FLOCK
FORMATION
TO CYCLICAL
OF TWO PARIDS IN RELATION
SEED PRODUCTION
IN A
PINYON-JUNIPER
WOODLAND
KIMBERLY A. WITH L3 AND MICHAEL L. MORRISON 2
•Departmentof Biological
Sciences,
NorthernArizonaUniversity,
Flagstaff,
Arizona86011USA,and
2Department
of ForestryandResource
Management,
Universityof California,
Berkeley,California94720USA
AI•STRACT.--Cyclical
productionof seedsin a pinyon-juniper(Pinusedulis-Juniperus
monosperma) woodland enabledus to assessthe effectsof resourceabundanceon the winter foraging
ecologyand flock formation of two avian seed predators,the Mountain Chickadee(Parus
gambeli)and the Plain Titmouse(P. inornatus).During the winter of low seed abundance
(1986/1987), chickadeesand titmice convergedin microhabitatuse. Chickadeesexhibited a
coarse-grainedresponseby selectivelyforaging in areaswith greater ponderosapine (Pinus
ponderosa)
densityduring the winter of high seedproduction,but shiftedto a fine-grained
useof microhabitatduring the following yearby randomlyforagingin differenttree species.
Titmicedisplayeda fine-grainedresponsein microhabitatuseirrespectiveof seedabundance,
but foraged significantlymore in ponderosapine during the winter of low seedproduction
(becomingmore like chickadeesin use of tree species).Conversely,foraging behavior was
consistentbetween years,and the two specieswere separablebasedupon the use of juniper
substrates(e.g. chickadeesforagedmore on juniper needlesthan titmice,titmice foragedmore
on the ground beneathjuniper).
The convergencein microhabitatuseby chickadeesand titmice during the winter of low
seedabundancemay be attributedto the prevalenceof mixed-species
flocks.During the year
of a mastseedcrop, chickadeesand titmice foragedsingly or in pairs 80%of the time and
were never observedtogether. Half of all individuals were observedin flocksthe following
year,and two thirds of flockingchickadeesand nearly all (88%)gregarioustitmiceparticipated
in mixed flocks.Monospecificflocksof chickadeesused lessjuniper and foraged distinctly
from titmice(e.g.probedmore,peckedless,gleanedfrom an inverted position),but converged
in these characteristics
in the presenceof titmice. Only two titmice ever occurredin mixedspeciesflocks,yet up to six chickadeesformed thesemixed-speciesflocks.Chickadeesapparentlyjoin titmice,assubstantiated
by the observedshiftsin foragingecologyby chickadees
in the presenceof titmice, and perhapsgain knowledgeof resourcelocationsfrom resident
titmice (chickadeesare potentiallyaltitudinal migrantswithin our studyarea).We observed
an increasein socialityduring periodsof low seedabundance,which supportsthe proximate
role of resourcelevels in promoting flock formation, but does not preclude the possibility
that other factorsthat are a consequenceof low resourceabundance(e.g. decreasedtime
available for vigilance) provide the primary impetus for flocking behavior. Received18 July
1989,accepted
18 January1990.
PINYON-JUNIPERwoodlands exhibit marked
cyclesin the production of seedsand berries.
Massproductionof seedsoccursevery 5-6 years
in pinyon pine (Pinusedulis)and every 2-3 years
in juniper (luniperusmonosperma),
such that in
some years no seedsor berries are produced
(Baldaand Masters1980). This local synchrony
in seed production may be related to escaping
depletion by overwhelming birds and mammals that feed upon seeds("flooding the system" [e.g. Balda 1987]).
Two avian seedpredatorsthat winter in the
pinyon-juniperwoodlandsof northernArizona
are the Mountain Chickadee(Parusgambeli)and
the Plain Titmouse (P. inornatus)(Shrout 1977,
Balda and Masters 1980). Given the extreme an-
nual variation in resourceproduction,we were
interested in the responsesof these two seed
3Presentaddress:Departmentof Biology,Colorado predators in terms of flock formation and forState University, Fort Collins, Colorado 80523 USA.
aging ecology(microhabitatuse, foraging be522
The Auk 107:522-532.July 1990
July1990]
Flocking
Behavior
ofTwoParids
havior) during a winter (1985/1986) with a mast
crop of both pinyon seedsand juniper berries,
and during a winter (1986/1987) with almost
no seedand berry production. Specifically,we
examined the flocking behavior of chickadees
and titmice in relation to seed production during each winter, to determine whether differencesin foragingecologyexistedbetweenthese
congeners,and to ascertainwhether foraging
ecologychangedbetween winters in response
523
tributed to the increased advantage of heterospecificover monospecificflocksbecausetotal
scanningrange of the flock could be increased
becauseof the presenceof different speciesthat
foraged in different locations.
Thus, we made severalpredictionsin regard
to flock formation and foraging ecology of
Mountain
Chickadees
and Plain
Titmice
in re-
sponseto changesin annual productivity of
seeds:(1) chickadees and titmice will alter their
foraging ecologyin responseto differencesin
If one subscribesto the "increasedforaging food abundance between winters; (2) both
efficiency" hypothesisas the primary conse- specieswill exhibit increasedsociality during
to differences
in food abundance.
quence of flock formation (Krebs et al. 1972;
Caraco1979a,b), then it is expectedthat Moun-
the winter of low seed abundance; and (3) con-
sequently,each specieswould be expectedto
divergein foragingcharacteristics,
particularly
flocksduring the winter of low seedabundance when participating in mixed-speciesflocks as
to increasethe likelihood of locatingscarceand comparedwith when foraging in monospecific
patchily distributed resources.Formation of flocksor alone.Alternatively,if socialityfacilflockswill bring individuals and speciesinto itatesresourceacquisition,then convergencein
closercontact,and competitiveinteractionsmay foraging ecologywould be expectedof species
counterpotential benefitsdue to increasedfor- in mixed-speciesflocks. It should be possible,
aging efficiency.Speciesthat flock during times therefore, to distinguishbetween the contrastof low resourcelevels ("ecologicalcrunches," ing predictionsof the "competition"hypothesensuWiens 1977)therefore should foragemore sis and "social facilitation" hypothesisregardflock formation.
distinctly in mixed-speciesflocks than when ing mixed-species
flockingwith conspecifics.
Specieswithin flocks
METHODS
of oakwoodlandbirdsin Arizona were spatially
segregatedin microhabitat use. For example,
tain
Chickadees
and
Plain
Titmice
will
form
Bridled Titmice (P. wollweberi)altered their foraging ecology (foraging stance,substrateuse,
and position in canopy)when in the presence
of Bushtits (Psaltriparusminimus)(Austin and
Smith 1972). Willow Tits (Parus montanus)
avoided trees in which Crested Tits (P. cristatus)
foraged, and shifted their position within the
canopy of the tree in the presence of either
Crestedor Great (P. major)tits (Alatalo 1981).
Conversely,socialfacilitationor "copyingbehavior" predictsincreasedsimilaritiesbetween
specieswhen foraging in mixed flocks. Morse
(1978) observedseveralunambiguouscasesof
copying behavior in Blue Tits (P. caeruleus)
attracted to sites previously or concurrently occupied by other foraging individuals (conspecifics as well as other species).Krebs (1973)
experimented with mixed-species flocks of
Black-capped(P. atricapillus)and Chestnutbacked(P. rufescens)
chickadeesin an aviary to
demonstrate
thatflockmembers(of bothspecies)
convergedin foraging behavior in responseto
successfulindividuals (of either species).This
led Krebsto proposethat sociallearningplayed
an important role in flock behavior and con-
Our study was conductedduring the winters (No-
vember-March) of 1985/1986 and 1986/1987 in a 20-
ha areaca.26 km north of Flagstaff,CoconinoCounty,
Arizona. Vegetation on the study area was representative of a transitionbetweenpinyon-juniperwoodlandscharacteristic
of lower elevations(1,670m; upper sonoran life zone) and ponderosa pine (Pinus
ponderosa)
forestscharacteristicof higher elevations
(2,100 m; transition life zone) in the southwest.
MICROHABITAT
ABUNDANCE
We quantifiedvegetationabundanceand structure
(microhabitat)to characterizethe habitat and to compare bird speciesuse relative to vegetation within
this pinyon-juniper habitat. Thirty-five 20-m-radius
(0.13 ha) plots were established50 m apart from an
arbitrary starting point in the study area in 1987. In
each plot we counted all trees <2 m, 2-7 m, and >7
m in height(estimated
visually).Thesecategories
were
selectedto separateyoung (reproductivelyimmature)
trees(< 2 m) and pinyon and juniper trees(generally
<7 m in height) from ponderosapines.A 30-m transectbisectedeach plot in a randomly selecteddirection from which we estimatedthe height of live foliage that intercepted an imaginary vertical line
extendingup from the groundat 3-m intervalsalong
the transect.Height categoriesfor foliage cover were
524
WITHANDMORRISON
0-1 m, 1.1-3 m, 3.1-5 m, 5.1-7 m, 7.1-10 re, and >10
m. Variables related to undergrowth were not recordedbecauseshrubswere rare, and grassand forb
[Auk,Vol.107
represent increasinglevels of successby the model
in distinguishingbetween the groups.A high percentageof species-centered
points(microhabitatuse)
coverwassparse
or coveredby snowduringourstudy. classifiedincorrectlyas randomly centeredpoints
(microhabitat abundance) would indicate that the
speciesis using microhabitatin proportion to abundance(fine-grainedresponse).
Theconceptof "grain"
The locationof foraging birds (explainedbelow) isusedtodescribespeciesresponses
toenvironmental
servedas the centerof a 20-m-radiusplot. Microhab- heterogeneityto provide an understandingof how
itat usewasquantifiedasdescribedfor quantification different species"perceive"their environment (AdMICROHABITAT
of microhabitat
USE
abundance.
dicott et al. 1987).
Comparisons
betweenspeciesand years.--Two-tailed
t-tests were used to evaluate differences
in microhab-
FORAGING BEHAVIOR
itat use between specieswithin and between winters
as well as for both years combined(overall species
comparison).The assumptionof homogeneityof variancebetweensampleswasevaluatedby Levene'stest;
few comparisons
departedfrom this assumptionand
a pooled variance estimate subsequentlywas used.
Comparisonswere consideredsignificantlydifferent
if P < 0.05.
We performeddiscriminantanalysiswith forward
stepwiseinclusionof variablesto characterizemicrohabitatuseof the two species.Variableswere entered
initially into the equationif the variable significantly
(P < 0.1,F-test)improvedthe discriminationbetween
groups. We accepteda significancelevel of P < 0.1
for entry of variablesinto discriminantanalysissoas
not to exclude variablesthat were nonsignificant(P
> 0.05) between speciesin the univariate tests(i.e.
t-tests),but that may have provided significantcontributionsin this multivariateanalysisat later steps
in the procedure.Multicollinearitybetweenvariables
was reducedby eliminating one of any pair of variables with
an r-value
of >0.7.
The variable
retained
for possibleinclusion in discriminant analysiswas
the one with the highestF-value for separationbetween groups.
Discriminantfunction equationswere developed
for each of the two winters of study and for both
winters combined.To determinethe temporalvalidity of these equations,we examined the ability of
equations developed from one winter to correctly
classifydata (as to one of the two species)collected
during the other winter. We validated the discriminant functionsfor eachwinter by randomly dividing
the data set for each winter (and the overall data set
for both winters combined),and using this random
subsetto develop the discriminant function and the
remaining data to determine the accuracyof classification. The assumptionunderlying discriminant
analysis of equal variance-covariance matrices between groupswas evaluatedwith Box's-M statistic.
Useversus
abundance
ofmicrohabitat.--A
three-group
discriminantanalysiswas run to determine whether
each speciesused microhabitatin proportion to microhabitat
abundance.
Because a classification
accu-
racyof 33%is expectedby chancefor eachgroupin
a three-group
discriminant
analysis,
percentages
> 33%
Foragingactivity increasedafter 0800, peaked between 1000-1300 (85% of all observations),and de-
creasedsharply after 1400. Admittedly, this pattern
of diurnal variationin foragingintensitywasamplified further within our data set by the concentration
of our efforts during the periods of the day when
birdsapparently were mostactive. We systematically
traversedthe study area, and when a foraging bird
was encountered, we waited 10 s (to reduce bias due
to observer disturbance
and towards individuals
for-
aging in conspicuouslocations) and then recorded
activityfor 10-60 s.To describethe foragingactivities
and locationsof thesetwo species,we recordedforaging substrate(where foraging motions were directed;e.g. twig, smallbranch,mediumbranch,large
branch, and trunk); foraging activity (e.g. glean,
probe);distance(m) movedby flying or hovering;
distance(m) moved by hopping, using wings only
for balance;verticalforagingheight (m); tree height
(m); and horizontallocationof bird in canopymeasured as percent distancefrom tree bole (all heights
and distanceswere visually estimated as we were
trained to make visual estimates;Block et al. 1987).
Data for substrateuseand foragingmode were converted to percent use for each individual to standardize observationperiodsof different lengths.
Once we recorded data on an individual, the bird
was not followed. We recordeddata on only one individualper flockto minimizebiasdue to potentially
correlatedactivities of flocking individuals. The observer resumed traversing the study area in the direction opposite to that in which the bird or flock
disappeared.This minimized encounteringthe same
individual or flockagain.Encounterratesof birdsby
observerswere calculatedfor each species,and statisticalcomparisons
(t-tests;comparisons
significantly
different if P < 0.05)were madebetweenspeciesfor
eachwinter and betweenwintersfor eachspeciesto
assess the distribution
and abundance
of individuals
(or flocks,becausedata on only one individual per
flockwere recorded)throughoutthe studyarea.
Comparisons
betweenspecies
and years.--Differences
between speciesin foraging behavior within each
winter and for bothwinterscombined(overallspecies
comparison)were evaluated using two-tailed t-tests
July1990]
Flocking
Behavior
ofTwoVarids
525
as for analysisof microhabitatuse.Similarly, differencesbetween yearsin foraging behavior for each
specieswasassessed
with a two-tailed t-test.Discriminant analysiswasemployedto characterizeforaging
behavior of the specieswith the same criteria and
validation proceduresas describedpreviously for
analysisof microhabitatuse.
FLOCKING BEHAVIOR
When we encountereda foragingbird, we recorded
whether the bird wasforagingsingly,asa pair, or in
a flock (>-3 birds),and the speciescompositionand
number of individuals if foraging in a flock. Percentageof time observedforaging singly, in pairs, or
in monospecificor heterospecific
flockswasassessed
for eachspeciesand betweenwinters.We usedt-tests
to test for differences
in mean
flock size between
speciesand betweenyears(comparisonssignificantly
different if P < 0.05). Differences in microhabitat use
and foragingbehaviorbetweenthe two specieswere
assessed
separatelyfor monospecificand heterospecific flocks using Mann-Whitney tests. To examine
ßwhether chickadeesand titmice foraged differently
in mixed-species
flocksthan when in the presenceof
conspecifics,
the foragingecologyof eachspecieswas
contrastedbetween these different assemblages.
RESULTS
MICROHABITAT
ABUNDANCE
Theunderstorycomprisedmostly(95%)small
(<2 m) junipersand pinyons,each of which
occurredin densitiesof approximately86 stems/
ha (Appendix 1). The study area was predominated (63%)by trees2-7 m in height. Pinyon
pine (45%,163stems/ha)andjuniper (40%,146
stems/ha) occurredin roughly equal numbers.
Ponderosapine represented13%(72 stems/ha)
of this woodland overall, and 83% of the trees
>7-m tall were ponderosapines(10 stems/ha).
Live foliagewasconcentratedmainly (63%)below 3 m.
MICROHABITAT
USE
Comparisons
betweenspecies
andyears.--Mountain Chickadeesforagedin areasof significantly
(P < 0.05) denser ponderosapine than Plain
Titmice, and in areaswith greater foliage cover
between 3 and 7 m in height during winter
1985/1986 (Appendix 1). During winter 1986/
1987, chickadeesused significantly denserju-
niper in the midcanopy(2-7 m) than titmice.
Chickadeesdifferedbetweenyearsonly by for-
eqV
V
526
WrrHANDMOP,
RISON
[Auk,Vol. 107
TABLE2. Percentsuccess,as classifiedfrom discriminant analysisof Mountain Chickadees(MOCH) and Plain
Titmice (PLTI) microhabitatuse, and use of microhabitatvs. abundance,during winters 1985/1986 and
1986/1987 in northern Arizona. Values in parenthesesare resultsof subgroupclassification(validation)
usinga randomsubsetof 50%of that year'sobservations;
NA = not applicable.
Predictedgroup
Winter
Year / actual
group
1985/1986
Winter 1986/1987
MOCH
PLTI
Abundance
69 (82)
32 (53)
3 (0)
72 (91)
23 (53)
22 (0)
26 (12)
11 (6)
9 (18)
42 (35)
86 (94)
MOCH
PLTI
Abundance
1985/1986
MOCH
PLTI
Abundance
MOCH
PLTI
NA
28 (9)
77 (47)
37
48
63
52
23 (36)
28 (47)
11 (50)
56 (64)
81 (73)
40 (0)
52 (13)
3 (0)
44 (36)
19 (27)
1986/1987
MOCH
PLTI
Abundance
MOCH
PLTI
NA
70
24
30
76
37 (64)
21 (40)
86 (50)
aging in areaswith lessdensefoliage coverbe-
tain Chickadeeswere more specificin their use
tween 3 and 5 m during the second winter.
of microhabitatthan Plain Titmice during the
Titmicesignificantlyreduceduseof juniper (but
juniper was still used in proportion to its abundance) and increaseduse of 2- to 7-m tall ponderosapines between years.
Discriminant analysis identified four variables that significantly separatedmicrohabitat
usebetween speciesduring the first winter (Table 1); three variablesrelated to use of ponderosapine and areaswith foliage cover between
3 and 7 m in height were previously identified
by univariate tests and were discussedabove.
The discriminant function classifiedcorrectly
first winter.
The discriminant
function
distin-
guished between chickadee microhabitat use
and microhabitat
abundance data, but was un-
able to successfullyclassifytitmice (Table 2).
Validation proceduresusing a subsetof the data
from
the first winter
resulted
in about
three
quarters of the chickadees (70%) and titmice
(76%)being identified correctly.
ca. 75% of individuals based upon microhabitat
use data from the first winter (Table 2). Vali-
During the second winter, the discriminant
function was less successfulin discriminating
the speciesand randomly centered points, although microhabitat abundancedata remained
a distinct group (86% classificationsuccess;
Table 2). These relationships were considerably
dation of the discriminantfunction usinga sub-
weakened
set of the data resulted
validate the model as speciesmicrohabitat use
in 50% of the titmice
being incorrectlyclassifiedas chickadeeswith
respectto microhabitatuse.The model derived
from the secondwinter and applied to habitat
use data from the first winter correctly identified •nany (70-76%) of the chickadeesand tit-
and
when
microhabitat
a subset of data was used to
abundance
data
became
in-
separable.
FORAGING BEHAVIOR
mice.
Comparisons
betweenspecies
andyears.--We enDiscriminantanalysisseparatedspeciesby use countered individuals of either speciesat 35of 2- to 7-m tall juniper during the secondwin- min intervals (SD = 18.9) over both winters
ter (Table 1). Classificationwas poor, however, (1985/1986:27 = 34 + 18.9 min; 1986/1987:27 =
as 81% of titmice and 44% of chickadees were
36 + 19.0 min; P > 0.05, t-test). No significant
misclassified.
The discriminant
function
from
differences
in encounter
rates were
found
be-
the first year misclassified60% of chickadees tween species for either winter (1985/1986:
Mountain Chickadee 27= 36 + 28.2 min, Plain
and approximately50% of titmice (Table 2).
Use versusabundance
of microhabitat.--Moun- Titmouse 27= 41 + 14.7 min; 1986/1987: Moun-
July1990]
Flocking
Behavior
of TwoVarids
527
TABLE3. Discriminantanalysisof Mountain Chickadeeand Plain Titmouseforagingbehaviorduring two
winters in northernArizona.Variablesretainedfor discriminantanalysisresultedfrom the stepwiseinclusionof variablesthat significantly(P < 0.1, F-test)improvedthe discriminationbetweengroups(see
text for details).
Winter 1985/1986
Eigenvalue
Canonical
correlation
Chi-square (df)
1.229
0.743
19.328 (5)
P
Variables • and correlation
Winter 1986/1987
0.383
0.527
47.702 (9)
0.002
JUTW
JUSE
JUHOP
JUNE
JUSBMB
Box's-M (P)
<0.001
0.56
0.49
0.43
-0.36
0.33
JUSE
JUTW
JUGR
PPFLY
JUSBMB
<0.001
0.38
0.32
0.30
-0.26
0.25
<0.05
"Abbreviationsfor variables:Juniper(JU) twigs (JUTW),search(JUSE),hoppingrate (JUHOP),needles(JUNE),small and mediumbranches
(JUSBMB),groundbeneathtree (JUGR),foragingrate usingwingsin ponderosa
pine (PPFLY);seealsoAppendix2 for full descriptions
of
variables.
tain Chickadee 5' = 53 + 34.1 min, Plain Titmouse 5, = 40 + 16.1 min). Neither were there
validated on a subset of the data (Table 4). The
discriminant
function
from
the second
winter
significant differences between winters for correctlyclassifiedmost(91%)of the titmice but
either species,despitethe fact that chickadees only approximately50%of the chickadeesfrom
were encounteredless frequently (ca. 17 min the first winter. Three of the five variables of
more between encounters)during the second the discriminantanalysisrelatedto juniper subwinter.
strateswere the samebetween years (Table 3).
During the first winter, chickadeesdiffered The discriminant function for the second winsignificantly from titmice in substate use. ter separated most chickadeesand titmice (91%
Chickadees
foragedmoreuponponderosa
twigs and 82%)into groupsbasedupon their foraging
of the outer canopyand lesson juniper twigs behavior; this categorizationheld after valida(where titmice foraged 44% of the time, but tion procedureswith half of the data set (Table
which fell significantlyto 18%during the sec- 4). The model from the first winter correctly
ond winter; Appendix 2). Chickadeesforaged identified almost all (97%) chickadees,but corsignificantlymoreon juniper needlesthan did rectly identified only approximately50%of titmice.
titmice, and chickadeesgleaned from an inverted position, a behavior not employed by
titmice. Chickadeesused juniper needlessig- FLOCKING BEHAVIOR
nificantly lessduring the secondwinter. ForMountain
Chickadees and Plain Titmice foraging behavior differed between thesespecies
during the second winter becausetitmice foraged singly (78%) or in pairs (85%) during the
aged in significantlysmaller juniper and pin- first winter (Table 5). Mean flock size did not
yon thandid chickadees,
andtitmicespentmore differ significantlybetween species(chickadee:
(22% vs. 4%) time on the ground. Overall, tit5,= 1.91 + 0.995 [SD], n = 32; titmouse: œ= 1.91
mice spent significantly more time in and + 1.04, n = 33). The two specieswere never
around juniper than chickadeesdid, and chick- observedtogether, although a chickadee foradeesusually foraged more on needlesand in aged with 1-2 Golden-crownedKinglets (Regtaller trees.
ulus satrapa)on three occasions.During the
Discriminant analysescorroboratedthe find- secondwinter, both speciesoccurred in flocks
ings of the univariate tests.Foragingbehavior (three or more birds) in ca. 60% of all obserwas separablebetween chickadeesand titmice vations.They occurredsingly or in pairsin only
during the first winter by variablesrelated to
36% of the observations
juniper substratesand movement within juniper (Table 3). Classificationsuccesswas high
for titmice (Table 5). About half of all foraging
for chickadees
and 38%
(chickadees= 77%, titmice = 76%), but was little
individuals (47.2% of chickadees and 42.5% of
titmice) were observed in mixed flocks. Further,
better than 50% for titmice when the model was
heterospecificflockswere more prevalentthan
528
WITHANDMORRISON
TABLE 4.
Percent success, as classified from discrim-
TABLE5. Monospecificflock formation during two
winters in a pinyon-juniper woodland of northern
Arizona. Percentagesrepresent frequency of foraging individuals observedeither singly, in pairs,
inant analysisof the foraging ecologyof Mountain
Chickadees (MOCH) and Plain Titmice (PLTI) during two winters in northern Arizona. Values in
parenthesesare results of subgroup classification
or •n monospecific flocks.
(validation) using a random subsetof 50% of that
year's data.
Mountain
Chickadees
Predicted group
Year/ Winter
1985/1986Winter
1986/1987
actual
group
MOCH
PLTI
MOCH
PLTI
Winter 1985/1986
MOCH
PLTI
77 (75)
24 (50)
23 (25)
76 (50)
97
46
3
54
55
91
91 (69)
18 (31)
9 (31)
82 (69)
Winter 1986/1987
MOCH
PLTI
45
9
Plain
Titmice
Flock
1985/
1986/
1985/
1986/
size
1986 a
1987 b
1986 a
1987 b
1
2
3
4
5
6
40.6
37.5
9.4
3.0
3.0
0.0
8.3
27.8
8.3
5.6
2.8
0.0
36.4
48.5
9.0
3.0
0.0
3.0
7.5
30.0
5.0
0.0
2.5
0.0
Sample sizesare 32 observationsfor chickadeesand 33 for titmice..
Sample sizes are 36 observationsfor chickadeesand 40 for titmice.
monospecificflocks.Nearly two thirds (74%) of
flocking chickadeesand nearly all (88%) gregarious titmice participated in mixed-species
flocks. The mixed flocks generally (60%) involved
[Auk,Vol. 107
one or two titmice and one or two chick-
adees; Red-breasted Nuthatches (Sitta canaden-
sis) and Brown Creepers (Certhia americana)
participatedon a few occasions(ca. 10%of all
observations). Mean flock size did not differ
between species(chickadee:• = 3.33 + 1.67
[SD], n = 36 individuals; titmouse: œ= 3.28 +
1.65, n = 40), but flock sizeswere significantly
different between winters for both species(P <
0.001, t-test).
coarse-grained
responsein microhabitatuseby
selectivelyforaging in areaswith greater density of ponderosa pine during the winter of
mastseedabundance,yet they shiftedto a finegrained use by randomly foraging in different
tree speciesduring the following winter. Titmice exhibited a fine-grained responsein microhabitatuse irrespectiveof seed abundance.
Yet titmice significantlyincreaseduse of ponderosapine during the secondwinter and became more like chickadeesin vegetation use.
Chickadees,like titmice, foraged in the lower
canopyof junipers during the secondyear, and
the discriminant function developed from the
Monospecificflocks of chickadeesused sig- first winter misclassified most (two thirds) of
astitmice.Thissupportsour connificantlylessjuniper and foragedin a manner the chickadees
distinctivefrom titmice (e.g. chickadeesprobed
tention that speciesconvergencein microhab-
more, pecked less,and gleaned from an in- itat use (especiallyby chickadees)occurredin
vertedposition;Table6). A convergence
in these a winter of low resource abundance.
Shifts in foraging ecology in responseto
characteristicswas observedbetween speciesin
heterospecificflocks. Chickadeessignificantly changesin productivityare not unprecedented.
altered their foragingbehaviorin the presence Specifically, Wagner (1981) found that Plain
of titmice (Table 6). Titmice foraged in signif-
icantlytaller treeswhen in heterospecific
flocks
than when in monospecificflocks, although
monospecificflocksof either specieswere not
significantlydifferent in selectionof tree stat-
Titmice significantlyshifted tree-speciesuseand
foraged lower one winter relative to the other.
Differencesin the foragingecologiesof several
parids between winters also have been related
to resourceabundancein an alpine larch forest
(Laurent 1986). Production of larch seedsvaried
ure.
betweenwinters and the primary seedconsumer, the Willow
DISCUSSION
Tit, increased use of other sub-
During the winter of low seedabundancein
the pinyon-juniper woodland, Mountain
strates(e.g. pine cones,branches,and ground)
during yearsof low larch seedabundance.Microhabitatuse(e.g.useof larch twigs or needles)
Chickadeesand Plain Titmice convergedin mi-
also differed
crohabitat
other speciesand was correlatedwith the abun-
use. Chickadees
had
exhibited
a
between
winters
for a number
of
July1990]
Flocking
Behavior
of TwoPan'ds
529
TABLE6. Foraging ecology of Mountain Chickadees(MOCH) and Plain Titmice (PLTI) in monospecificand
heterospecificflocksduring winter 1986/1897 in northern Arizona. Microhabitatuseand foraging behavior
(œ+ SD) are compared between specieswithin each assemblage(interspeciescomparisons),as well as
between monospecificand heterospecificflocksfor eachspecies(interflock comparisons;
Pc:chickadees,P•:
titmice). Sample sizesare 17 chickadeeand 19 titmouse monospecificflocks,and 13 chickadeesand 7 titmice
observedin heterospecificflocks.
Monospecificflocks.
Variable
MOCH
Heterospecificflocks
PLTI
MOCH
Microhabitat
Tree species(%)
Ponderosa pine
Pinyon pine
Juniper
Tree height (m)
Foragingheight (m)
Rel. foraging ht.b (%)
30.5 + 46.46
48.3 + 46.68
14.9 + 34.21
PLTI
Pc
28.6 + 48.80
14.3 + 37.80
57.1 + 53.45
NS
NS
NS
NS
NS
NS
use
11.2 + 28.01
22.8 + 32.96
64.3 + 40.75***
18.5 + 37.85
40.7 + 45.47
33.6 + 43.65
6.0 + 3.94
4.1 + 2.52
4.3 + 0.75
5.4 + 2.50
NS
0.03
3.3 + 2.59
57.4 + 22.86
1.9 + 1.42
48.2 + 27.20
2.4 + 1.39
61.0 + 23.07
2.1 + 1.54
40.0 + 29.17
NS
NS
NS
NS
7.0
32.5
22.4
28.5
20.4
21.4
21.1
14.4
31.64
38.43
37.04
31.86
5.9
28.4
29.4
36.3
+
+
+
+
15.57
46.60
38.85
46.94
NS
NS
NS
NS
NS
NS
NS
NS
+ 22.22
+ 31.21
+ 32.63
_+ 00.00
+ 38.13
16.8
00.0
37.7
1.0
52.89
+
+
+
+
+
26.97
00.00
47.70
2.65
43.94
NS
0.01
NS
0.04
NS
NS
NS
NS
NS
NS
NS
NS
Substrate use (%)
Needles
10.7 + 27.33
Twigs
49.5 + 45.39
Small branches
Ground
12.9 + 29.10
9.0 + 22.20
+
+
+
+
23.76
43.16
28.60
34.95
+
+
+
+
Foraging behavior
Foragingactivity (%)
Probe
Peck
Glean
Invert glean
Search
Foragingrate (perches/s)
24.2 + 36.25
00.0 + 00.00
25.9 + 33.94
39.5 + 41.63
5.6
15.1
19.7
00.0
59.6
0.29 + 0.145
0.37 + 0.356
3.8 + 7.14
+
+
+
+
+
18.20'
32.92*
32.01
00.00'*
41.99
8.4
16.2
21.4
00.0
35.58
0.28 + 0.241
0.28 + 0.204
Significancelevels: * = P < 0.05, ** = P < 0.01, *** = P < 0.001.
% tree height/bird height.
dance of arthropod prey (e.g. caterpillars) on
these substrates.
The foraging behavior of chickadeesand titmice appearedto be consistentbetween years.
The modelsdevelopedfor eachof the two winters are similar, and speciesare separablebased
upon their use of juniper substrates.Morpho-
chickadeesand titmice may be attributedto the
prevalenceof heterospecific
flocksduring the
winter
of low seed abundance.
Half
of all in-
dividualswere observedin flocksduring the
secondwinter (as opposedto none during the
previousyear of a mastseedcrop),and nearly
two thirdsof flockingchickadeesand almostall
logical constraintsimposedby body size and
bill shapemay restrictforagingbehavior,and
thus microhabitatuse may represent a more
gregarious titmice were members of mixed
plastic responseto variations in resourceabundanceand distributionasevidencedby the shift
from specificto random use of tree speciesbetween winters by Mountain Chickadees. In-
foraging behavior in the presenceof titmice.
deed, Alatalo (1982) found an inverse relation-
shipbetweenbodysizeandversatilityof feeding
postures(a measurerelated to both foraging
technique and use of vegetation structure) in
tits during winter, leading him to assertthat
suchversatility enabledbirds to use scarceresourcesduring winter.
The convergence in microhabitat use in
flocks. Further, chickadeessignificantlyincreasedtheir use of juniper and convergedin
This convergencein foraging ecologylends
supportto the socialfacilitationhypothesis.Both
speciesmay extend their visual field in the
search for scarceand patchily distributed resourcesby forming flocks.The preponderance
of mixed flocksimplies a potential advantage
of heterospecific
flocksovermonospecific
flocks.
Mixed-speciesflocksmay generatea composite
searchpattern basedon the assemblageof different species,which leadsto an increasedsearch
efficiency(but see Hutto 1988 for an example
530
WITHANDMORRISON
[Auk, Vol. 107
flocking is primarily to protect against predators, then flock formation should be independent of food abundance(e.g. Berner and Grubb
1985). As Hutto has suggested(pers. comm.),
this assumptiondoes not follow logically be1989). We note that no more than two titmice causethere is ample evidencethat foraging efwere ever observedin a mixed-speciesflock, ficiencyand vigilanceare inversely related (e.g.
whereasup to six chickadeesformed thesehet- Caraco 1979b, 1982; Sullivan 1984). Thus, the
erospecificassemblages.We believe that the two hypothesesare inextricablylinked. If predchickadees join titmice, especially because ator protection served as the ultimate motivachickadeesconverged in foraging ecology in tion for flocking behavior, then formation of
the presenceof titmice. The gain to chickadees flocks would not be independent of resource
from titmice over conspecificsis not clear. Tit- abundancebecausea reduction in vigilance is
a direct consequenceof the increasein search
mice are obligatory to the pinyon-juniper
woodlands of northern Arizona and are per- time for food, necessitatedby scarceresources.
manent residents within this habitat (Balda and Individuals that aggregateto compensatefor
Masters 1980). Mountain Chickadees, in con- reductionin vigilance should disperseonceretrast, occurand breed in a variety of habitatsin sourcelevels surpasssome threshold that enaddition to pinyon-juniper. Given the prox- ablesindividuals to effectively scanfor predaimity of the San FranciscoPeaks (elevation = totswhile foraging.This providesan alternative
3,950 m) to our study area and that chickadees interpretation consistentwith the reduction in
selectivelyforaged in areasof greater ponder- flockingbehavior observedin food supplemenosa pine density within the pinyon-juniper tationexperiments(e.g.Bernerand Grubb 1985).
woodland, and foraged significantly more in Food abundance may provide a proximate
ponderosapine than titmice, we hypothesize mechanismfor flock formation, yet other facthat many of the chickadeeswere altitudinal tors that are a consequenceof reduced resource
migrantsthat breedin the mountainsand win- levels (such as decreasedscan time) may proter in the pinyon-juniper woodlands. These vide the ultimate motivation for flocking. Thus,
chickadeesmight benefit from learning the lo- the occurrenceof flocks we observed during
cationsof seed-bearingtrees (which are scarce periods of low resourcelevels does not necesduring the year of almost no seed production) sarily supportthe food-mediatedhypothesisas
from resident titmice.
the primaryimpetusfor flockformationin birds.
Increasedsocialityof chickadeesand titmice
during the winter of low seedabundancesugACKNOWLEDGMENTS
geststhat increasedsearchefficiencymaybe the
We are indebted to Richard L. Hutto, whose comprimary motivation for their flock formation.
ments
on earlier drafts significantly improved this
Morse (1967) documenteddecreasedflock participationin anotherseedpredator,the Brown- manuscript.We alsothank Leonard A. Brennan, Kimberly A. Sullivan, and an anonymousrefereefor their
headed Nuthatch (Sitta pusilia),in responseto
helpful reviews, and Lori Merkle for manuscript
decreasedabundanceof long-leaf pine seedsin preparation.
Louisiana. Berner and Grubb (1985) supplemented food during the winter in deciduous
LITERATURE CITED
of a potentialdecreasein foragingefficiencyfor
specieswithin mixed flocks). Further, mixed
flocks may accruebenefits from increasedvigilance and decreasedcompetitionand aggression relative to flocksof conspecifics(Metcalfe
woodlands,
which
resulted in reduced soci-
ality in a number of bird speciesthat included
Carolina Chickadees (Parus carolinensis)and
Tufted Titmice (P. bicolor).
The antithesis of the foraging efficiency ex-
ADDICOTT,J. F., J. M. AHO, M. F. ANTOLIN, D. K. PADILLA, J. S. RICHARDSON,& D. A. SOLUK. 1987. Eco-
logical neighborhoods:scaling environmental
patterns. Oikos 49: 340-346.
ALATALO,R. V. 1981. Interspecific competition in
tits Parusspp. and the goldcrestRegulusregulus:
foraging shifts in multispecificflocks.Oikos 37:
planationfor the advantagesof flockformation
is the antipredation hypothesis,whereby increaseddetection of predators(e.g. Siegfried
335-344.
and Underhill 1975) or diffusion of predation
1982. Multidimensional foraging niche orrisk (Hamilton 1971) is the primary impetus to
ganizationof foliage-gleaningbirds in northern
Finland. Ornis Scandinavica
13: 56-71.
flock.The antipredationand foragingefficiency
hypothesesdo not producemutually exclusive AUSTIN,G. T., & E. L. SMITH. 1972. Winter foraging
ecologyof mixedinsectivorous
bird flocksin oak
predictions.The general suppositionis that if
July1990]
woodland
Flocking
Behavior
ofTwoParids
in southern
Arizona.
Condor
74: 17-
531
, M. H. MACROBERTS,
& J. M. CULLEN. 1972.
Flocking and feeding in the Great Tit Parusma-
24.
jor-an experimentalstudy.Ibis 114:507-530.
BALI•A,
R.P. 1987. Avian impactson pinyon-juniper
woodlands. Pp. 525-533 in Proceedings--pinyon-juniperconference.Ogden, Utah, U.S. Dep.
Agric. For. Serv. Gen. Tech. Rep. INT-215.
LAURENT,
J.L. 1986. Winter foraging behaviour and
resourceavailability for a guild of insectivorous
gleaningbirds in a southernalpine larch forest.
Ornis
-, & N. MASTERS. 1980. Avian communities in
Scandinavica
17: 347-355.
the pinyon-juniperwoodland:a descriptiveanalysis.Pp. 146-167 in Managementof western forestsand grasslandsfor nongame birds. Ogden,
Utah, U.S. Dep. Agric. For.Serv.Gen. Tech.Rep.
METCALFE,
N. B. 1989. Flocking preferencesin relation to vigilance benefitsand aggressioncosts
in mixed-species
shorebirdflocks.Oikos 56: 91-
INT-86.
MORsE,
D.H. 1967. Foragingrelationshipsof Brownheaded Nuthatchesand Pine Warblers. Ecology
98.
BERNER,
T. O., & T. C. GRUBB
JR. 1985. An experimentalanalysisof mixed-species
flockingin birds
of deciduouswoodland. Ecology66: 1229-1236.
48: 94-103.
1978. Structure and foraging patterns of
flocksof tits and associatedspeciesin an English
woodland during the winter. Ibis 120: 298-312.
SHROUT,
D.A. 1977. Flocking and foraging behavior
BLOCK,W. M., K. A. WITH, & M. L. MORRISON. 1987.
On measuringbird habitat:influenceof observer
variability and samplesize. Condor89: 241-251.
CARACO,
T. 1979a. Time budgeting and group size:
a theory. Ecology60: 611-617.
1979b. Time budgeting and group size: a
test of theory. Ecology60: 618-627.
ß 1982. Flocksizeand the organizationof behavioral sequencesin the junco.Condor 84: 101-
of winter
birds in three habitats of northern
Ar-
izona. Ph.D. dissertation.Flagstaff,Northern Arizona
Univ.
SIEGFRIED,
W. R., & L. G. UNDERHILL.1975. Flocking
as an anti-predatorstrategyin doves.Anim. Behav. 23: 504-508.
SULLIVAN,
K. A. 1984. The advantagesof socialforaging in Downy Woodpeckers.Anim. Behav.32:
105.
HAMILTON,W. D. 1971. Geometry for the selfish
16-22.
herd. J. Theor. Biol. 31: 295-311.
WAGNER,J. L. 1981. Seasonalchange in guild structure: oak woodland insectivorousbirds. Ecology
HUTTO,R.L. 1988. Foragingpatternssuggesta possible costassociated
with participationin mixedspeciesbird flocks.Oikos 51: 79-83.
KREI•S,J.R.1973. Sociallearningand the significance
of mixed-species
flocksof chickadees
(Parusspp.).
62: 973-981.
WIENS,J. A. 1977. On competitionand variableenvironments.
Am. Sci. 65: 590-597.
Can. J. Zool. 51: 1275-1288.
APPENDIX1. Microhabitat abundanceand use of microhabitatby Mountain Chickadees(MOCH) and Plain
Titmice (PLTI) during two winters in northern Arizona. Samplesizesare in parentheses;
all valuesare œ
_+SD; * = significant(P < 0.05) interspecificcomparison,** = significant(P < 0.01) interspecificcomparisons,and •' = significant(P < 0.05) interyear comparisonfor the species.
1985/1986
Variable
MOCH (32)
I986/1987
Micro-
Overall
habitat
abundance
PLTI (31)
MOCH (30)
PLTI (29)
MOCH (62)
PLTI (60)
20.7 + 9.91
12.4 _+7.44
0.4 _+0.80'
13.4 + 7.41
10.6 + 6.20
0.5 + 1.11
10.4+ 7.06•'
9.3 + 6.58
1.4 + 2.82
15.7 _+11.34
11.5 + 6.98
0.9 + 1.89
15.7 ñ 10.03
10.9 _+7.15
0.9 + 2.09
11.2 + 5.02
11.2 _+4.74
1.1 -+ 1.90
29.6 _+8.27
25.2 + 14.29
3.3 _+4.43**
25.6 -+ 11.29
28.9 + 12.59
6.9 + 7.52
19.4 + 11.19*'J'
31.4 + 17.55
10.4 + 15.12•'
25.4 _+13.53
30.1 _+16.30
7.9 _+8.50
24.7 _+10.98
28.2 _+16.12
6.7 _+11.46
19.0 + 8.28
21.2 _+9.74
6.9 _+7.28
Trees <2-m-tall per 20-m radius
Juniper(JUHT1)
Pinyon (PIHT1)
Ponderosa(PPHT1)
17.8 + 13.85
12.3 ñ 7.64
1.3 _+2.37
Trees2-7-m-tall per 20-m radius
Juniper (JUHT2)
Pinyon (PIHT2)
Ponderosa(PPHT2)
25.2 + 15.52
31.3 + 19.28
8.8 + 9.34
Trees >7-m-tall per 20-m radius
Juniper (JUHT3)
Pinyon (PIHT3)
Ponderosa(PPHT3)
0.1 + 0.18
0.6 + 1.37
3.7 _+8.78
0.1 + 0.18
0.1 + 0.34
1.1 _+1.39
0.0 + 0.00
0.0 + 0.00
0.1 -+ 0.13
0.1 _+ 0.13
0.1 _+ 0.68
0.0 + 0.00
0.0 + 0.00
0.3 + 1.01
0.1 + 0.25
0.1 _+ 0.51
1.2 + 1.29
2.6 + 3.92
2.5 _+ 6.45
1.8 _+ 2.98
1.3 + 1.38
30.3 + 26.19
21.4 + 22.63
28.4 _+ 20.90
26.3 _+ 21.86
35.7 + 21.73
29.0 + 21.71
25.5 + 22.45
31.5 _+ 20.47
27.7 _+ 20.51
26.9 _+ 1.45
17.3 _+19.93
10.7 _+15.82'
6.3 -+ 9.73
6.1 _+ 12.19
3.8 _+ 10.43
2.3 _+ 5.47
Foliageheight categories
0-1 m (HT1)
1.1-3 m (HT2)
3.1-5 m (HT3)
5.1-7 m (HT4)
7.1-10 m (HT5)
>10 m (HT6)
26.6 _+14.50
33.8 + 19.30
24.7 + 19.67
8.8 + 14.54
0.9 + 2.96
0.0 + 0.00
31.0
28.7
9.4
1.9
0.0
0.0
_+20.39
+ 18.75
_+ 12.09'*
_+5.43*
_+0.00
_+0.00
9.3 + 17.21•' 12.1 + 19.16
3.3 + 8.44
5.9 + 13.76
2.7 + 6.40
3.1 + 10.04
1.8 + 4.97
1.5 _+ 7.09
0.0 + 0.00
0.7 + 2.54
0.7 + 3.71
0.3 + 1.78
0.3 -+ 2.58
0.0 _+ 0.00
APPENDIX
2. Foraging ecology (œ+ SD) of Mountain Chickadees(MOCH) and Plain Titmouse (PLTI) during
two winters in a pinyon-juniper woodland of northern Arizona. Asterisksdenote significantinterspecific
comparisons:
* = P < 0.05,** = P < 0.01,*** = P < 0.001;•' = significant(P < 0.05)interyearcomparison
for the species.
Winter 1985/1986
Variable
MOCH
PLTI
Winter 1986/1987
MOCH
Overall
PLTI
MOCH
PLTI
Tree height (m)
Juniper
4.4 + 0.68
3.9 ñ 1.12
3.9 ñ 0.57
3.1 _+1.10','[
4.2 + 0.68
3.5 ñ 1.18'*
Pinyon
4,4 + 1.92
3.6 + 1.14
3.7 + 0.96
3.0 _+0.67*
3.9 + 1.32
3.2 + 0.86
Ponderosa
7.2 + 1.79
0.0 ñ 0.00
8.9 ñ 3.02
9.1 + 3.38
8.9 + 2,80
Juniper
Pinyon
2.4 + 1.20
2.5 ñ 1.62
2.2 _+1.22
0.0 _+0.00
2.3 + 0.82
2.4 + 1.00
1.8 ñ 1.18
1.8 + 0.46
2.3 + 1.06
2.5 ñ 1.18
2.0 ñ 1.21
1.8 ñ 0.41
Ponderosa
3.6 ñ 1.52
0.0 _+ 0.00
6.5 + 4.23
4.6 ñ 2.70
5.6 + 3.81
4.5 _+ 2.51
10.0 + 3.63
Foragingheight (m)
Relative foraging height (% tree height/bird height)
Juniper
Pinyon
56 + 31.4
54 + 23.4
56 + 25.8
0 ñ 00.0
Ponderosa
51 + 21.9
0 + 00.0
62 + 27.7
65 + 18.6
60 + 28.4
61 + 21.3
58 + 29.6
62 + 20.2
57 + 26.7
60 + 18.6
65 + 30.2
52 + 19.9
60 ñ 28.0
51 ñ 18.6
20 _+35.4'[
29 _+36.4'[
43 _+37,0*
25 _+37.0p
32 + 39.7
20 + 31.5
46 + 34.2*
16 + 30.5
Locationin canopy(% from tree bole)
Juniper
Pinyon
Ponderosa
44 + 40.7
11 _+22.4
50 + 31.4
8 + 19.3
8 + 20.6
1 + 4.0*
21 _+33.2
11 _+25.9'[
15 + 28.3
6 ñ 19.0'
Foraging rate using wings (m/s)
Juniper
Pinyon
0.09 + 0.06
0.04 _+0.05
0.08 + 0.06
0.00 + 0.00
0.06 + 0.08
0.05 + 0.03
0.03 -+ 0.03'[
0.02 + 0.02'*
0.07 _+0.07
0.05 _+0.04
0.06 + 0.05
0.02 ñ 0.02*
Ponderosa
0.00 + 0.00
0.00 + 0.00
0.06 + 0.04
0.03 + 0.01'
0.06 _+ 0.04
0.04 ñ 0.03
0.06 _+0.03
0.08 + 0.05
0.13 ñ 0.05
0.11 + 0.10'
0.13 _+0.08
0.00 + 0.00
0.07 + 0,05
0.07 ñ 0.05
0.05 _+0.05'[
0.09 + 0.06
0.12 + 0.08*
0.10 + 0.08
0.06 _+0.04
0.07 _+0.05
0.08 ñ 0.06
0.10 ñ 0.08*
9.12 _+0.07*
0.10 + 0.07
25 _+40.5
6 + 20.0
8 ñ 22.9*
0 _+00.0
9 _+23.5'[
14 ñ 28.0
9 _+25.4
4 _+12.7'[
17 + 33.6
10 + 24.6
Foragingrate while hopping (m/s)
Juniper
Pinyon
Ponderosa
Needle use (%)
Juniper
Pinyon
Ponderosa
3 + 11.8
1 -+ 5.4
5 + 18.8
9 + 24,0
2 + 9.1'
0 + 00.0
8 +_ 23.5
<1 + 3.8*
0 _+00.0'[
11 ñ 29.5
18 _+37.7'[
3 + 16.2
8 ñ 25.6
10 + 27.1
5 + 20.9
6 + 21.2
8 + 25.1
3 +_ 15.2
10 + 23.0*
3 + 11.0
2 ñ 7.0
1 ñ 2.3
6 + 18.0
3 + 10.0
3 ñ 10.7
4 + 18.7
Twig use (%)
Juniper
Pinyon
17 + 34.9
9 ñ 24.7
44 + 41.5'*
11 + 27.7
Ponderosa
12 + 28.9
<1 + 1.6'
31 + 41.3'**
7 + 22.9
Small (<5 cm) branch use (%)
Juniper
Pinyon
1 + 6.1
1 + 2.6
3 _+10.1
2 _+9.0
2 + 7.9
<1 + 2.1
Ponderosa
2 _+ 10.2
3 + 15.8
Juniper
Pinyon
17 _+33.8
8 + 22.5
20 ñ 32.7
3 + 15.8
Ponderosa
0 + 00.0
0 + 00.0
Juniper
22 + 37.9
15 _+34.3
16 + 31.2
15 _+•30.5
Pinyon
8 + 26.7
6 -+ 23.3
9 + 22.9
6 + 20.4
9 + 24.6
6 + 21.7
Ponderosa
5 + 20.2
3 -+ 15.8
5 -+ 16.8
3 + 17.2
5 + 18.4
3 + 16.4
18 + 34.1
19 -+ 32.9
3 -+ 13.8'[
10 ñ 26.7
9 -+ 24.9
1 + 5.6
1 + 4.3
0 _+00.0'
1 + 6.2
3 + 11.2
5 + 21.3
Ground use (%)
4 _+15.8'[
5 _+17.1
3 + 15.0
22 _+35.2*
9 + 22.9
10 + 26.7
6 + 19.8
21 + 33.7*
6 + 19.8
0 _+ 00.0
1 ñ 10.8
0 ñ 00.0
Glean (%)
Ground
9 -+ 19.9.[
<1 + 1.0
16 + 32.5
Invert glean (%)
Juniper
Pinyon
3 + 7.8
1 + 3.8
0 + 00,0'
0 _+00.0
0 _+00.0'[
2 ñ 4.7
0 ñ 00.0
2 + 8.7
Ponderosa
0 ñ 00.0
0 + 00.0
1 + 3.0
Juniper
13 ñ 29.5
18 + 34.8
5 ñ 18.5
9 _+24.8
15 ñ 30.9
Pinyon
0 + 00.0
1 ñ 8.2
15 _+33.1'[
3 + 10.3'
8 ñ 24.8
2 + 9.2
Ponderosa
0 + 00.0
0 _+00.0
12 + 27,1'[
3 + 13.7'
6 + 20.3
2 + 9.7
13 + 30.2
7 + 22.1
9 + 26.0
33 ñ 41.2'
5 _+17.2
0 + 00.0'
9 + 21.0
17 + 33.9
36 + 42.4"
9 + 22.1
10 _+28.3'[
11 _+25.7
12 _+29.0
10 + 27.1
<1 + 1.2
<1 +_ 2.1
<1 ñ 0.9
Peck-probe(%)
12 + 26.5
Search (%)
Juniper
Pinyon
Ponderosa
11 + 28.3
34 + 41.5'**
7 + 19.7
5 ñ 20.5