Long-tailed Manakins, in contrast, eat primarily fruits
as adults and feed their nestlings a fruit-dominated
diet (Sargent 1993, D. McDonald, unpubl. data). Even
some carnivorous birds, such as vultures, kites,
antbirds, and wrens, sometimes eat fruit (Morton
1973, McDiarmid et al. 1977, Buskirk and Lechner
1978, Willis 1980, Keeler-Wolf 1986). Common Bush-
Tanagers choose some of the fruits they eat based on
the fly larvae infesting the fruits (Valburg 1992a,b,c;
Valburg, "Do Fruit-Eating Birds," p. 210). Other spe-
cies, such as Brown Jays, eat a wide variety of fruit
and invertebrate and vertebrate food (Lawton 1983).
The majority of bird species in Monteverde are pri-
marily insectivorous. Slate-throated and Collared Red-
starts eat a diet of dipterans, homopterans, coleopterans,
orthopterans, odonates, and lepidopterans. Foraging
mode and diet vary depending on whether the indi-
viduals are in the presence of a mixed-species forag-
ing flock (Shopland 1985; Fig. 6.7). House Wrens have
a diet similar to that of redstarts, except that they eat
arachnids and some hemipterans but ignore coleopter-
ans (Young 1994a; see Winnett-Murray, "Choosiness
and Productivity in Wrens," pp. 208-210).
Epiphytic plants are important resources for both
frugivores and insectivores in Monteverde. These
plants, which can contribute up to 63% of the plant
species richness and 40% of the aboveground foliar
biomass, provide habitat for arthropods, small verte-
brate prey, fruit, and nectar resources (Nadkarni and
Matelson 1989). In Zone 2, 59% of 56 bird species seen
in trees foraged in epiphytes (Nadkarni and Matelson
1989, Nadkarni 1994).
Digestive physiology can influence foraging behav-
ior. In Steely-vented Hummingbirds and Canivet's
Emeralds (formally "Fork-tailed Emeralds"), the crop
empties itself of nectar as fast as the nectar passes
through the gut. Gut passage rate may be a time-
limiting factor when hummingbirds consume large
amounts of nectar during periods of high food de-
mand. Under such conditions, hummingbirds may be
forced to perch often to process nectar in their crops.
During times of normal demand, however, food is
passed rapidly enough that birds need not suspend
foraging to process gut contents (Tiebout 1989).6.2.3. Social Behavior
Two long-term studies of social behavior have been
conducted in Monteverde. One is the study of Brown
Jays initiated by Marcy and Robert Lawton and con-
tinued by Dean Williams (see Williams and Lawton,
"Brown Jays," pp. 212-213). The second is the study
of Long-tailed Manakins by Dave McDonald (see
McDonald, "Cooperation between Male Long-Tailed
Manakins," pp. 204-205). Other studies have addressed
social behavior in Emerald Toucanets (Riley 1986; see
Riley and Smith, "Ecology and Sexual Dimorphism,"
p. 214) and Three-wattled Bellbirds (Snow 1977).Figure 6.7. Slate-throated Redstart preparing to hawk for insects. Photograph by Gregory Dimijian.188 Birds