appearing for varying periods throughout the year.
These two guilds are apparently stable even in habi-
tats that sustain disturbance. Community makeup was
similar in forest understory, tree-fall gaps, and experi-
mental cut-over plots that mimicked larger scale dis-
turbances such as landslides (Feinsinger et al. 1986).
Guilds of frugivorous birds in Monteverde have
been studied to a lesser degree. A compilation of all
feeding records of frugivorous birds in Monteverde
(Wheelwright et al. 1984) provides a springboard for
biologists who study guilds of birds that feed on spe-
cific groups of plants. Large-gaped birds can eat
larger fruits than small-gaped birds; however, large-
gaped birds can also eat fruits just as small as those
eaten by small-gaped birds, which results in less spe-
cialized diets for large-gaped birds (Wheelwright
1985). The behavior of individual members of a guild
of frugivorous birds may be similar. The four major
species that feed on the fruits of trees in the family
Lauraceae (Resplendent Quetzal, Emerald Toucanet,
Three-wattled Bellbird, and Mountain Robin) spend
similar amounts of time (2.9-4.7 min) in a tree
and eat about the same number (1—3) of fruits per
visit, although their foraging methods and locations
may differ (Santana and Milligan 1984, Wheelwright
1991). Members of large- and small-gaped frugiv-
ore guilds show greater overlap in both diet and
behavior than do guilds of long- and short-billed
hummingbirds.
Two guilds of small-gaped frugivores consist of spe-
cies with unusually narrow diets (Sargent 1994). One
guild consists of four tanager species (Golden-browed
Chlorophonia, Blue-hooded Euphonia, Yellow-throated
Euphonia, Yellow-crowned Euphonia) that feed on
five species of mistletoe (family Viscaceae). The
sticky seeds of these fruits force the birds to engage
in bill or vent-wiping behaviors that glue the seed
to the branch where the bird is perched (see Sargent,
"Specialized seed dispersal," p. 288). There the seed
germinates and infests a new host. Another guild of
10 species includes a diverse group of pigeons, fly-
catchers, and vireos that feed on six species of mistle-
toes (families Loranthaceae and Eremolepidaceae).
These families have single-seeded fruits whose pulp
separates more easily from the seed than does the
pulp of fruits in the Viscaceae. The diet of the Blue-
hooded Euphonia consists of 81% mistletoes in the
Viscaceae; 77% of the diet of the Paltry Tyrannulet
is made up of mistletoes in the Loranthaceae and
Eremolepidaceae. These diets are unusually restricted
compared to frugivores that typically eat dozens of
species of fruits (Wheelwright et al. 1984). Having
evolved the ability to handle mistletoe fruits more ef-
ficiently than other frugivores, Blue-hooded Euphonias
and Paltry Tyrannulets may not require very broad
diets (Sargent 1994).
Guilds can be defined based on the use of resources
other than food. One promising group for future study
is the guild of cavity-nesting birds (Table 6.3). Snag
density in Monteverde (Zone 4) is less than the snag
density of many temperate forests, but 2.5 times more
species nest in these cavities (Gibbs et al. 1993). Study
of how cavity-nesting birds divide up this resource
or compete for suitable nesting sites will help explain
the coexistence of species and provide information on
how to best manage for these species. Cavity-nesting
birds are especially amenable to experimental study
because of the ease of manipulating the resource with
artificial cavities.6.6.2. Interactions with Other Taxa
Bird-plant interactions have been the primary focus
of bird research at Monteverde. There is strong depen-
dence of Monteverde's plants on birds for seed dis-
persal (Table 6.7), but Monteverde's flora is much less
dependent on birds for pollination. About 9% of the
flora is bird-pollinated (Stiles 1981; see Chap. 8,
Plant-Animal Interactions). Nevertheless, 29 species
of hummingbirds, plus other nectar-feeders (Tennes-
see Warblers, Bananaquits, Baltimore Orioles, Slaty
Flower-Piercers, Scarlet-thighed Dacnis, and three
species of honeycreepers) feed on nectar during at
least part of the year.Table 6.7. Dependence of Monteverde's flora on birds for seed dispersal compared to other
habitats in Costa Rica (based on Stiles 1985b).Site (Forest Type)Percentage of Plant Species That
Rely on Birds for Seed Dispersal
Shrubs and Small Trees Canopy Trees
Monteverde (middle elevation cloud forest)
Palo Verde (lowland dry forest)
La Selva (lowland rain forest)
Cerro de la Muerte (highland wet forest)77
45
63
8263
35
50
72200 Birds