tain an average of 4.4 species per flock (range 2-20)
and an average of 6.4 individuals per flock (range 2-
26; Shopland 1985). Flocks in Monteverde generally
have more furnariids (ovenbirds) and fewer antbirds
and thamnophilids (antshrikes) than lowland flocks,
in which antbirds and thamnophilids are often a major
component (Buskirk 1976, Powell 1979, Munn 1985,
Shopland 1985). Lowland forests also tend to have a
greater variety of flock types than higher elevation
forests (Munn and Terborgh 1979, Levey and Stiles
1994).
Common Bush-Tanagers are major components of
flocks in Zones 3-5 (Valburg 1992c). In the presence
of conspecifics, they eat primarily fruit; in the pres-
ence of flocks, they tend to switch to arthropod prey.
Flocks may either facilitate access to arthropods that
are normally unavailable, or provide assistance in
predator vigilance that is unnecessary when feeding
on fruit (Valburg 1992c; see Valburg, "Why Join Mixed-
Species Flocks?," pp. 205-206). Some birds (Collared
and Slate-throated Redstarts) appear to join flocks pri-
marily as a means of territory defense or vigilance
against predators; any foraging is simply "making the
best of a bad lot" (Shopland 1985; see Shopland, "The
Cost of Social Forging," pp. 206-207).
North temperate migrants vary in their propensity
to join flocks (Powell 1980, Tramer and Kemp 1980).
Black-and-white Warblers and Golden-winged War-
blers often join flocks; Philadelphia Vireos and Ten-
nessee Warblers frequently forage with conspecifics
(Tramer and Kemp 1979,1980). Black-and-white and
Golden-winged Warblers use foraging methods that
prevent them from simultaneously watching for
predators (foraging on tree trunks and dead leaf
clumps, respectively; Tramer and Kemp 1980), which
may make them more dependent on the predator vigi-
lance of flocks than other north temperate migrants
with generalized feeding methods.
For many species, the advantage of joining insec-
tivorous flocks appears to be avoidance of predation
(Buskirk 1976,1981, Powell 1985, Munn 1986, Hutto
1994). Flock-joiners tend to be species that are vul-
nerable to predation, use resources that are difficult
to defend, and occur in pairs or small family groups.
Species that do not join flocks are less vulnerable to
predation (e.g., sentinel foragers that are adept at de-
tecting predators themselves, hummingbirds that feed
on the wing and are constantly vigilant when perched,
and ground foragers that are protected from direct
approaches by winged predators because of low foli-
age), form single-species flocks, or feed on clumped
resources (Buskirk 1976). Observations that species
attending flocks do not show positive associations,
that flocks move relatively constantly through the
forest without slowing at clumps of prey, and that
members forage on a wide variety of prey, support the
suggestion that most species join flocks for early de-
tection of predators (Powell 1985, Hutto 1994; see
Shopland, "The Cost of Social Foraging," pp. 206-
207). Predation avoidance is not the exclusive rea-
son for joining flocks, as the Monteverde redstart
study (Shopland 1985) and others in the neotropics
have shown (Munn 1986). Some species probably join
flocks to enhance foraging efficiency (Krebs 1973,
Valburg 1992a; see Valburg, "Why Join Mixed-Spe-
cies Flocks?," pp. 205-206). Further studies will pro-
vide details of how various members of a flock ben-
efit in different ways (Latta and Wunderle 1996).6.2.2. Foraging Behavior
Birds use similar resources in different ways. Quetzals,
toucanets, and bellbirds feed on the fruits of laura-
ceous trees. Toucanets take fruits while perched,
quetzals pluck fruits during sallies, and bellbirds use
both techniques. Toucanets and bellbirds tend to for-
age high in the trees, whereas quetzals feed primarily
in lower parts of the crown (Santana and Milligan
1984). Carnivorous birds also vary in their prey cap-
ture techniques. When foraging at army ant swarms,
for example, Collared Forest-Falcons perch low to
capture insects scared up by the ants (Mays 1985).
Collared Forest-Falcons also chase lizards by running
along the ground with partially opened wings (Mays
1985). Sunbitterns feed frogs, fish, lizards, and insects
to their nestlings (Lyon and Fogden 1989). The par-
ents often wash food items before presenting them to
nestlings or after nestlings refuse a food item (Lyon
and Fogden 1989). House Wrens and Gray-breasted
Wood-Wrens (Fig. 6.5) forage faster, attack prey at a
higher rate, change their foraging position faster, and
use a greater variety of foraging sites (ground level
through forest understory, woody tangles, and trees)
than the larger Plain Wrens and Rufous-and-white
Wrens, which forage in more specialized microhabi-
tats such as aerial leaf clusters (Winnett-Murray 1986;
see Winnett-Murray, "Choosiness and Productivity
in Wrens," pp. 208-210).
Plants in Monteverde offer a wide variety of fruits
as rewards for seed dispersal (Stiles 1985b), and fruits
are important in the diets of a wide range of birds.
Quetzals feed on the fruits of over 40 species of trees
(Wheelwright 1983). Emerald Toucanets feed on the
fruits of at least 109 species of plants and the flowers
ofErythrina lanceolata, Saurauia sp., and Macleania
sp. (Wheelwright et al. 1984, Riley 1986, Riley and
Smith 1986; Fig. 6.6). Although they eat mostly fruit
as adults, both quetzals and toucanets feed their nest-
lings a diet of primarily arthropods (Wheelwright
1983, Riley 1986). Yellow-throated Euphonias and186 Birds