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j Raptor Res. 34(3):196-202
¸ 2000 The Raptor Research Foundation, Inc.
THE FOOD HABITS OF SYMPATRIC FOREST-FALCONS DURING
THE BREEDING SEASON IN NORTHEASTERN GUATEMALA
RUSSELL THORSTROM
The Peregrine Fund, 5666 West Flying Hawk Lane, Boise, ID 83709 U.S.A.
ABSTRACT.--The food habits of Barred (Micrastur ruficollis) and Collared Forest-Falcons (M. semitorquatus)
were studied in Tikal National Park, Guatemala. On a numerical basis for 405 identified prey for Barred
Forest-Falcons, lizards (Anolis spp., Ameiva or Cnemidophorus spp., Laemanctus spp., and Corytophanes spp.)
were the most numerous prey type comprising 61.5% of the diet. For Collared Forest-Falcons, on a
numerical basis of 170 identified prey, mammals represented the greatest proportion at 45.9%. On a
biomass basis, lizards (37.3%) and birds (36.8%) were equally important in the diet of Barred Forest-
Falcons but, for Collared Forest-Falcons, mammals (47%) and birds (45.4%) were the most important
prey. Food-niche overlap was 0.49 between the two forest-falcons and prey that overlapped were mice,
rats, bats, birds (Momotus spp., Dendrocincla spp.), and lizards ( Corytophanes spp.). The wider food breadth
of the Collared Forest-Falcon was probably attributable to the greater diversity of bird species in its diet.
The Collared Forest-Falcon is approximately 3 times the size of Barred Forest-Falcons but the mean
weight of its prey (MWP) was 10 times greater (• = 239 g) than that of Barred Forest-Falcons (/= 24 g).
KEY WORDS: Barred Forest-Falcon; Micrastur ruficollis; Collared Forest-Falcon; Micrastur semitorquatus; food
habits; niche overlap; niche breadth.
H•tbitos alimenticios de dos halcones de bosque simpatricos durante la estaci6n reproductiva en el
noreste de Guatemala
RESUMEN.--Los h/tbitos alimenticios de Micrastur ruficollis y Micrastur semitorquatus rueton estudiados en
Parque Nacional Tikal, Guatemala. En una base num6rica de 405 presas identificadas para Micrastur
ruficollis, las lagartijas (Anolis spp., Ameiva o Cnemidaphorus spp., Laemanctus spp., y Corytaphanes spp.)
rueton el tipo de presa m/rs numeroso o sea el 61.5% de la dieta. Para Micrastur semitorquatus, en una
base num6rica de 170 presas identificadas, los mamiferos representaron la proporci6n mayor con el
45.9%. En relaci6n a la biomasa, las lagartijas (37.3%) y aves (36.8%) fueron igualmente importantes
en la dieta de Micrastur ruficollis, pero para Micrastur semitorquatus, los mamiferos (47%) y aves (45.4%),
fueron las presas mgs importantes. El traslape del nicho alimenticio rue de 0.49 entre los dos halcones
de bosque y las presas que se traslaparon rueton ratones, ratas, murci•lagos, aves (Momotus spp., Den-
drocincla spp.), y lagartijas (Corytophanes spp.). El espectro m/rs amplio de la dieta de Micrastur semitor-
quatus fue probablemente atribuible a la mayor diversidad de especies de aves en su dieta. Micrastur
semitorquatus es 3 veces el tamafio de Micrastur ruficollis pero su peso medio fue 10 veces mayor (• =
239g) que el de Micrastur ruficollis (/= 24 g).
[Traducci6n de C•sar M/trquez]
Neotropical birds of prey are poorly known, es-
pecially the forest-dependent species which are in-
conspicuous in their habits. The secretive forest
raptors of the genus Micrastur are among the least-
studied raptors and most accounts of their diets
come from stomach contents of museum speci-
mens or incidental observations (Dickey and van
Rossem 1938, Friedmann 1948, Smith 1969, Izawa
1978, Mader 1981, Willis et al. 1983, Mays 1985,
Trail 1987, Rappole et al. 1989, Thorstrom et al.
1990). The most detailed account of the food hab-
its of this genus is given by Robinson (1994), but
it too is limited to incidental observations.
The Barred Forest-Falcon (Micrastur ruficollis) is
perhaps the most common raptor in Neotropical
forests. It has the widest distribution of any forest-
falcon, occurring from southeastern Mexico to
northern Argentina, Paraguay, and east through
Brazil and the Guianas (Brown and Amadon 1989,
del Hoyo et al. 1994). It ranges from humid low-
land and foothill forests to higher subtropical and
montane forests reaching its limit near 2500 m. In-
196
SEPTEMBER 2000 FOOD HABITS OF FOREST-FALCONS 197
formation on the diet of the Barred Forest-Falcon
suggests that it feeds mainly on lizards (Thorstrom
et al. 1990, Thorstrom 1993, del Hoyo et al. 1994).
The Collared Forest-Falcon (M. semitorquatus)
also has a broad distribution, ranging from central
Mexico to eastern Bolivia, northern Argentina, and
Paraguay (Brown and Amadon 1989). It occupies
dense primary and secondary forests from sea level
to 2500 m. A recent sighting in Texas (Lasley et al.
1994) extended its northern distribution to the
southwestern U.S. Food of the Collared Forest-Fal-
con includes birds, mammals, lizards, snakes, and
insects (Brown and Amadon 1989, Thorstrom
1993).
In this paper, I compare the diet of Barred For-
est-Falcons and Collared Forest-Falcons based on
several years of nest observations of prey deliveries,
and direct observations at and away from nests dur-
ing breeding seasons from 1988-92 in northeast-
ern Guatemala. My objectives were to compare
prey frequency and biomass and to assess the
amount of overlap in diet among the two species
and compare food-niche parameters and differenc-
es as potential mechanisms for coexistence of these
two forest-falcons.
STUDY AREA AND METHODS
I studied Barred and Collared Forest-Falcons in Tikal
National Park, Pet6n, Guatemala from 1988-92. The park
encompasses 576 km 9 in northeastern Guatemala and its
center lies at 17ø13'N, 89ø36'W. Vegetation in the park is
semideciduous tropical forest with lowland rolling hills
ranging from 200-450 m elevation.
Schulze and Whitacre (1999) described several forest
types that occur along topographical drainage, soil type,
and moisture gradients within the park. The two ex-
tremes of this forest-type continuum are upland or high-
ground forests (tall, semi-evergreen forests on well-
drained, shallow soils) and "bajo" forests (low in stature,
with open canopy and dense understory, occurring in
low-lying sites of deep, clay-rich soils subject to seasonal
flooding and drought). Tikal National Park is covered
mostly by unbroken primary forest, except for some areas
where light selective logging occurred prior to 1969.
The climate has pronounced wet and dry seasons with
rains usually beginning in May or June and ending by
December. Between 1989-95, monthly precipitation
ranged from 1.0 mm in April to 302.5 mm during Sep-
tember with an annual mean rainfall of 1309 mm (pers.
obs.). Mean monthly temperatures ranged from a low of
15øC in January to a high of 35øC in May.
The forest and known forest-falcon territories were
searched daily from February through August to docu-
ment nesting activity and potential breeding pairs. Nests
of Barred Forest-Falcons were observed primarily from
the ground and those of the Collared Forest-Falcon were
occasionally observed from tree platforms. Observations
were made using 7-10X binoculars at distances of 25-50
m. During the breeding season, observations of prey
items were recorded during prey deliveries and away
from nests during radiotracking sessions. All prey was
identified to the most accurate taxonomic level possible
with the exception of amphibians and insects, which were
not identifiable to the species level and were assigned to
larger taxonomic groupings. The resulting tabulauon
produced a total of 37 prey categories for both species.
Only observed prey delivered and captured were includ-
ed in biomass estimates to avoid possible bias from prey
found in nests (Snyder and Wiley 1976, Wiley and Wiley
1981, Marti 1987). Anolis lizards were separated in small
(<20 cm) and large catagories (>20 cm).
To estimate mean weight of prey (MWP), I multiplied
each prey item by its average weight (Table 1), summed
the products and divided the sum by the total number
of prey observed. Mammal weights follow Emmons and
Feer (1997), bird weights come from Smithe (1966) and
Dunning (1993), and reptile weights were taken in the
field.
Food-niche breadths (FNB) were calculated using Lev-
ins' (1968) equation: FNB = 1/EP, j 2, where P• is the
proportion of the ith prey category of species j. For com-
parison among raptors with different number of prey cat-
egories, a standardized niche breadth value (FNBs) was
also calculated as follows: FNBs = (FNB - 1)/(n - 1),
where n is the number of prey categories (Levins 1968)
Niche overlap was calculated using Schoener's (1970) in-
dex of symmetrical overlap: overlap = 1 - («)(•[P•j -
P•a[), where P• is the proportion of the ith prey category
for species j and k. Linton et al. (1981 ) found this overlap
formula to be the only index that accurately measures
real overlap between 7-85%.
The Collared Forest-Falcon is the largest of the two
species with a body mass of 467-511 g for males (Dickey
and van Rossem 1938) and 556-750 g for unknown sexes
(Haverschmidt 1968). Males I weighed averaged 587 ñ
17.6 g (ñSD, range = 563-605 g, N = 4) and females
averaged 869 g ñ 63 g (range = 792-940 g, N= 6)
Barred Forest-Falcons averaged 167.8 _+ 10.6 g (range =
144-184 g, N = 25) for males and 233.2 g -+ 23.9 (range
= 200-322 g, N = 34) for females.
RESULTS
Barred Forest-Falcon. I recorded lizards (Anohs
spp., Ameiva spp. or Cnemidophorus spp., Laemanctus
spp., and Corytophanes spp.), birds (Momotus spp.,
Aulacorhynchus spp., Turdus spp., Leptotila spp., Den-
drocincla spp., Thryothorus spp., and Tyrannidae),
amphibians, mammals, snakes, and insects (Blatti-
dae) in the diet of Barred Forest-Falcons during
the nesting season.
I observed a total of 600 prey items being deliv-
ered to females, nesdings, and fledglings from
1988-92. On a numerical basis, reptiles were the
predominant prey comprising 61.5% of the diet
(249 prey items), followed by birds 22% (89), in-
sects 8.2% (33), mammals 5.9% (24), and amphib-
ians 2.5% (10) (Fig. 1). Nearly one third (195) of
198 THORSTROM VOL. 34, NO. 3
Table 1. Weights used to estimate prey biomass of Barred and Collared Forest-Falcons at Tikal National Park,
Guatemala.
WEIGHT
PREY (g) SOURCE
Insects
Blattaria 1.5 This study
Reptiles
Anolis <20 cm 3.9 This study
Anolis large >20 cm 13.8 This study
Ameiva or Cnemidophorus 25 This study
Laemanctus 15 This study
Corytophanes 45 This study
Birds
Crypturellus 440 Smithe 1966
Penelope 600 Smithe 1966
Crax 500 Smithe 1966
Ortalis 450 Smithe 1966
Agriocharis 3000 Smithe 1966
Odontophorus 300 Smithe 1966
Leptotila 160 Smithe 1966
Ciccaba 240 Smithe 1966
Momotus 133 Dunning 1993
Ramphastos 350 Dunning 1993
Pteroglossus 220 Dunning 1993
Aulacorhynchus 150 Smithe 1966
Melanerpes 81 Dunning 1993
Celeus 85 Dunning 1993
Tyrannidae 15 Smithe 1966
Cyanocorax 200 Dunning 1993
Troglodytidae 15 Smithe 1966
Muscicapidae 75 Smithe 1966
Mammals
Sciurus small 205
Sciurus large 400
Artibeus 50
Unidentified bat 20
Unidentified mouse (Heteromys) 76
Unidentified rat ( Rattus, Oryzomys, Sigmodon) 150
Emmons and Feer 1997
Emmons and Feer 1997
Emmons and Feer 1997
This study
This study, Emmons and Feer 1997
This study, Emmons and Feer 1997
the items were unidentified, especially late in the
nestling period, because male forest-falcons flew
secretively into their nests without calling their
mates to receive prey, and females flew into the
nests quickly and directly without vocalizations. It
was unlikely, however, that the unidentified prey
items differed from those actually identified. The
most detailed dietary information was obtained
during 1989 when 267 of 380 items delivered to
nests were identified. Again, most (64.0%, N =
171) were lizards and were represented by 57 small
Anolis spp., 21 large Anolis spp., 28 teiids (most like-
ly Ameiva spp. or Cnemidophorus spp.), 11 Laemanc-
ms spp., 5 Corytophanes spp., and 49 unidentified
lizards. Snakes included 1 coral snake or mimic
(Lampropeltis sp. or Micrurus sp.) and 2 other
snakes. Eleven of the 267 identified prey (4%)
were frogs (Rana spp. and/or Hyla spp.). Only 21
arthropods (8 cockroaches and 13 other items in-
cluding spiders and beetles, 8% of the diet) were
identified. Birds contributed 52 prey items (19.5 %
of the diet) and included five Blue-crowned Mot-
mots (Momotus momota), two flycatchers (Tyranni-
dae), two Emerald Toucanets (Aulacorhynchus pra-
SEPTEMBER 2000 FOOD I-IABITS OF FOREST-F,•LCONS 199
a)
Barred Forest-Falcon (n:267)
..,•'•:'•' •
':Z •.
Collared Forest-Falcon (n=170)
-.
'[• Mammals
ß Birds
[] Reptiles
ii[] Amphibians
[] Insects
Barred Forest-Falcon biomass Collared Forest-Falcon biomass
-. , ,•,• • ' ............ 5,,...•,
•,:. .•'
,.'
: :. .•'• • Mammals
:• ' ß Birds
...• •...? ...... .- • [] Reptiles
'•=• '"•:;•'"'"""•'-- • Amphibians
[] Insects
.:• •'•,'•-& '7 ='=':'='=':':'-.
.,:•.•:+?=? • ß
, ,,•=:4 -..-.=.-
• ,.,• ,'
"•!=• •x•='• ............... •..•ii •?="
..... ; :•..•;;::.:•.•.:.• •,..
Figure 1. Comparison of the diets of Barred Forest-Falcons and Collared Forest-Falcons as (a) the percent prey of
individuals and (b) the biomass composition (% weight of prey individuals).
sinus), one Gray-fronted Dove (Leptotila rufaxilla),
one woodcreeper (Dendrocincla sp.), one Spot-
breasted Wren (Thry0th0rus maculipectus), and one
Clay-colored Robin (Turdus grayi). Birds taken
ranged in size from an unidentified warbler (Den-
droica sp.) at 9 g to a Gray-fronted Dove at 160 g
(Smithe 1966, Dunning 1993). The nine mammals
I identified represented only 3% of the diet.
Among them were seven rodents, one bat, and one
other mammal. The rodents were possibly mem-
bers of the genera Heteromys and Oryzomys. Snakes
accounted for 3 prey items or 1.1% of the diet.
Biomass estimates were made for 267 identified
prey items delivered during the 1989 breeding sea-
son. On a biomass basis, reptiles (37.3%), birds
(36.8%), and mammals (20.2%) comprised 94.3%
of the estimated biomass (Fig. 1). Males delivered
more prey items and prey biomass than females
during the breeding season. Of the 267 identified
prey delivered in 1989, five males brought in 3.8
kg (75.7%) and five females delivered 1.2 kg
(24.3%) of the biomass during the breeding sea-
son.
Collared Forest-Falcon. I found squirrels (Sciu-
rus spp.), bats (Artibeus spp.), rats (Sigmodon spp.),
mice (Heter0mys spp.), birds (Crypturellus spp., Pe-
nelope spp., Crax spp., Ortalis spp., Agriocharis spp.,
Odontophorus spp., Leptotila spp., Ciccaba spp., Mom-
otus spp., Ramphastos spp., Pteroglossus spp., Aulaco-
rhynchus spp., Melanerpes spp., Celeus spp., Cyanocor-
ax spp., Dendrocolaptidae), snakes (Coluber sp.),
and lizards (C0ryt0phanes spp.).
From 1990-92, 222 prey items were delivered to
females, nestlings, and fledglings and 170 of these
were identified. On a numerical basis, 45.9% were
mammals (78 prey items), 34.7% birds (59), 18.8%
reptiles (13 lizards and 19 snakes), and 0.6% am-
phibians (1 frog) (Fig. 1). The 52 unidentified
200 THORSTROM VO•.. 34, NO. 3
prey items were presumed to have been similar to
those that were identified. In addition, 36 items
were given to two fledglings by an extra adult be-
lieved to be a male. This male specialized in catch-
ing toucans so I calculated the diet of Collared For-
est-Falcons both with and without this male's
contribution.
Prey of Collared Forest-Falcons ranged in size
from a frog estimated at 20 g to an Ocellated Tur-
key (Ag•iocharis ocellata) weighing about 3 kg. The
two largest prey were the adult female turkey and
a young Crested Guan (Penelope purpurascens). Of
the 13 lizards taken, 12 were in species belonging
to the genus Corytophanes. The 19 snakes I ob-
served were most likely colubrids. The 78 mam-
mals identified included 42 Deppe's squirrels (Sciu-
rus deppei; 190-220 g), 11 Yucatan squirrels (S.
yucatanensis; 420 g), two fruit bats (Artibeus spp.),
14 unidentified bats, 7 rat-sized rodents including
the hispid cotton rat ( Sigmodon hispidus), and 2
mice believed to be spiny pocket mice (Heteromys
spp.). Among the 59 birds identified, the most nu-
merous were Collared Aracari (Pteroglossus torqua-
tus, N = 9), Plain Chachalaca ( Ortalis vetula, N =
7), Great Curassow (Crax rubra, N = 7), Keel-billed
Toucans (Ramphastos sulfuratus, N = 6), Ruddy
Woodcreepers (Dendrocincla homochroa, N = 4),
Tinamous (Crypturellus spp., N -- 3) , and Brown
Jays (Cyanocorax morio, N = 3).
In 1990, a third adult forest-falcon, probably a
male, began delivering prey items to two young, 4
wk after they fledged. We observed this adult de-
liver 36 prey items until 11 weeks after fledging. It
appeared to prefer Keel-billed Toucans delivering
27 toucans, two Collared Aracari, two unidentified
birds, four squirrels (S. deppeO, and one unidenti-
fied prey item. Sometimes it delivered two Keel-
billed Toucans a day. When this contribution was
included in the overall diet of Collared Forest-Fal-
cons, the diet was dominated by birds (43.9%, 90
individuals) followed by mammals (40.0%, 82),
reptiles (15.6%, 32), and amphibians (0.5%, 1). In
terms of biomass, this extra adult delivered 12.6 kg
of prey during the post-fledging period.
Biomass estimates were based on the 170 iden-
tified prey items delivered during the breeding sea-
sons. On this basis, 47.0% of the prey were mam-
mals, 45.4% birds and 6.5% reptiles (Fig. 1).
Squirrels represented 66.7% of the mammalian
biomass. Males delivered 11.4 kg (65.7%) and fe-
males 5.9 kg (34.3%) of the biomass.
Food-niche Parameters. Lizards, especially Anolis
Table 2. Food-niche breadth, dietary overlap, and esti-
mated mean weights (g) of prey (MWP) and of birds
(MW) of Barred and Collared Forest-Falcons during the
nesting season. All calculations based on prey at the ge-
neric or family level. Mean _+ SE (N).
BARRED COLLARED
FOOD-NICHE FOREST- FOREST-
PARAMETERS FALCON FALCON
Total identified prey
items
Mammal species
richness
Bird species richness
Lizard species richness
MW birds
FNB
FNBs
Dietary overlap
267 170
3 6
7 15
5 1
23.7 +-- 2.5 238.9 _+ 18.9
(267) (170)
62.1 _+ 15.3 373.4 +_ 49.5
(52) (59)
7.9 13.8
0.33 0.49
0.49
spp., dominated the Barred Forest-Falcon diet and,
as a result, it had a narrower niche breadth than
did the Collared Forest-Falcon. Collared Forest-Fal-
cons took a higher richness of bird and mammal
species (Table 2). The standardized FNB of the
Barred Forest-Falcon was lower (0.33) than the
Collared Forest-Falcon (0.49). Dietary overlap be-
tween the two forest-falcons was 0.49. Estimated
MWP captured by Collared Forest-Falcons was sig-
nificantly heavier than that of Barred Forest-Fal-
cons (Table 2). The larger Collared Forest-Falcon
captured larger avian (/= 373.9 + 49.5 g, +SE, N
= 59) and mammalian (/= 179 + 12.5, N = 78)
prey than did the Barred Forest-Falcon which took
mostly lizards (œ = 13.8 + 0.6, N = 122) and birds
(œ = 62.1 + 4.9, N = 52).
DISCUSSION
Barred and Collared Forest-Falcons are moder-
ately dimorphic with Collared Forest-Falcons 3-4
times larger than Barred Forest-Falcons. Optimal
foraging theory predicts that larger predators
should have a wider food niche than smaller ones
(Schoener 1970). I found this to be true for these
two forest-falcons. Collared Forest-Falcons cap-
tured a higher proportion of medium-sized mam-
mals, especially squirrels, and they had a greater
diversity of birds in their diet giving them a broad-
er food-niche breadth (13.8) compared to Barred
SEPTEMBER 2000 FOOI• HABITS OF FOREST-FALCONS 201
Forest-Falcons (7.9). Barred Forest-Falcons preyed
predominantly on lizards, mainly Anolis spp., con-
tributing to its narrower food-niche breadth, and
birds were of secondary importance in their diet.
Collared Forest-Falcons preyed on a wider range of
animal sizes, ranging from a small frog (20 g) to
large birds (3 kg) whereas Barred Forest-Falcons
caught prey ranging in size from insects (1.5 g) to
a dove (160 g).
In terms of biomass, Barred Forest-Falcons cap-
tured nearly equal proportions of lizards (37.3%)
and birds (36.8%) during the breeding season.
This was attributed to the smaller mean weight of
lizards (13.8 g) vs. the mean weight of birds (93.5
g). Birds were approximately seven times heavier
but three times fewer in numbers. Prey biomass of
Collared Forest-Falcons was distributed nearly
equally between mammals (47%) and birds
(45.4%), but the mean weight of birds (368 g) was
twice that of mammals (179 g). However, fewer
birds (59) than mammals (78) were delivered dur-
ing the nesting season, contributing to the nearly
equal frequency of prey biomass of Collared For-
est-Falcons.
The food-niche overlap was relatively high be-
tween these two congeners and almost near the
competition threshold level of 0.6 which was pro-
posed as biologically significant by Zaret and Rand
(1971). Schoener (1984) and Temeles (1985) pre-
dicted that similar morphological features of rap-
tors can be found among congeners which affect
their hunting ability and food habits. However, Bo-
sakowski and Smith (1992) showed that larger dif-
ferences in body size limit food overlap below the
competition threshold. Thus, while the two forest-
falcons exhibited overlap on a few prey species, I
suspect that the effect on overall prey availability
was probably insignificant. Both species have a
broad diet with Barred Forest-Falcons relying more
on lizards and Collared Forest-Falcons preying
mainly on squirrels.
The Barred Forest-Falcon is dependent on ma-
ture forests while the Collared Forest-Falcon oc-
cupies mature forests, forest edge, and secondary
woodlands and thickets. Both species use a short
stay "perch-hunting" technique, a common meth-
od found in forest or woodland-adapted species
(Kenward 1982, Newton 1986). The higher con-
sumption of avian prey by the Collared Forest-Fal-
con may be enhanced by its great maneuverability,
owing to its long legs and long-arched tail which
are morphological adaptations for chasing prey by
foot. Collared Forest-Falcons were observed chas-
ing prey by running on the ground, around tree
trunks, and along large branches, whereas Barred
Forest-Falcons usually attacked prey by surprise
from concealed perches.
The information provided here is limited to ob-
servations during the nesting season and may not
accurately reflect the overall diet of these two spe-
cies. There may be seasonal shifts in the diet of
these forest-falcons or certain prey types may be
taken preferentially due to experience or ability as
observed in the extra adult Collared Forest-Falcon
that delivered 75% of its prey as Keel-billed Tou-
cans. This particular bird apparently had a special
ability or learned behavior for capturing toucans.
More information is needed from other regions in
the Neotropics and during the nonbreeding sea-
son to determine the extent of niche breadth and
dietary overlap between these two species.
ACKNOWLEDGMENTS
This study was part of The Peregrine Fund's Maya Pro-
ject, in cooperation with the Instituto Nacional de Antro-
pologia y Historia (IDAEH), Centro de Estudios Conser-
vationistas (CECON), Guatemala, and Consejo Nacional
de Areas Protegidas (CONAP), Guatemala. A specml
thanks to B. Burnham, J.P. Jenny, L. Kiff, and D. Whitacre
of The Peregrine Fund. Thanks to Boise State University
for providing assistance. I kindly thank the staff of Tikal
National Park, Guatemala for their assistance. For assist-
ing in the field I would like to thank E.M. Ramirez, J.D.
Ramos, C.M. Morales, J.M. Castillo, H. de J.G. Manzane-
ro, and C.S. Mateo.
LITERATURE CITED
BOS•KOWSKI, t. AND D.G. SMITH. 1992. Comparative diets
of sympatric nesting raptors in the eastern deciduous
forest biome. Can. J. Zool. 70:984-992.
BROWN, L. AND D. AMADON. 1989. Eagles, hawks, and fal-
cons of the world. Wellfleet Press, Seacaucus, NJ
U.S.A.
DICKEY, D.R. AND AJ. vAN ROSSEM. 1938. The birds of E1
Salvador. Zool. Ser. 23, Field Mus. Nat. Hist., Chicago,
IL U.S.A.
DUNNING, J.B., J}•. 1993. CRC handbook of avian body
masses. CRC Press Inc., Boca Raton, FL U.S.A.
EMMONS, L.H. AND F. FEER. 1997. Neotropical rainforest
mammals. Univ. Chicago Press, Chicago, IL U.S.A.
FreEDMANN, H. 1948. Birds collected by the National Geo-
graphic Society's Expedition to northern Brazil and
southern Venezuela. Proc. U.S. Natl. Mus. 97:373-569
HAVERSCHMIDT, F. 1968. Birds of Surinam. Oliver & Boyd,
London, U.K.
DEE HOYO, J., A. ELIOT, AND J. SAP, AG^TAL. 1994. Hand-
book of the Birds of the World. Vol. 2. New World
202 THORSTROM VOL. 34, No. 3
vultures to guineafowl. Lynx Edicions, Barcelona,
Spain.
IZAWA, K. 1978. A field study of the ecology and behavior
of the black-mantle tamarin (Sanguinus nigricollis). Fo-
lia Primates 19:241-274.
K•NWAm), R.E. 1982. Goshawk hunting behaviour and
range size as a function of food and habitat availabil-
ity. J. Anim. Ecol. 51:69-80.
L•SLEY, G.W., C. SEXTON, )•ND G.D. LUCKNER. 1994. Win-
ter season, December 1, 1993-February 28, 1994, Tex-
as Region. Natl. Audubon Soc. Field Notes 48:224-228.
LEVINS, R. 1968. Evolution in changing environments.
Princeton Univ. Press, Princeton, NJ U.S.A.
LINTON, L.R., R.W. DAVIES, AND FJ. WRONA. 1981. Re-
source utilization indices: an assessment. J. Anim. Ecol.
50:283-292.
M•ER, W. 1981. Notes of nesting raptors in the llanos
of Venezuela. Condor 83:48-51.
M•TI, C.D. 1987. Raptor food habits studies. Pages 67-
79 in B.G. Pendleton, B.A. Millsap, K.W. Kline, and
D.A. Bird [EDS.], Raptor management techniques
manual. Natl. Wildl. Fed. Sci. Tech. Ser. No. 10. Na-
tional Wildlife Federation, Washington, DC U.S.A.
M•s, N.M. 1985. Ants and foraging behavior of the Col-
lared Forest-Falcon. Wilson Bull. 97:231-232.
NEWTON, I. 1986. The Sparrowhawk. T. & A.D. Poyser,
Calton, U.K.
R•POLE, J.H, M.A. P,•d•os, •qI) K. WINKER. 1989. Win-
tering wood thrush movements and mortality in
southern Veracruz. Auk 106:401-410.
ROBINSON, S.K. 1994. Habitat selection and foraging ecol-
ogy of raptors in Amazonian Peru. Biotropica 26:443-
458.
SCHOENE•, T.W. 1970. Nonsynchronous spatial overlap of
lizards in patchy habitats. Ecology 51:408-418.
ß 1984. Size differences among sympatric bird-eat-
ing hawks: a worldwide survey. Pages 254-281 in D.R.
Strong, D. Simberloff, L.G. Abele, and A.B. Thistle
[EDS.], Ecological communities conceptual issues and
the evidence. Princeton Univ. Press, Princeton, NJ
U.S.A.
SCHULZE, M. AND D.F. WHITACRE. 1999. A classification
and ordination of the tree community of Tikal Na-
tional Park, Petfin, Guatemala. Bull. Fla. Mus. Nat.
Hist. 41:169-297.
SNYDER, N.ER. ^ND J.W. WILEY. 1976. Sexual size dimor-
phism in hawks and owls of North America. Ornithol
Monogr. 20.
SMITH, N.G. 1969. Provoked release of mobbing--a hunt-
ing technique of Micrastur falcons. Ib/s 111:241-243.
SMITHE, F.B. 1966. The birds of Tikal. The Natural His-
tory Press, Garden City, NY U.S.A.
TEMELES, EJ. 1985. Sexual size dimorphism of bird-eating
hawks and the effect of prey vulnerability. Am. Nat.
125:485-499.
THORSTROM, R.K. 1993. Breeding ecology of two species
of forest-falcons (Micrastur) in northeastern Guate-
mala. M.S. thesis, Boise State Univ., Boise, ID U.S.A.
--, C.W. TURLEY, F.G. RAMIREZ, AND B.A. GILROY.
1990. Description of nest, eggs, and young of the
Barred Forest-falcon (Micrastur ruficollis) and of the
Collared Forest-falcon ( Micrastur semitorquatus). Con-
dor 90:237-239.
TRAIL, P.W. 1987. Predation and antipredator behavior at
Guiana. Auh 104:495-507.
WILEY, J.W. AND B.N. WILEY. 1981. Breeding season ecol-
ogy and behavior of Ridgway's Hawk (Buteo ridgwayi).
Condor 83:132-151.
WILLIS, E.O., D. WECHSLE•, ANI) F.G. STILES. 1983. Forest-
falcons, hawks, and pygmy-owl as ant followers. Rev.
Bras. Biol. 43:23-28.
ZARET, T.M. AND A.S. RAND. 1971. Competition in tropical
stream fishes: support for the competitive exclusion
principle. Ecology 52:336-342.
Received 19 July 1999; accepted 19 March 2000
