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The Condor89:201-204 0 The CooperOrnithological Society 1987

FORAGING BEHAVIOR OF BARK-FORAGING IN THE SIERRA NEVADA1

MICHAELL. MORRISON Department of Forestryand ResourceManagement. Universityof California, Berkeley, CA 94720

KIMBERLY A. WITH Department of Biological Sciences,Northern Arizona University, Flagstaft;AZ 86011

IRENE C. TIMOSSI AND WILLIAM M. BLOCK Department of Forestryand ResourceManagement, Universityof California, Berkeley, CA 94720

KATHLEEN A. MILNE Pacific SouthwestForest and Range Experiment Station, USDA Forest Service, Fresno, CA 93710

Key words: Bark-foraging birds;foraging behavior; STUDY AREA Sierra Nevada. The study area was the Blodgett Forest Research Sta- tion of the Universitv of California-Berkelev. El Do- Data on foragingbehavior are often usedfor examining rado County, California. This 1,200-ha forestis located use of habitat and describing community structure in the mixed-conifer zone (see Griffin and Critchfield among co-occurring species of birds using the same 1972) at about 1350 m elevation in the central-western resourcebase (e.g., Johnson 1966, Eckhardt 1979, Rus- Sierra Nevada. The forest consisted of five predomi- terholz 1981). Differencesin tree species,foliage mor- nant conifer species:incense cedar (Calocedrusdecur- phology, and bark structuremay influence the types of rens; 25% of total basal area, unpubl. data); white fir prey taken and the speciesof using the substrate (Abiesconcolor, 21%); ponderosapine (Pinus ponder- (e.g., Jackson1979, Holmes and Robinson 1981, Rob- osa, 19%); sugar pine (P. lambertiana, 10%); and inson and Holmes 1984). Elucidation of foranine.be- Douglas-fir (Pseudotsugamenziesii, 15%);and one de- haviors and tree speciespreferences is import&t Tf we ciduousspecies, California black oak (Quercuskellog- are to determine the role of birds in forest ecosystems, gii, 8%). The forest has been divided into 5- to 37-ha and make informed decisions regarding management compartments to be managed under various silvicul- of these forests. In this study we describe the use of tural systems and is now mostly mature (>70 years tree species,foraging modes, and foraging substrates old) conifer that is at or near rotation (cutting) age. by a group (or “guild,” sensuRoot 1967) of bark-for- During 1983 and 1984 we selected24 compartments aging birds breeding in the western Sierra Nevada. A (of mature conifer); compartments totalled about 420 similar study on foliage insectivores was conducted ha (range = 7-37 ha). Durine. 1985 we selected nine previously near our study area (see Airola and Barrett compa

This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. 202 SHORT COMMUNICATIONS

We defined the following foraging motions: glean- prey taken from the surfaceof the substratewhile the bird was perched; hover-glean-prey taken from the surfaceof the substratewhile the bird was flying; sal- lying (hawk, flycatch)-bird leaves a perch, attempts to catch flying prey on the wing, and returns to a perch (see also Eckhardt 1979); probe-prey taken from an opening, such as bark crevices; and peck-bill force- fully struckat the surface.Other foragingmotions (e.g., flush-pursuit, aerial flight, chase) were used little and are reported here as a combined “other” category. Flaking of bark was noted primarily during winter at BlodgettForest (seeMorrison et al. 1985) and was not included as a separatecategory herein. Data for males and females were combined for this analysis. Although intersexual differences in foraging behavior are known for many (e.g., JSil- ham 1965, Grubb 1975, Williams 1980) we combined sexesbecause: (1) we only had marginally enoughdata for intersexual comparisonsin two of the woodpecker species;and (2) we could not usually differentiate sex in the nonwoodpeckerspecies. Except for the Pileated Woodpecker, our sample sizes greatly exceededthose necessaryfor analysisof avian foraging behavior (i.e., observationson > 35 individuals; Morrison 1984). The distribution of foragingactivities for useof tree species, FIGURE 1. Use (%) of tree species(upper bar), for- modes, and substrateswere examined separately for agingmodes (middle bar), and foragingsubstrates (low- each bird speciesby chi-square analysis.All data were er bar) for birds at Blodgett Forest during the summers analyzed using SPSSX (SPSS 1983). of 1983 to 1985.

RESULTS tential problem of correlated activities of co-occurring In the results that follow, the distribution of foraging individuals of the same species. activities for each bird speciesfor use of tree species, Once encountered,the foraging activity of an indi- and foraging modes and substrates,were significantly vidual was recorded for a minimum of 10 set to a different (chi-squareanalysis, P c 0.05) except as not- maximum of 30 sec. The observer either timed the ed. activity period with a stopwatch or by counting; only Differences were evident in the percent use of dif- birds actively foraging are analyzed herein. The fol- ferent tree speciesby birds; all distributions were sig- lowing data were recordedfor each individual: species; nificantly different exceptfor the Pileated Woodpecker, sex; type of foraging motion (defined below); species which used all tree species with roughly equal fre- of plant; substrateto which the foraging motion was quency. All speciesexcept the White-headed Wood- directed; foraging substrate; perch height; and plant peckerand (to a lesserextent) the Brown Creeper dem- height. Heights and distanceswere visually estimated. onstrated high use of white fir, and all speciesexcept The dominant foragingactivity (e.g., motion, substrate, the Red-breasted Sapsuckershowed high use of pon- height) observed during the timed period for each in- derosa pine (Fig. 1). Relative to the other species,the dividual was recorded. Pileated Woodpecker occurred more on oak, and the

TABLE 1. Foragingheight and dbh of foragingtrees for birds at Blodgett during the summersof 1983 to 1985.” Values with same letter (A, B, C, or D) are not significantly different (P z 0.05) as determined by Duncan’s new multiple range test.

Foragingheight (m) Foragingtree dbh (cm) Species ??a R SD + SD

Red-breasted Sapsucker 91 11.5 6.63C 41.8 23.55C Hairy Woodpecker 89 10.6 6.68C 44.5 24.3OC White-headed Woodpecker 116 11.6 6.59C 58.9 27.56B Pileated Woodpecker 48 17.4 8.99A 90.0 42.72A Red-breasted Nuthatch 126 13.9 6.49B 47.7 22.63C Brown Creeper 123 6.7 4.46D 47.9 25.64C

aSample sizes = numberof individuals observed. SHORT COMMUNICATIONS 203

TABLE 2. Vigor of foraging trees and foraging substratesused by birds at Blodgett Forest during the summers of 1983 to 1985. Sample sizes in Table 1.

Foraging tree (%) Foraging substrate (%) Species Alive Dead Alive Dead

Red-breasted Sapsucker 16.4 23.6 72.0 28.0 Hairy Woodpecker 46.6 53.4 32.0 68.0 White-headed Woodpecker 81.1 18.9 61.0 39.0 Pileated Woodpecker 55.3 44.1 21.5 72.5 Red-breasted Nuthatch 82.8 11.2 65.0 35.0 Brown Creeper 86.2 13.8 18.3 21.7

White-headed Woodpecker and creeper were unique the Hairy and White-headed woodpeckers). Differ- in exhibiting a greater-useof incensecedar. In addition, ences in foraging height, type of substrateused, and the White-headed Woodvecker had a higher use of tree and substratevigor also differed among species. sugarpine than the other-species. For example,although the creeperand nuthatchgleaned Only the sapsuckerand nuthatch were observedfly- extensively (in contrast to most woodpeckers), they catching. The creeper foraged primarily by gleaning foraged at different average heights, and the nuthatch and probing. The nuthatch gleaned extensively, but used a wider range of foraging substrates.The sap- spent roughly equal time probing and pecking. In con- sucker differed from the other woodpecker speciesin trast to the other woodpeckers, the sapsuckerspent severalways, most notably in its higher useof gleaning. similar amounts of time pecking and gleaning. The The use of living substratesby the sapsucker,in con- other woodpeckerspecked and probed >70% of the trast to the predominate use of dead wood by other time (Fig. 1). woodpeckers,is likely a reflection of the use of sap as All speciesexcept the nuthatchconcentrated foraging a food source by the sapsucker(see Vemer and Boss activities on trunks (Fig. 1). The nuthatch used a wide 1980). variety of substrates,including twigs and small-sized A change in the speciesor size composition of the branches.Although concentratingactivities on trunks, forest would likely alter the pattern of resourceuse by the Pileated Woodpecker also showed substantialuse the birds we observed. Such changesare, in fact, un- of medium-sized branches (Fig. 1). derway in the Sierra Nevada due to silvicultural activ- Pileated Woodpeckers foraged significantly higher ities (see Morrison et al. 1985). For example, external than all other species,the nuthatch foragedhigher than bark structurechanges with tree diameter (e.g., usually all remaining species,and the creeper foraged signifi- becomingthicker and more deeply furrowed with age), cantly lower than all other species(Table 1). The re- which affects prey abundanceand the ability of birds maining three species, the White-headed and Hairy to obtain the prey (Jackson1979, also Morrison et al. woodpeckers,and Red-breasted Sapsucker,had mean 1985). The current trend in the western Sierra Nevada foraging heights of about 10 to 11 m. is conversionfrom mixed-conifer to monotypic stands The Pileated Woodpecker used significantly larger of ponderosapine, or mixed standsof ponderosapine (by diameter at breast height; dbh) trees than all other and lesseramounts of Douglas-fir and fir (R. C. Heald, species;the White-headedWoodpecker used larger trees Manager, Blodgett Forest, pers. comm.). Precommer- than the remaining species(Table 1). The averagedbh cial- and commercial-thinning activities also remove of foraging trees used by the other specieswas similar much of the understory of small, commercially un- (about 42 to 48 cm dbh). desirable species or less-vigorous individuals. Al- The sapsucker, White-headed Woodpecker, nut- though most speciesin our study usedponderosa pine, hatch, and creeper spent the majority of their time they also showed high use of white fir. Further, several foraging on the live parts of trees (Table 2). The Hairy speciesexhibited high use of incense cedar. Although and Pileated woodpeckers foraged in live and dead we would predict that conversion to ponderosa pine trees with similar frequency, but about 70% of their would probably not eliminate any specieswe studied, foraging time was on dead substrates(Table 2). such a changewould likely alter the abundanceof the birds. The ramifications of such changeson interac- DISCUSSION tions within the bird community, and within the forest Our results indicated that the birds we studied used ecosystemitself (e.g., bird- interactions), are un- different combinations of the foraging behaviors ex- known. Our results, as well as those of Airola and amined. All speciesexcept the sapsuckershowed high Barrett (1985) for foliage insectivores, indicate that use of ponderosa pine; high use of white fir was ob- efforts should be made to maintain a high diversity of served for all except the creeper and White-headed tree species and size classesthroughout the mixed- Woodpecker;the white-headed and creeperhad higher conifer zone of the Sierra Nevada. use of incense cedar, and the white-headed also used This paperelucidates foraging behaviors used by bark more sugarpine than the other species.Where overlap foragersduring spring and summer. In a related study, occurredin tree speciesuse (ponderosapine and white Morrison et al. (1986) found a significant increase in fir), the bird speciesused different foragingmodes and/ the use of incensecedar by many birds during winter. or other tree speciesto a greater extent (e.g., compare This change was related to prey availability and was 204 SHORT COMMUNICATIONS

accompaniedby changesin foragingmode. Therefore, JACKSON,J. A. 1979. Tree surfacesas foraging sub- biologistsand resourcemanagers should view the pres- stratesfor insectivorousbirds, p. 69-93: Zi J. G. ent paper in light of suchseasonal changes in behavior. Dickson. R. N. Conner. R. R. Fleet. J. C. Kroll. Unfortunately, we did not measure prey availability and J. A: Jackson[eds.]: The role of insectivorous during the breedingperiod, a shortcomingthat should birds in forest ecosystems.Academic Press, New be eliminated in future studies.Detailed analysisof the York. surfacearea of bark available to birds, including limbs JOHNSON,N. K. 1966. Bill size and the question of and twigs, should also be quantified in the future. competition in allopatric and sympatric popula- tions of Dusky and Gray flycatchers. Syst. Zool. We appreciate field assistanceduring parts of this 15:70-87. study provided by P. N. Manley, S. A. Laymon, R. E. K~LHAM,L. 1965. Differences in feeding behavior of Etemad, and J. M. Anderson. R. C. Heald and staff at male and female Hairy Woodpeckers.Wilson Bull. Blodgett Forest are thanked for logistical support. J. 77:134-145. Vemer, C. J. Ralph, R. T. Holmes, and an anonymous MORRISON,M. L. 1984. Influence of sample size and referee gave very useful comments on an earlier draft sampling design on analysisof avian foraging be- of this manuscript. Comments made on another but havior. Condor 86: 146-150. related manuscript given by R. L. Hutto, F. C. James, MORRISON,M. L., I. C. TIMOSSI,K. A. WITH, AND P. and T. W. Schoenerplayed a substantialrole in eval- N. MANLEY. 1985. Use of tree speciesby forest uation of the results reported herein. birds during winter and summer. J. Wildl. Man- LITERATURE CITED age. 49:1098-l 102. MORRISON.M. L.. K. A. WITH. AND I. C. TIMOSSI. AIROLA,D. A., AND R. H. BARRETT. 1985. Foraging 1986. ’ The structure of a forest bird community and habitat relationships of insect-gleaningbirds during winter and summer. Wilson Bull. 98:214- in a Sierra Nevada mixed-conifer forest. Condor 230. 87:205-216. ROBINSON,S. K., AND R. T. HOLMES. 1984. Effects ECKHARDT,R. C. 1979. The adaptive syndromes of of plant speciesand foliage structure on the for- two guilds of insectivorousbirds in the Colorado aaina behavior of forest birds. Auk 101:672-684. Rocky Mountains. Ecol. Monogr. 49:129-149. RooT~R~B. 1967. The niche exploitation pattern of GRIFFIN,J. R., AND W. B. CRITCHL~ELD.1972. The the Blue-grayGnatcatcher. Ecol. Monogr. 37:3 17- distribution of forest trees in California. USDA 350. For. Serv. Res. Pap. PSW-82. RUSTERHOLZ,K. A. 1981. Competition and the struc- GRINNELL,J., AND A. H. MILLER. 1944. The distri- ture of an avian foragingguild. Am. Nat. 118:173- bution of the birds of California. Pacific Coast 190. Avifauna No. 27. SPSS, INC. 1983. SPSSX user’s guide. McGraw-Hill GRUBB,T. C., JR. 1975. Weather-dependentforaging Book Company, New York. behavior of some birds wintering in a deciduous VERNER,J., ANDA. S. Boss. 1980. California wildlife woodland. Condor 77: 175-l 82. and their habitats: western Sierra Nevada. USDA HOLMES,R. T., AND S. K. ROBINSON. 1981. Tree For. Serv. Gen. Tech. Rep. PSW-37. speciespreferences of foraginginsectivorous birds WILLIAMS,J. B. 1980. Intersexual niche partitioning in a northern hardwood forest. Oecologia 48:3 l- in Downy Woodpeckers.Wilson Bull. 92:439-45 1. 35.