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The Condor89:201-204
0 The CooperOrnithologicalSociety1987
FORAGING
BEHAVIOR OF BARK-FORAGING
SIERRA NEVADA1
BIRDS IN THE
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
PacificSouthwestForest and Range Experiment Station, USDA ForestService,
Fresno, CA 93710
Key words: Bark-foragingbirds;foraging behavior;
Sierra Nevada.
STUDY AREA
The study area was the Blodgett Forest ResearchStation of the Universitv of California-Berkelev. El DoData on foragingbehavior are often usedfor examining rado County, California. This 1,200-ha forest 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.,Johnson1966, Eckhardt 1979, Rus- Sierra Nevada. The forest consistedof five predominant conifer species:incensecedar (Calocedrusdecurterholz 1981). Differencesin tree species,foliage morrens; 25% of total basal area, unpubl. data); white fir
phology, and bark structuremay influence the types of
prey taken and the speciesof bird using the substrate (Abiesconcolor,21%); ponderosapine (Pinus ponderosa, 19%); sugar pine (P. lambertiana, 10%); and
(e.g.,Jackson1979, Holmes and Robinson 1981, Robmenziesii, 15%);and one deinson and Holmes 1984). Elucidation of foranine.beDouglas-fir (Pseudotsuga
haviors and tree speciespreferencesis import&t Tf we ciduousspecies,California black oak (Quercuskellogare to determine the role of birds in forest ecosystems, gii, 8%). The forest has been divided into 5- to 37-ha
and make informed decisionsregardingmanagement compartments to be managed under various silviculof these forests. In this study we describe the use of tural systemsand 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-forDuring 1983 and 1984 we selected24 compartments
agingbirds 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<ments, seven’ofwhichdiffered from thoseused
during 1983 and 1984;the 1985 compartmentstotalled
1985).
The speciesanalyzed, in order of decreasingabun- 210 ha (range = 15-37 ha). Selection was not comdance, were: Red-breasted Nuthatch (Sitta canaden- pletely randomized becauseforest silvicultural plans
sis),Brown Creeper(Certhia americana), Red-breasted often determined which compartmentswere available
Sapsucker(Sphyrapicusruber), White-headed Woodfor use.In most casescompartmentswere not adjacent
pecker (Picoidesalbolarvatus),Hairy Woodpecker (P. to each other.
villosus),and Pileated Woodpecker (Dryocopuspileatus) (seeMorrison et al. 1986). All are typical inhab- METHODS
About 1250 person-hr were spent observing foraging
itants of coniferousforestsin the Sierra Nevada (Grinnell and Miller 1944). The Downy Woodpecker
behavior during the summers (May to mid-July) of
(Picoidespubescens)and Northern Flicker (Colaptes 1983 to 1985. We tried to collect data for all species
auratus) occurredrarely in our study area and are not at an equal rate throughouteach summer to lessenthe
influence of possible temporal variation in behavior
analyzed herein.
among species. Observers systematically walked
through a compartment recordingdata on birds as en’ Received 23 June 1986. Final acceptance29 Sep- countered. Only one individual of a specieswas obtember 1986.
served at a particular place and time to avoid the po-
is
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FIGURE 1. Use (%) of tree species(upper bar), foragingmodes(middle bar), and foragingsubstrates(lower bar) for birds at BlodgettForest during the summers
of 1983 to 1985.
We defined the following foraging motions: gleanprey taken from the surfaceof the substratewhile the
bird was perched; hover-glean-prey taken from the
surfaceof the substratewhile the bird was flying; sallying (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 forcefully 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 separatecategoryherein.
Data for males and females were combined for this
analysis. Although intersexual differencesin foraging
behavior are known for many woodpeckers(e.g., JSilham 1965, Grubb 1975, Williams 1980) we combined
sexesbecause:(1) we only had marginally enoughdata
for intersexualcomparisonsin two of the woodpecker
species;and (2) we could not usually differentiate sex
in the nonwoodpeckerspecies.Except for the Pileated
Woodpecker, our sample sizesgreatly exceededthose
necessaryfor analysisof avian foragingbehavior (i.e.,
observationson > 35 individuals; Morrison 1984). The
distribution of foragingactivities for useof tree species,
modes, and substrateswere examined separately for
each bird speciesby chi-squareanalysis.All data were
analyzed using SPSSX (SPSS 1983).
RESULTS
tential problem of correlatedactivities of co-occurring
individuals of the same species.
Once encountered,the foraging activity of an individual was recorded for a minimum of 10 set to a
maximum of 30 sec. The observer either timed the
activity period with a stopwatchor by counting; only
birds actively foraging are analyzed herein. The following data were recordedfor eachindividual: species;
sex; type of foraging motion (defined below); species
of plant; substrateto which the foraging motion was
directed; foraging substrate;perch height; and plant
height. Heights and distanceswere visually estimated.
The dominant foragingactivity (e.g.,motion, substrate,
height) observedduring the timed period for each individual was recorded.
In the results that follow, the distribution of foraging
activities for each bird speciesfor use of tree species,
and foraging modes and substrates,were significantly
different (chi-squareanalysis,P c 0.05) except as noted.
Differences were evident in the percent use of different tree speciesby birds; all distributions were significantly different exceptfor the Pileated Woodpecker,
which used all tree species with roughly equal frequency. All speciesexcept the White-headed Woodpeckerand (to a lesserextent) the Brown Creeper demonstrated high use of white fir, and all speciesexcept
the Red-breasted Sapsuckershowed high use of ponderosapine (Fig. 1). Relative to the other species,the
Pileated Woodpecker occurred more on oak, and the
TABLE 1. Foragingheight and dbh of foragingtreesfor 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.
Species
Red-breasted Sapsucker
Hairy Woodpecker
White-headed Woodpecker
Pileated Woodpecker
Red-breasted Nuthatch
Brown Creeper
aSamplesizes= numberof individuals observed.
??a
91
89
116
48
126
123
Foragingheight (m)
R
11.5
10.6
11.6
17.4
13.9
6.7
SD
6.63C
6.68C
6.59C
8.99A
6.49B
4.46D
Foragingtree dbh (cm)
+
SD
41.8
44.5
58.9
90.0
47.7
47.9
23.55C
24.3OC
27.56B
42.72A
22.63C
25.64C
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203
TABLE 2. Vigor of foragingtrees and foraging substratesused by birds at Blodgett Forest during the summers
of 1983 to 1985. Sample sizes in Table 1.
Foraging tree (%)
Species
Red-breasted Sapsucker
Hairy Woodpecker
White-headed Woodpecker
Pileated Woodpecker
Red-breastedNuthatch
Brown Creeper
Foraging substrate (%)
Alive
Dead
Alive
Dead
16.4
46.6
81.1
55.3
82.8
86.2
23.6
53.4
18.9
44.1
11.2
13.8
72.0
32.0
61.0
21.5
65.0
18.3
28.0
68.0
39.0
72.5
35.0
21.7
White-headed Woodpecker and creeper were unique
in exhibiting a greater-useof incensecedar. In addition,
the White-headed Woodvecker had a higher use of
sugarpine than the other-species.
Only the sapsuckerand nuthatch were observedflycatching. The creeper foraged primarily by gleaning
and probing. The nuthatch gleaned extensively, but
spentroughly equal time probing and pecking.In contrast to the other woodpeckers, the sapsuckerspent
similar amounts of time pecking and gleaning. The
other woodpeckerspecked and probed >70% of the
time (Fig. 1).
All speciesexceptthe nuthatchconcentratedforaging
activities on trunks (Fig. 1). The nuthatch used a wide
variety of substrates,including twigs and small-sized
branches.Although concentratingactivities on trunks,
the Pileated Woodpecker also showed substantialuse
of medium-sized branches(Fig. 1).
Pileated Woodpeckers foraged significantly higher
than all other species,the nuthatchforagedhigher than
all remaining species,and the creeper foraged significantly lower than all other species(Table 1). The remaining three species, the White-headed and Hairy
woodpeckers,and Red-breasted Sapsucker,had mean
foragingheights of about 10 to 11 m.
The Pileated Woodpecker used significantly larger
(by diameter at breast height; dbh) trees than all other
species;the White-headedWoodpeckerusedlargertrees
than the remaining species(Table 1). The averagedbh
of foraging trees used by the other specieswas similar
(about 42 to 48 cm dbh).
The sapsucker, White-headed Woodpecker, nuthatch, and creeper spent the majority of their time
foragingon the live parts of trees (Table 2). The Hairy
and Pileated woodpeckers foraged in live and dead
trees with similar frequency, but about 70% of their
foragingtime was on dead substrates(Table 2).
DISCUSSION
Our results indicated that the birds we studied used
different combinations of the foraging behaviors examined. All speciesexcept the sapsuckershowedhigh
use of ponderosa pine; high use of white fir was observed for all except the creeper and White-headed
Woodpecker;the white-headed and creeperhad higher
use of incense cedar, and the white-headed also used
more sugarpine than the other species.Where overlap
occurredin tree speciesuse (ponderosapine and white
fir), the bird speciesuseddifferent foragingmodesand/
or other tree speciesto a greater extent (e.g., compare
the Hairy and White-headed woodpeckers). Differences in foraging height, type of substrateused, and
tree and substratevigor also differed among species.
For example,althoughthe creeperand nuthatchgleaned
extensively (in contrast to most woodpeckers), they
foraged at different averageheights, and the nuthatch
used a wider range of foraging substrates.The sapsuckerdiffered from the other woodpecker speciesin
severalways,most notably in its higher useof gleaning.
The use of living substratesby the sapsucker,in contrast to the predominate use of dead wood by other
woodpeckers,is likely a reflection of the use of sap as
a food sourceby the sapsucker(see Vemer and Boss
1980).
A changein the speciesor size composition of the
forest would likely alter the pattern of resourceuse by
the birds we observed. Such changesare, in fact, underway in the Sierra Nevada due to silvicultural activities (see Morrison et al. 1985). For example, external
bark structurechangeswith tree diameter (e.g., usually
becomingthicker and more deeply furrowed with age),
which affectsprey abundanceand the ability of birds
to obtain the prey (Jackson1979, also Morrison et al.
1985). The current trend in the western Sierra Nevada
is conversionfrom mixed-conifer to monotypic stands
of ponderosapine, or mixed standsof ponderosapine
and lesseramounts of Douglas-fir and fir (R. C. Heald,
Manager, Blodgett Forest, pers. comm.). Precommercial- and commercial-thinning activities also remove
much of the understory of small, commercially undesirable species or less-vigorous individuals. Althoughmost speciesin our study usedponderosapine,
they also showedhigh useof white fir. Further, several
speciesexhibited high use of incense cedar. Although
we would predict that conversion to ponderosa pine
would probably not eliminate any specieswe studied,
sucha changewould likely alter the abundanceof the
birds. The ramifications of such changeson interactions within the bird community, and within the forest
ecosystemitself (e.g., bird-insect interactions),are unknown. Our results, as well as those of Airola and
Barrett (1985) for foliage insectivores, indicate that
efforts should be made to maintain a high diversity of
tree species and size classesthroughout the mixedconifer zone of the Sierra Nevada.
This paperelucidatesforagingbehaviorsusedby bark
foragersduring springand summer. In a related study,
Morrison et al. (1986) found a significant increase in
the use of incensecedar by many birds during winter.
This changewas related to prey availability and was
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accompaniedby changesin foragingmode. Therefore,
biologistsand resourcemanagersshouldview the present paper in light of suchseasonalchangesin behavior.
Unfortunately, we did not measure prey availability
during the breedingperiod, a shortcomingthat should
be eliminated in future studies.Detailed analysisof the
surfacearea of bark available to birds, including limbs
and twigs, should also be quantified in the future.
We appreciate field assistanceduring parts of this
study provided by P. N. Manley, S. A. Laymon, R. E.
Etemad, and J. M. Anderson. R. C. Heald and staff at
Blodgett Forest are thanked for logistical support. J.
Vemer, C. J. Ralph, R. T. Holmes, and an anonymous
referee gave very useful comments on an earlier draft
of this manuscript. Comments made on another but
related manuscriptgiven by R. L. Hutto, F. C. James,
and T. W. Schoenerplayed a substantialrole in evaluation of the resultsreported herein.
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