Figure 6.3. Species accumulation
curves of birds caught in mist nets
for two life zones at Monteverde
(B. Young and D. McDonald,
unpubl. data).cation of the level of species richness at which the
curves will flatten out. Species accumulation in Zone
6 is similar to that of Mami, Peru, the most diverse of
four well-studied lowland neotropical sites (Karr et al.
1990a).
Most research at Monteverde has occurred in the
Pacific slope (Zones 2—4) where species richness is
intermediate (121—202 species per zone). These zones
receive considerably less precipitation than the Ca-
ribbean slope zones (2000-2500 mm vs. 5000-6000
mm, respectively) and dry forests generally support
fewer bird species than wet forest (Lack and Moreau
1965, Stiles 1983). Also, elevational migration is
much less pronounced on the Pacific slope than on
the Caribbean slope. Breeding species tend to remain
resident year-round on their territories; middle-
elevation sites receive fewer elevational migrants
than similar sites on the Caribbean slope. Ecologi-
cal differences between dry forests and the moist
highland forests are greater than between wet Ca-
ribbean slope forests and the highland forests, which
may limit migration. On the lower Pacific slope, sea-
sonal migration is more common locally between
adjacent dry and gallery forests, rather than along
elevational gradients. Deforestation is most advanced
on the Pacific slope, especially at lower elevations,
and some species (e.g., the Scarlet Macaw, Am macao)
have gone locally extinct.
Most species that occur in Monteverde are re-
stricted to a subset of the six principal life zones.
Thirty-four species occur in just a single life zone. The
effect of these restricted ranges is that as one travels
along a transect up one slope across the Continental
Divide and down the other slope, a series of distinct
communities of birds is encountered, so the number
of species increases throughout the journey. Thus,
Monteverde has both high alpha species diversity
(many species in one habitat type) and high beta spe-
cies diversity (a high turnover in species across adja-
cent habitats).
The pattern of species occurring only in specific
life zones is remarkable because habitat specialization
occurs on a very small spatial scale. The life zones
occur in narrow bands paralleling the ridge top of
the Cordillera de Tilaran (Fig. 1.5). One way to
appreciate this compression of habitat diversity is
to consider the spatial scale of a standard 24-km-
diameter Audubon Christmas Count circle. These
circles are used by volunteer birdwatchers through-
out North and Central America to sample winter bird
densities. Count circles typically sample one or at
most two life zones; by contrast, the count circle for
the Monteverde Christmas Bird Count encompasses
all six of Monteverde's life zones. Thus, the Monte-
verde counters typically record 20-30 more species
than the most diverse of the other 1,700 Audubon
Christmas Counts conducted annually.6.1.2. Migration
Monteverde's birds consist of permanent residents,
long-distance migrants, and elevational migrants,
which is typical for locations on or near mountains
in the neotropics (Wetmore 1926, Ridgely 1976, Stiles
1985a, 1988, Hilty and Brown 1986, Loiselle and Blake
1991). Long-distance migrants either breed primarily
in the north temperate region during its summer and
migrate to the neotropical region during the north
temperate winter, or breed in Central America and
migrate to South America during the nonbreeding
season. Elevational migrants breed at one elevation
on mountain slopes and migrate to another elevation
during the nonbreeding season. Radio-tracking stud-
ies at Monteverde show these "elevational" migra-
tions entail complex patterns of seasonal movement
that can span international boundaries (G. Powell,183 Birds