Diets of Four Sympatric Amazonian Woodcreepers (Dendrocolaptidae)

12 Pages·2002·0.8 MB·English

The Condor 93:904-915
0 The CooperO rnithologicaSl ocietyI 99 I
DIETS OF FOUR SYMPATRIC AMAZONIAN WOODCREEPERS
(DENDROCOLAPTIDAE)’
ANGELAC HAPMAN~ AND KENNETH V. ROSENBERG
Museum of Natural Sciencea nd Department of Zoology and PhysiologyL, ouisiana State University,
Baton Rouge, LA 70803-3216
Abstract. Contents of 78 stomachsf rom four widespread and sympatric specieso f Am-
azonian woodcreepers( Dendrocolaptidae)w ere examined to assessth e role of diet in resource
partitioning. Orthopterans (2%35%) and beetles (lO-32%) dominated the diets of all four
species,d espite large differencesi n foraging behavior. A higher proportion of spiders in the
diet of Dendrocinclam erula was associatedw ith specialized ground-foraginga t army ant
swarms; however, specialization on dead leaves by Xiphorhynchusg uttatusr esulted in no
significant dietary differences from the substrateg eneralist X spixii. Overlap in diet was
much greater than overlap in behavior for all speciesp airs, and the degree of diet special-
ization was unrelated to behavioral specialization. Taxonomic representation of prey in
stomach contents differed significantly from field sampling of available prey in the three
speciest ested, with orthopterans apparently selectedb y all speciesa nd beetles selectedb y
D. fuliginosa. We suggestt hat behavioral differences may have evolved to reduce overt
aggression( interference competition) but may not serve to reduce diffuse competition for
food among syntopic species.S egregationi n substrateu se, however, may allow high diet
overlap, suggestinga case of niche complementarity among these species.
Key words: Dendrocolaptidaed; iet;foraginge cologyr;e source partitioning;s pecialization;
woodcreeperstr;o picalf orest birds.
INTRODUCTION Munn 1985). Others follow swarms ofarmy ants
How large numbers of closely related speciesc o- (E&on spp.) to feed on escaping arthropods
exist in diverse tropical communities has in- (Willis and Oniki 1978). In addition, many spe-
trigued ecologistsf or decades.A lthough studies cies do not feed directly on trunk or branch sur-
of resource partitioning among potential avian faces,b ut investigatec lusterso f dead leaves,v ine-
competitors are numerous, studies that comple- tangles, or palm fronds, or sally after flying prey
ment data on habitat selection and foraging be- (e.g., Pierpont 1986). The extwenhti ctho these
havior with data on diet are few (e.g., Rotenbebrerhy avioral differences may result in differences
1980, Robinson and Holmes 1982, Rosenbine rdgi et is unknown.
et al. 1982, Dahlsten et al. 1985). This is esB-e causeu p to 17 specieso f woodcreepersm ay
pecially true for species-richt ropical forestg uicldose;x ist in parts of the Amazon Basin (e.g., Ter-
only Sherry (1984) has investigated the dietbaoryrg h et al. 1984) their foraging relationships
relationships among sympatric neotropical airne- a significant component of the overall com-
sectivores,u sing 16 flycatcher speciesf rom Comsutan ity dynamics. In this paper, we describe the
Rica. diets of four widespread speciesa t two sites in
The woodcreepers (Dendrocolaptidae) arseo uat hwesternA mazonia. We chose Dendrocincla
large and characteristic family of neotropical fuliginosa (Plain-brown Woodcreeper), D. mer-
forest birds. Although superficially similar in ex- ula (White-chinned Woodcreeper), Xiphorhyn-
ternal morphology and in their tendency to climb thus guttatus (Buff-throated Woodcreeper), and
vertical trunks and branches, woodcreeper spe- X. spixii (Spix’s Woodcreeper)b ecauset hesew ere
cies exhibit a wide range of foraging behaviors. the most common large (> 30 g) woodcreepers
Many species typically join mixed-species for- at each of these two well-sampled sites. Both
aging flocks in the understory or canopy (e.g., species of Dendrocincla are “professional” fol-
lowers of army-ant swarms throughout much of
Amazonia (Willis 1972, 1979), although D. fu-
’ 12 February 1991. Final acceptance 7
liginosa also forages away from ants. Both Xi-
June 1991.
z Presenta ddress:D epartment of Biology, Universiti phorhynchus spp. rarely follow army ants but
Brunei Darussalam, Bandar Seri Begawan, 3 186 Bru- routinely join mixed-species flocks in the un-
nei, Darussalam. derstory and subcanopy (Munn 1985, Pierpont
[9041
DIETS OF AMAZONIAN WOODCREEPERS 905
1986; Rosenberg, unpubl. data). To address such as beetles. We believe, therefore, that with
whether diet was important in niche partitioning knowledge of the particular fragments repre-
by these species, we asked the following ques- senting each food-type, we could detect hard-
tions: (1) do differencesi n foraging behavior cor- and soft-bodied prey equally well. Further ex-
respond to differencesi n diet? (2) do theses pecies perimentation would be needed to verify our
specialize on particular prey? and (3) how do impression.
species’ diets relate to food availability? The proportion of each prey category was de-
termined separatelyf or each stomach; diets were
METHODS
then determined as the average of the propor-
Diets were determined by direct observation of tions in the individual stomachs of each species
stomach contents from 78 individual birds: 32 (i.e., samples were not pooled). Diet overlap be-
X. guttatus,1 8 X. spixii, 11 D. jiuliginosaa, nd tween pairs of woodcreepersw as calculated as:
17 D. merula( Table 1). Specimensw ere collected 0, = ~(P,,P,,)/\/(~Pz,.)(~Pz,~), where Pi, and P,.
from June through August, mainly with mist nets, are the proportions of prey category “a” in the
near Cobija, Dpto. Pando, extreme northwestern diets of species“ i” and “j” respectively (Pianka
Bolivia, in 1986 (see Parker and Remsen 1987) 1974, May 1975). Niche breadth, or prey-type
and near Abujao, Dpto. Ucayali, eastern Peru, diversity, was calculated for every stomach using
in 1987 by field parties from Louisiana State B = (Zp,Z))‘, where p, is the proportion of taxon
University Museum of Natural Science “i” in the stomach (Levins 1968). An average
(LSUMNS). The two sites are in continuous, niche breadth was then calculated for each spe-
lowland rainforest, separatedb y roughly 200 km. cies. It is unclear whether pooled frequency dis-
Additional foraging and insect availability data tributions of prey items from multiple stomach
were gathered at the Tambopata Reserve, Dpto. samples can be compared statistically, because
Madre de Dios, southeastern Peru, from 1987- of potential pseudoreplication (Hurlbert 1984).
1989. Although in most casesin dividual arthropod prey
Stomachs were preserved in 70% ethanol and captured by woodcreepersp robably representi n-
housed permanently in the LSUMNS collection, dependent events (except for rare attacks on ant
along with references kins or skeletons.C ontents nests),w e have conservatively chosen not to ap-
of each stomach were examined using a stereo- ply goodness-of-fit statistics to compare diets
microscope( 6-25 x) with a micrometer. Because among species. We believe, however, that the
only a small number of items could be identified patterns in dietary overlap are clear enough to
to family, most prey were identified to order or indicate biologically significant trends, and our
suborder, using Borror and White (1970), Borror overall conclusions would not be altered.
et al. (1981), and Ralph et al. (1985). Prey items For comparisons with the foraging behavior
were sorted, measured, and counted, and sketch- of these species,w e drew upon data from several
es of identified material were made to facilitate sources.F irst, woodcreepersw ere observed for-
later identification. Prey sizew asd etermined from aging as part of a general community study at
measurable fragments using regressione quations the Bolivia study site and at the Tambopata Re-
in Calver and Wooller (1982) or computed from serve in southeastern Peru (J. V. Remsen and
arthropods collected in the study areas (Rosen- Rosenberg, unpubl. data). These observations
berg, unpubl. data). Reference series of identi- consisted of opportunistic sightings along forest
fied, mounted fragments, drawings, and photo- trails, for each of which we recorded foraging
graphs (Fig. 1) were prepared for future com- height (estimated to the nearest 1 m), foraging
parisons. substrate, perch site, prey-capture method, and
The question of differential digestion of hard- an estimate of foliage density around the bird,
and soft-bodied prey is pertinent to any analysis as well as associateds pecies( e.g., mixed-species
of stomach contents (Rosenberg and Cooper flocks) and general habitat features. Data nota-
1990, and referencest herein). Hard-bodied items tion followed Remsen and Robinson (1990).
take longer to digest and may persist longer in Usually, fewer than five consecutive observa-
the stomach. However, it seemed that the hard tions were made on each bird, although repeated
fragments of soft-bodied prey, such as orthop- sightings of (probably) the same individuals in
teran mandibles and spider fangs, were as prev- the same mixed-speciesf locks were included. Be-
alent in our samplesa s thoseo f hard-bodied prey, cause these efforts focused on mixed-species-
1.
FIGURE 1. Fragmented prey items found in the stomachso f four woodcreepesrp eciesA. . Homoptera( head);
B. Lepidopterala rva (mandible);C . Coleoptera(e lytron); D. lizard (jaw); E. Curculionidae (head); F. ant (head
with mandibles); G. roach (head with mandibles); H. spider (fang); I. Coleoptera (head with mandibles); .I.
Orthoptera (mandible part); K. spider (chelicerae with fangs); L. Orthoptera (mandible). Black bar indicates
I mm.
flocking species,o ur samples are adequate only then computed using the same procedure as for
for the two Xiphorhynchuss pecies.F or the two diets, based on the proportional use of seven
species of Dendrocincla, we rely primarily on categoriesc ommon to all studies (see Fig. 7).
published accounts of their behavior in Willis In these comparisons, we assume that geo-
(1972, 1979). Willis (1972) studied D. jiiliginosa graphic variation in each species’ behavior
in lowland rainforest in Panama from 1960-l 97 1 throughout the lowland rainforests is consider-
by observing behavior both at and away from ably less than variation among the four species
army ants. Observations consisted of quantifi- at our study sites. Although quantitative data on
cations of behavioral characteristics, including geographic variation are lacking, our observa-
height, diameter, and angle of perches,a nd sub- tions suggestt hat these data sets represent the
stratesf rom which prey were taken. Willis (1979) foraging roles of these four speciesi n Bolivia and
used identical procedures to study D. me&a in southern Peru. For example, D. merula is known
northern and eastern Brazil (Manaus, Belem) in to be an obligate army-ant follower throughout
1973 and 1974. To compare these data with our its range, and its behavior was found to vary little
observations of Xiphorhynchus spp., we calcu- among 19 Amazonian study sites (Willis 1979);
lated frequency distributions of foraging height our few observations of this speciesw ere within
and substrate use from Willis’ published tables. 1 m of the ground at ant swarms. Our small
Overlaps in substrate use among species were sample of observations of D. fuliginosa, both at
DIETS OF AMAZONIAN WOODCREEPERS 907
TABLE 1. Characteristicosf four Amazonianw oodcreepersW. eightsa nd measurementasr e averageso f five
male and five female specimenso f each speciesa t LSUMNS. Bill lengthi s the exposedc ulmen; bill width
measureda t nares( both in mm).
Number of stomachs
Soecies Bill lenath Bill width PWU Bolivia
Dendrocinclam erula 44.6 25.9 5.9 4 13
Dendrocinclaf ulieinosa 30.4 27.2 6.4 6 5
Xiphorhynchls s&ii 36.4 32.3 5.6 13 5
Xiphorhynchusg uttatus 57.8 38.3 6.1 19 13
and away from army ants, also is qualitatively speciesc onsisted of Coleoptera and Orthoptera
similar to thoseo fwillis in Panama (seeR esults). (Fig. 2). D. merula had the highest percentageo f
We compared diets with data on prey avail- spiders (21%) and the lowest percentage of Co-
ability for three of the four woodcreeper species. leoptera (10%). Spidersi n the other three species
For the two ant-following Dendrocincla spp., we ranged from 10-l 1% of total items in the diet.
used data on arthropods flushed by army ants in Ants were eaten consistently by all speciese xcept
Costa Rica (Otis et al. 1986). This study con- D. jiiliginosa. Weevils (Curculionidae) were ab-
sistedo f paired samples of leaf-litter plots before sent from D. merula but were particularly well
and after the passing of an ant swarm and also representedin the two Xiphorhynchus species( 33-
direct observationso f arthropodsf leeing the ants. 50% of beetles). Minor prey items (l-10%) in all
We estimated prey availability to birds by com- four species included roaches, lizards or frogs,
bining counts of fleeing arthropods (from their bugs (Heteroptera), and insect larvae.
Table 1) with the number of insects and spiders The four specieso verlapped considerably with
reduced on the paired plots. Other arthropods respectt o diet composition (Table 2). The great-
(e.g., isopods, ticks) found in the leaf-litter sam- est overlap was between X. guttatus and X. spixii,
ples were not included because they were not and the least was between D. jiiliginosa and D.
observed fleeing from ants nor were they found me&a. Dendrocincla merula ate more soft-bod-
in the diets of the woodcreepers.T he diet of X. ied spiders, roaches and orthopterans, whereas
guttatus was compared with arthropods found in its congener preyed more on beetles and verte-
suspendedd ead leaves at the Bolivian study site brates. Overall dietary diversity was similar in
(Rosenberg1 990). Samplesc onsistedo f 275 dead these species,r anging from 4.8 in D. merula to
leaves,i ndividually placed in zip-lock plastic bags 6.2 in X. guttatus, out of a possible 8.0.
and sprayed with insecticide; arthropods were To assesst he adequacy of our samples, we
then collected and later identified to lowest tax- examined the increasei n the number of prey taxa
onomic level possible. No data were available represented in cumulative samples of randomly
on arthropods in the microhabitats used by X. selecteds tomachs of each species( Fig. 3). In all
spixii. These comparisons are necessarily crude, species,t he important arthropod orders and sub-
but they give a first approximation of the selec- orders considered in this study were represented
tivity of prey items by these species. after samples of only one to five stomachs. The
additional prey taxa that accumulated in each
species’ diet included rare items such as earwigs,
RESULTS
flies, scorpions, and centipedes, which together
DIET COMPOSITION
never exceeded 5% of the diet. To test further
Diet composition within a speciesd id not differ our sample sizes, we determined the number of
significantly between the two sites (G-test, all P stomachs necessary to estimate the proportion
> 0.06), except for D. me&a, in which the Bo- of each prey category within + or -0.05 of the
livian samples (n = 13) contained a greater pro- total diet for each speciesa t each site. In most
portion of spiders (24% vs. 10%) and fewer ants samples, the cumulative estimates of diet com-
(9% vs. 3 1%) than those from Peru (n = 4). For position was within 0.05 for all prey categories
further comparisonsa mong species,d ata for both after five stomachs( Fig. 4). The exceptions were
sites were pooled. for ants in X. guttatus (up to eight stomachsn eed-
The majority (54-60%) of the diet of all four ed) and in the small sample of D. me&a from
908 ANGELA CHAPMAN ANDK ENNETH V. ROSENBERG
n Beetles 0 Roaches q Heteroptera q Larvae El Other
Q Oflhopteraq Spiders q Ants II0 Vertebrates
D. menila D. fuliginosa X. guttafus x. spixii Amy ants Dead leaves
(171236) (11 1107) (32 1430) (18 1357) (157) (275)
Species
FIGURE 2. Diet compositiono f four woodcreepesrp eciesc omparedw ith prey flushedb y army ants (data
from Otis et al. 1986) and prey availablei n deadl eaves.F or bird speciesn, umbersi n parentheseasr e number
of stomachs/numbeorf prey items;f or availability samplesn, umbero f arthropods.
Peru, in which total diet may not have been well liginosa foraged as high as 10 m (Fig. 6) and
represented. These results indicate low levels of sallied to trunks, vines, foliage, and air, as well
individual variation in these speciesa nd suggest as to the ground (Fig. 7; data from Willis 1972).
that our pooled samples of 10 or more stomachs At our study sites, D. filiginosa was seen mostly
for each speciesa dequately represent their diets. from 5 to 8 m above ground (n = 34 observa-
tions), and it foraged from a variety of substrates
PREY SIZE including trunks (3 lo/o), air (3 lo/o), live foliage
(23%) and epiphytes (8%). Therefore, the char-
All four woodcreeper speciesa te a similar size
acterization of this species as an arboreal gen-
range of the most important prey (Fig. 5) in spite
eralist, compared to the more restricted ground-
of considerable differences among the wood-
foraging of D. merula, appears representative of
creepersi n bill size and body weight (Table 1).
their foraging at our study sites as well as those
Dendrocincla merula took, on average, slightly
of Willis. In Peru and Bolivia, X. guttatus stayed
larger beetles and spiders than the other species,
mostly above 6 m (Fig. 6), and about two-thirds
but none of the differences were significant (t-
of its foraging was at suspended dead leaves,
tests; all P > 0.10). The sizes of caterpillars, liz-
especially large Cecropia leaves and palm fronds
ards, and other uncommon items could not be
(Fig. 7). Xiphorhynchus spixii foraged generally
determined from digested fragments; however,
from 2 to 10 m above ground (Fig. 6), usually
they probably contributed little, if any, to dif-
ferencesi n prey size among theses pecies.I n spite
of substantial differencesi n bill length there was TABLE 2. Dietary and behavioralo verlapsa mong
no relationship to prey size. four woodcreepesrp eciesV. aluesa bove diagonala re
for diet composition;v alues below diagonala re for
substrateu se.E quationf or overlapg iven in text.
COMPARISON WITH FORAGING
BEHAVIOR
X. X. D. fili- D.
In Brazil, D. merula perched usually within 1 m gun**us spixri ginosa merula
of the ground (Fig. 6) and about 90% of its for- X. guttatus - 0.999 0.986 0.906
aging consistedo f sallies to the ground in pursuit X. spixii 0.419 0.957 0.830
of arthropods fleeing the ants (Fig. 7; data from D. jiiliginosa 0.196 0.649 0.780
D. met-da 0.017 0.034 0.444 -
Willis 1979). At ant swarms in Panama, D. fu-
DIETS OF AMAZONIAN WOODCREEPERS 909
A. Bolivia
16
1
I”-;
4-
0 I
0 4 8 12 16 20
B. Peru
v D. fuliginosa
+ X. guttatus
0; -.
1. .‘. ‘1. ‘1.. ’
0 4 8 12 16 20
Number of pooled stomachs
FIGURE 3. Number of prey taxa in cumulatives ampleso f stomachsfr om four woodcreepesrp eciesa t two
pecking at trunks or probing under vine stems terms of prey orders, from D. merula and D.
or epiphytic moss and lichens (Fig. 7). fuliginosa but was nearly identical to X. spixii.
The overlap in diets among these four species Diversity of feeding behaviors was not con-
in all casesw as much greater than their overlap sistently related to diet diversity. Dendrocincla
in foraging substrate use (Table 2). In the two merula, with the most restricted mode of for-
ant-following species,a lmost total segregationi n aging, also had the least diverse diet, whereast he
foraging height and substrate use may have re- somewhat specialized X. guttatus had a slightly
sulted in a shift in the proportions of main prey more diverse diet than X. spixii.
categories,w ith more spiders and roachesi n the
PREY AVAILABILITY
diet associatedw ith the ground-foraging D. rner-
ulu and more beetles associatedw ith foraging on Basedo n the behavioral summary above, we were
trunks and vines in D. jiiliginosa. Xiphorhynchus able to compare the diets of three speciesw ith
guttutus,w hich forageso ften on suspendedd ead data on prey availability. Arthropod prey dis-
leaves, had a noticeably different diet, at least in placed by army ants in Costa Rica (Otis et al.
910 ANGELA CHAPMAN ANDK ENNETH V. ROSENBERG
PERU BOLIVIA
0. fuliginosa 0.6 D. fuliginosa
0.5 3
0.6 0.6 D. mew/a
0.5
0.4
0.3
0.2
0.1
0.0
0 5 10 15 20
0 5 10 15 20
0.6 X. gottatus 0.6 X. guttatus
0.5 0.5
.
0.4
0.6 x. spixii 0.6
0.5
0.4
- Orthoptera
0.2 - Ant
0.1
0.0
0 5 10 15 20
NUMBER OF POOLED STOMACHS
FIGURE 4. Proportionso f prey taxa in cumulatives ampleso f stomachsfo r four woodcreepesrp eciesa t two
sites.
1986) consisted mainly of spiders, roaches, and more beetlesa nd orthopterans than expecteda nd
ants, whereas prey available in suspendedd ead fewer roaches, spiders, and ants. In comparison
leaves consistedo f beetles, roaches, spiders, and with prey availability in dead leaves, X. guttatus
orthopterans (Fig. 2). In all comparisons, the ate slightly more orthopterans and fewer roaches;
woodcreepers’ diets were different from’ the dis- all other prey were eaten in proportion to their
tributions of available prey. The ant-following availability.
D. merula appeared to select orthopterans and
DISCUSSION
avoided small roaches, but took other prey ap-
proximately in proportion (+ or - 10%) to that In this study we have documented dietary dif-
flushed by army ants (Fig. 8). D. fuliginosaa te ferencesi n four woodcreepersw ith different for-
DIETS OF AMAZONIAN WOODCREEPERS 911
A. Beetles
XG
B. Orthoptera
35
30
25
20
15
10
5
xs XG
0
25 30 35
C. Spiders
2.5 -
2.0 -
1.5-
,
1 .o -
0.5
DM DF XG
25 30 35 40
Bill length (mm)
FIGURE 5. Comparison of bill length with prey size in four woodcreeper species.B ars indicate one standard
deviation; vertical lines indicate range. DM = Dendrocinclam e&a, DF = D. fuliginosa, XS = Xiphorhynchus
spixii, XG = X. guttatus.
aging habits. Although some differences in diet Dendrocincla merula and D. fuliginosa com-
were detected, they nonetheless represent only Pete for food at ant swarms (Willis 1979). D.
subtle shifts in the proportions of major prey merula is usually dominant, occupying the center
types. Dietary overlap was high for most species- of the swarm, close to the ground, where prey is
pairs, and all speciesw ere more similar in diet most abundant. D. merula also is less adept at
than in foraging behavior. arboreal foraging,w ith lesss tiffenedr ectricest han
912 ANGELA CHAPMAN AND KENNETH V. ROSENBERG
--(c LX menda (n= 1603)
- D. fU/i@KISa (n = 7966)
- x. gUft&flS (11~247)
* X. spixii (n= 105)
” .
0.0 0.2 0.4 0.6 0.6 1 .o
Proportion of observations
FIGURE 6. Foraging heights of four woodcreepers pecies.D ata for D. meruluf rom Willis (1979); data for D.
fuliginosa from Willis (1972); data for X g&tutus and X spixii from this study.
other woodcreepers and more “clumsy” sallies Dendrocincla spp. to be members of a sallying
to substrates above the ground (Willis 1979). guild of woodcreepers at Manu National Park,
Thus, the behavioral differencesa nd consequent Peru. She found D. merula to forage more fre-
dietary shifts between the two Dendrocincla are quently above the ground at palms and dead
actively maintained by a dominance hierarchy leaves, where it takes flushed prey normally hid-
at their sharedf oraging sites. The diet of D. mer- den inside these substrates.F oraging of D. fuli-
ula was more similar to our estimate of prey ginosa was more restricted to trunks and vine
availability at ant swarms than was that of D. surfaces,a nd less often at ants.
fuliginosa. This difference,a nd especiallyt he large Willis (1972, 1979) rarely observed Dendro-
number of beetles eaten by D. fuliginosa, may cincla spp. preying on beetles, although 30-42%
relate to increasedf oraging away from army ants of the observed prey in those studies (especially
by this species.P ierpont (1986) considered both small prey) were not identifiable. The small num-
n Ground q Vine stem 0 Air
El Trunk Live foliage PI Dead leaf Epiphyte
1 .oo
z
.o
0.80
5
z
3 0.60
8
‘ii
0.40
.r2E5- . 0.20 \\\\II\\ \\II\\ \\\\II\\ \\,, II II ,I ,, \\\\\\.\ \\,,,II\\\\I \\\\\\, I,I,\\\\, \\\\\\ \\\\ ,I,,\\\\ \\\.\\ ,,,, \\ 1,III \\, ,,,,, \\, ,, II \\ III, \\
a
0.00
D. menda D. Miginosa x. guttatus x. s&li_X/i
(1795) (1622) (217) (69)
Species
FIGURE 7. Foraging substrateu se by four woodcreeper species.S amples sizes in parenthesesa re number of
observations. Data sourcess ame as Figure 6.
DIETS OF AMAZONIAN WOODCREEPERS 913
A. Dendrocincla spp. versus army ants
33 1 w D. fnelu/a
0 D. fuliginosa
20
10
0
-10
-20
-30 ! 4
Beetle Orthopt Roach Spider Bugs Ant Larva
B. X. gulfatus versus dead leaves
-20 ! ,
Beetle Orthopt Roach Spider Bugs Ant Larva
Prey categories
FIGURE 8. Comparison of diet and prey availability for three woodcreeper species. Horizontal mid-line
indicatesu see qual to availability; bars above and below horizontal representp rey categoriess electedo r avoided,
respectively. A. D. merulaa nd D. fuliginosac ompared with arthropods flushed by army ants (Otis et al. 1986);
B. X. guttatusc ompared with arthropods in dead leaves in Pando, Bolivia.
ber of beetles seen at an army-ant swarm by Otis (x2 = 0.30, ns). Although these two speciesd if-
et al. (1986) also may not reflect their true avail- fered in substratess earchedf or food, overlap in
ability to woodcreepersD. ata from leaf-litter traps their diets was nearly complete. Xiphorhynchus
in the same area showed that beetles made up guttatus might be expectedt o compete more di-
about 12% of the arthropod fauna (Otis et al. rectly with other large-bodied flock members that
1986), and Pearson and Derr (1986) found that searchd ead leaves, such as foliage-gleaners( Au-
beetles made up 13% of the litter arthropods in tomolusa nd Philydor spp.) and barbets (Eubucco
terra jirme forest in Peru. spp.). These various species will occasionally
In the mixed-species flocks in which Xipho- search the same clusters of dead leaves without
rhynchusw oodcreepersf orage, overt aggression exhibiting overt aggression( Rosenberg 1990) al-
between X. guttatusa nd X. spixii is rare (Pierpont though Pierpont (1986) observed several dis-
1986), and competition for food, if present, is placements of foliage-gleaners by X. guttatus.
diffuse. Xiphorhynchusg uttatus and X. spixii do However, diet composition of X. guttatus over-
not avoid flocks in which the other species is lapped more with the other arboreal woodcreep-
present. Of 9 1 mixed-species flocks censusedb y ers (X. spixii and D. jiiliginosa) than with any
Rosenberg in Peru, 36 had X. guttatus, 14 had dead-leaf foraging species (Rosenberg 1990).
X. spixii, and 7 had both species;i .e., these spe- Furthermore, X. guttatus ate smaller prey, es-
cies were distributed independently among flocks pecially smaller orthopterans, than most of the