The consequences of plant interactions with birds
have been investigated repeatedly. For example, fruit
phenology, size, shape, color, and abundance on a
plant may be influenced by bird foraging patterns
(Wheelwright and Orians 1982, Wheelwright and
Janson 1985, Bronstein and Hoffman 1987, Murray
1987, Wheelwright 1988, Valburg 1992c, Murray et al.
1994). Flowering biology may also be influenced by
foraging in nectarivores (Feinsinger 1978, Feinsinger
et al. 1986, 1987, 1988, 1991a, Feinsinger and Busby
1987, Feinsinger and Tiebout 1991, Podolsky 1992,
Stiles and Freeman 1993). Plant flowering and fruit-
ing phenologies may be adapted to avoid competition
with other bird-pollinated or bird-dispersed plants
(see Wheelwright, "A Hypothesis," pp. 281-282). In
both pollination and seed-dispersal systems, there is
evidence for competition, but not for evolved shifts in
phenology to reduce this competition (Feinsinger 1978,
Wheelwright 1985, Feinsinger et al. 1986, 1987,
199lb, Busby 1987, Feinsinger and Busby 1987).
Birds are also influenced by how and when plants
offer their reproductive rewards. Links between the
migrations of hummingbirds and frugivores in and out
of Monteverde and seasonal nectar or fruit abundance
is one example (Feinsinger 1976, 1980, Wheelwright
1983, Powell and Bjork 1995). Another case is the
annual variation in fruit crops of some trees; wide
year-to-year variation exists in fruit production of
trees in the family Lauraceae, with 10-90% of indi-
viduals in a species producing fruit in a given year
(Wheelwright 1986b). The amount of fruit production
on a tree also varies considerably, resulting in annual
fruit resource levels that can vary by over an order of
magnitude. Birds dependent on these fruits may re-
spond by migrating, expanding their diets, or delay-
ing reproduction (Wheelwright 1986b). Plants also
vary in their annual production of flowers (P. Fein-
singer, unpubl. data), and nectarivorous birds may be
affected in the same way that frugivores are affected
by the variation in fruit production.
at the four sites to examine the structure of commu-
nities made up of different species.
We used bird lists from Monteverde (Fogden 1993);
Darien, Panama (Robbins et al. 1985); Huila Depart-
ment, Colombia (Ridgely and Gaulin 1980, Martin
1984); and the Cordillera Vilcabamba, Peru (Terborgh
1977), dividing the avifauna of each site into nec-
tarivores, frugivores, and insectivores (Hilty and
Brown 1986, Stiles and Skutch 1989). We ignored
carnivores because individuals are scarce (Terborgh
1977). We categorized species based on their primary
food, and split true omnivores evenly between the
frugivore and insectivore groups. We assumed that
where species lists were incomplete, overlooked spe-
cies belong to the three trophic guilds in the same
frequencies as the species that were detected.
The results show a remarkable similarity in the
relative abundance of species in the three trophic
groups across the four sites (Fig. 6.9). Insectivores
account for 53-63% of the faunas; frugivores make up
27-32% of the totals. The importance of nectarivores
varies by almost 40% in the four sites, but overall
they make up a small proportion of the total species.
Monteverde has intermediate numbers of all three
trophic groups relative to other sites, so the bird com-
munity at Monteverde appears to be typical of cloud
forest sites in the neotropics.
Compared to lowland forests, higher elevation for-
ests in the humid tropics tend to support relatively
fewer insectivore species and more frugivore species
(Terborgh 1977). In Costa Rica, insectivores make up
increasingly smaller fractions of the bird communi-
ties at higher elevation sites, for example, along a
transect including La Selva (35 m), Monteverde (1500
m), and Cerro de la Muerte (2900 m; Stiles 1983).
Lowland sites are especially rich in species of the
exclusively insectivorous families Dendrocolaptidae
(Woodcreepers) and Thamophilidae (antbirds) and6.7. Comparative Ecology
How typical is Monteverde's avifauna compared to
the avifauna of cloud forests elsewhere in the trop-
ics? We compared the fauna in Zone 3 (lower mon-
tane wet forest) of Monteverde to that of three other
sites in Central and South America that occur in the
same life zone. Because most species' ranges are
smaller than the scale of this comparison, comparing
species identity is inappropriate. A comparison of
species richness is also biased because the amount of
effort needed to identify birds varies among the sites
(Remsen 1994). Instead, we compared trophic guilds
Figure 6.9. Relative abundance of nectarivores,
frugivores, and insectivores in lower montane wet forest
(Zone 3) at four sites in Central and South America.201 Birds