ferences in their diet: insects are cryptic and difficult
to find, whereas fruits are often conspicuous (Moer-
mond and Denslow 1985). For insectivores, rates of
ingestion do not greatly exceed rates of digestion, so
they must forage continuously (Munn and Terborgh
1979). In contrast, frugivores can ingest fruit much
more rapidly than they can digest it, which results in
episodic searching for food. Exploitation of different
resources may therefore favor differences in foraging
behavior. For an omnivore, balancing the time con-
straints of insectivory and the spatial constraints of
frugivory requires compromises in both behavior and
diet.
Observations on foraging and social behavior of
Common Bush-Tanagers suggested that individuals
may alter their feeding behavior in the presence of
different social groups. For example, the diet of South
American primates varies depending on whether in-
dividuals forage in single-species or mixed-species
groups (Terborgh 1983). When birds join mixed-species
flocks in Monteverde, their feeding behavior con-
verges with the nuclear species in that flock (Buskirk
1976, Powell 1977b).
I studied Common Bush-Tanagers in lower montane
wet forest (Zone 3 of Holdridge 1967) in Campbell's
Woods, adjacent to the MCFP. Common Bush-Tanagers
mainly forage in understory and second-growth patches,
although many home ranges include primary forest.
Bush-tanagers use the heavily fruiting shrubs of second-
growth areas and forage mainly within their own
territories. They commonly join mixed-species flocks,
which include Three-striped Warblers, Slate-throated
Redstarts, Spotted Barbtails, Gray-breasted Wood-
Wrens (Fig. 6.5), and other species. The only major
avian predators of adult birds are Barred and Collared
Forest-Falcons. I mist-netted and color-banded 33
Common Bush-Tanagers. Transient birds were not
marked but were tallied in single-species flock counts.
Each individual's foraging behavior was recorded at
1-min intervals during 45-min observation periods
from March 1987 through June 1988.1 recorded data
on social grouping (flock composition and flock size),
and prey type (arthropod or fruit; Valburg 1992c).
The balance of fruits and insect prey consumed by
Common Bush-Tanagers changed significantly be-
tween mixed-species and single-species groups, with
members of single-species groups averaging 66% fruit
compared to an average of 28% for individuals in
mixed-species groups. Although bush-tanagers only
foraged with the mixed-species flocks for 30% of their
foraging day, they consumed the bulk of their arthro-
pod diet at that time. As flock size increased, fruit
consumption decreased, especially for flock sizes of
5—15 birds. These small flocks were either mixed-
species flocks or flocks made up of relatively large
numbers of bush-tanagers. The decrease in fruit con-
sumption was truly an increase in insect consump-
tion in small mixed-species flocks only, independent
of flock size and composition.
If increased group size alone resulted in decreased
risk of predation, as proposed by Powell (1985), there
should not have been such a marked difference be-
tween single-species and mixed-species flock forag-
ing where flock size was similar. This study supports
the hypothesis that foraging behavior of mixed-spe-
cies flock participants tends to converge regarding
food type consumed (Buskirk 1976) and suggests that
birds may join mixed-species groups to facilitate the
finding and capturing of arthropod prey. Common
Bush-Tanagers had access to all insect and fruit prey
in both solitary foraging and single-species group for-
aging modes but selected more insect prey per unit
time when in a mixed-species group. This switch to
insectivory may enhance foraging technique or copy-
ing of insectivorous flock members. For Common
Bush-Tanagers participating in mixed species flocks,
these results suggest that enhanced foraging for insects
may be a primary benefit of mixed-species flocking.THE COST OF SOCIAL FORAGING-IN MiXED-SPECIES BIRD FLOCKS
Jennifer Shoplandince the time of Henry Bates and Alfred
Wallace, naturalists have commented on multi-
species flocks around the world, from waterfowl
in wetlands to small songbirds in tropical forests.
Feeding with a mixed flock may increase the foraging
success of its members or protect them from preda-
tion (Morse 1977; see Valburg, "Why Join Mixed-Species Flocks?," pp. 205-206). Preliminary observa-
tions in Monteverde suggested that the potential
predators (primarily hawks) of adult flock members
were uncommon. I tested the foraging-success hy-
pothesis for three flocking species.
In lower-elevation woodlands and in the MCFP,
mixed flocks form when "nuclear species" travel206 BirdsS