckers. U.S. Natl. Mus. Bull. 174:1-334. Jehl, J. R., Jr. 1979. Pine conesas granariesfor Acorn Woodpeckers.W. Birds 10:219-220. Koenig,W. D., and R. L. Mumme. 1987. Populationecology of the cooperatively breedingAcorn Woodpecker.Princeton Univ. Press,Princeton, N. J. Lathrop, E. W., and Thorne, R. F. 1978. A flora of the Santa Ana Mountains, California. Aliso 9:197-278. Lathrop,E. W., andThorne, R. F. 1985. A new preserveon the SantaRosa Plateau. Fremontia 13(1): 15-19. Lathrop,E. W., andWong, B. 1986. Standcharacteristics of southernoak woodland on the Santa Rosa Plateau, southernCalifornia. Crossosoma12(4):1-7. Lathrop, E. W., and Zuill, H. A. 1984. Southernoak woodlandsof the Santa Rosa Plateau,Riverside County, California. Aliso 10:603-611. MacRoberts,M. H., and MacRoberts,B. R. 1976. Socialorganization and behavior of the AcornWoodpecker in centralcoastal California. Ornithol. Monogr. 21:1- 115. Ritter,W. E. 1938. The CaliforniaWoodpecker and I. Univ. of Calif.Press, Berkeley.

Accepted 11 February 1992

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