pp. 283-286) suggested that seasonal patterns of seed-
ling survival in three species of pioneer plants might
constrain fruiting phenology in these species more
tightly than seasonal patterns of flowering, disperser
abundance, or gap formation. Examining temporal
constraints on seedling survival is a priority for fu-
ture work.
Altitudinal migration of fruit-eating birds: causes
and consequences Frugivore populations in Monte-
verde shift spatially. Striking examples are the pro-
nounced seasonal migrations of Resplendent Quetzals
and Three-wattled Bellbirds (see Chap. 6, Birds). Cen-
suses of quetzals and their major food plants in the
Monteverde area (1350-1550 m), revealed that the
birds' movements tracked the ripening seasons of dif-
ferent species of food plants (Wheelwright 1983). Fur-
ther work confirmed these patterns, but found con-
siderable year-to-year variation in the timing and
pattern of quetzal migratory behavior (Powell and
Bjork 1995).
Another important Monteverde frugivore that
migrates altitudinally is the Black-faced Solitaire (Fig.
6.4, Murray 1986a; see Murray, "Fruiting Phenologies,"
pp. 283-286). Solitaires leave the upper elevations of
Monteverde and return at about the same time as
quetzals (Murray 1986a). The details of their movements
after they leave Monteverde are unclear; they have been
sighted in the Penas Blancas valley (L. Moreno, pers.
comm.). Many solitaires and other species of small fru-
givorous birds leave the lower montane forest (1000 m)
after the breeding season (September) and increase in
abundance in the foothills (500 m) (Loiselle and Blake
1991). Solitaires occur down to 50 m at La Selva dur-
ing the nonbreeding season. The timing of their altitu-
dinal migration does not coincide with the general
pattern of fruit availability at that elevation; solitaires
move out of lower montane forests despite sharp in-
creases in fruit availability.
Altitudinal migrations of seed dispersers probably
have important consequences for the plants that de-
pend on such animals. The predictable absence of
such important dispersers as quetzals, bellbirds, and
solitaires, even for a few months of the year, implies
strong selection pressure on fruiting phenologies, espe-
cially in plant species that have few other dispersers.
Reproductive phenology of fruit-eating bats Events
in the life histories of fruit-eating bats in Monteverde
seem tied to the phenology of their food plants. The
peak periods of lactation (the most energetically costly
part of the breeding cycle) of two species of fruit-
eating bats (Sturnira ludovici and Artibeus toltecus,
Phyllostomidae) coincided with peaks in the abun-
dance of their food plants (see Chap. 7, Mammals),
which lends additional support to the idea that their
breeding seasons have evolved to coincide with pe-
riods of maximum food availability (Dinerstein 1983,
1986).8.2.3. Consequences of Dispersal for
Reproductive Success in Monteverde Plants
Comparing the "quality" of dispersal by different ani-
mals necessitates knowing where the seeds go (the
"seed shadows") and their fate. The three advantages
of seed dispersal identified by Howe and Smallwood
(1982)—the "escape," "colonization," and "directed
dispersal" hypotheses)—provide a framework for
studies of seed movement and its consequences.Escape from density-dependent mortality: Lauraceae and
other large-seeded primary forest species. As in other
neotropical forests, many of the dominant trees of
primary forests in Monteverde produce large, single-
seeded fruits adapted for consumption by large fruit-
eating birds. Because the seeds of such species typi-
cally germinate within six weeks of deposition,
regardless of the type of habitat in which they are
deposited, escape from intense density-dependent
predation near the parent is presumed to be the pri-
mary advantage of dispersal. Predation rates on large
seeds are very high (Janzen 1971, Wheelwright 1988a),
and the few detailed studies of distance and density
effects demonstrate the positive consequences of dis-
persal (Clark and Clark 1984, Howe et al. 1985).
In Monteverde, large-seeded fruits adapted for dis-
persal by birds occur in many families (e.g., Myrtaceae,
Sapotaceae, and Simaroubaceae), but the Lauraceae is
conspicuous for its large number of species, its numeri-
cal importance among canopy and subcanopy trees, its
lipid- and protein-rich fruit pulp, and its consequent
importance to large fruit-eating birds. Although many
species of Lauraceae produce fruit crops in the tens
of thousands, fruit-eating birds such as Resplendent
Quetzals, Emerald Toucanets, Three-wattled Bellbirds,
and Mountain Robins do not merely remain sedentary
in tree crowns, dropping seeds directly beneath them.
Instead, birds typically leave trees after 3-5 min
(Wheelwright 1991), separating the pulp from the seed
in the gut and regurgitating the seed elsewhere. Be-
cause this internal processing takes an average of 30-
40 min, the majority of lauraceous seeds leave the
immediate vicinity of the parent tree. However, regur-
gitated seeds are commonly found beneath these same
trees; they may be brought by birds from either con-
specific trees nearby or from the same tree visited ear-
lier. In either case, at least some seeds dispersed by
birds are deposited in locations just as unfavorable as
the area directly beneath their parent.
Seed dispersal by some of these same birds may
vary markedly from season to season, sometimes re-263 Plant-Animal Interactions