unpubl. data; see Powell et al., "Altitudinal Migra-
tions and Habitat Linkages, pp. 439-442).Long-distance migration. Of Monteverde's bird fauna,
91 species (21%) are long-distance migrants (Tramer
1979, Stiles and Smith 1980, Tramer and Kemp 1982,
Fogden 1993). The majority breed in North America
and pass through Monteverde during migration or
overwinter in Monteverde. Four species (Swallow-
tailed Kite, Sulphur-bellied Flycatcher, Piratic Fly-
catcher, and Yellow-green Vireo) breed in Monteverde
and migrate to South America during the nonbreeding
season. In contrast to elevational migrants, long-dis-
tance migrants decrease in abundance with increas-
ing elevation, occurring most commonly in Zones 1-
2 and 5-6 (Tramer and Kemp 1982, Fogden 1993,
Young et al. 1998). This same pattern occurs through-
out Costa Rica (Stiles 1980b). We know little about
the ecology of North American migrants in Monte-
verde other than that they occur in old-growth and
secondary forest, some species join mixed-species
flocks of resident birds, and some species defend non-
breeding territories (Tramer and Kemp 1980, 1982).
Competition between migrant and resident birds may
not be very strong because individuals of the two
groups rarely fight or interact agonistically (Tramer
and Kemp 1980). Key questions such as overwinter
survivorship of long-distance migrants in different
habitats, population trends, effects of anthropogenic
changes on habitat availability, and resource use by
different species of migrants need study in Monte-
verde (Martin and Finch 1995, Rappole 1995).
Elevational migration. Resplendent Quetzals have long
been known to move seasonally from high-elevation
nesting sites to lower elevations on both slopes (Wheel-
wright 1983). In the early 1990s, radio-tracking data
provided greater precision concerning the timing and
extent of these movements (Powell and Bjork 1995).
The broad pattern is a four-stage annual movement.
Courtship and nesting occur in cloud forest habitats
(1500-1800 m) on both slopes between January and
June. After nesting, adults and their offspring move
to lower elevations (1100-1300 m) on the Pacific
slope. Four months later, the birds move back for a
few weeks to the higher elevation sites used for nest-
ing. They then move across the Continental Divide to
the Caribbean slope (700-1100 m), until the nesting
season approaches. The pronounced variability of
movement patterns among years is hypothesized to
be due to variation in the availability of their fruit
resources, particularly trees in the family Lauraceae
(Wheelwright 1983, Powell and Bjork 1995).
An even more complex pattern of movement
emerged from radio-tracking studies of Three-wattledBellbirds (Fig. 6.2). Bellbird migration begins similar
to quetzals with breeding near the Continental Divide
from March to July and a postbreeding movement
down the Pacific slope to Zone 2 during August and
September. They fly to the lowland rainforests of
southeastern Nicaragua where they spend October
through December. In January, bellbirds travel to the
Pacific coast of Costa Rica where they scatter from
Nicoya to the Osa Peninsula before returning to Monte-
verde in March (G. Powell, pers. comm.). Whether bell-
birds flew all the way to Nicaragua before most of the
northern lowlands of Costa Rica were cleared is
unknown.
Smaller birds also make elevational movements,
but they are harder to track with radios. For example,
Black-faced Solitaires (Fig. 6.4) nearly disappear from
Zone 4 during September-December when they mi-
grate down the Caribbean slope (Murray 1986). Juve-
nile (but not adult) Black-faced Solitaires regularly
appear in Zone 2 forests between July and Septem-
ber, suggesting age-related variation in migration (D.
McDonald, pers. obs.). Although the migratory pat-
terns of these smaller passerines have not been stud-
ied in detail, 68 of Monteverde's bird species are
elevational migrants in the Cordillera Central and
elsewhere in Costa Rica, and they probably undergo
similar seasonal movements in Monteverde (Appen-
dix 8). In general, frugivorous migrants nest at high
elevations and descend on the Caribbean slope dur-
ing the nonbreeding season. Hummingbirds migrate
up to breed during the wet season and down in the
dry season, although exceptions occur (Stiles 1980a,
1985c, Loiselle and Blake 1991).6.2. BehaviorThe first ornithologists in Monteverde studied the
behavior of birds in mixed-species flocks, and several
studies have followed up on this theme (Buskirk 1976,
Powell 1979, Shopland 1985, Valburg 1992c). Re-
searchers have examined the foraging behavior of
individual species in the context of ecological stud-
ies of species coexistence, seed dispersal, sexual di-
morphism (Winnett-Murray 1986, Riley and Smith
1992, Sargent 1994), and social behavior (Snow 1977,
Riley 1986, McDonald and Potts 1994, Williams et al.
1994).6.2.1. Mixed-Species Foraging Flocks
Many species in Monteverde forage while accompa-
nying a mixed-species flock of birds. These flocks
("flocks" in this section refers to mixed-species assem-
blages) are one of the most noticeable features of bird-184 Birds