Information on nest location, nest architecture, and
construction materials is difficult to generalize. Indi-
vidual species demonstrate a wide range of adapta-
tions to protect their reproductive investments from
the weather and predation. The Brown Jay, which
nests in isolated trees to reduce predation by arboreal
mammals, must nest in protected microsites to pre-
vent the wind from blowing its nest out of the tree
(Lawton and Lawton 1980). Nests in isolated trees
produce four times as many fledglings as nests in wind-
breaks or forest (Williams et al. 1994), but nests in
trees with high wind exposure are often blown down
in wind storms (Lawton and Lawton 1980).
Many species of birds take advantage of readily
available materials for nest construction. Moss grows
densely on the bark of most trees in the MCFP (Zones
3-5); birds nesting in these life zones frequently use
epiphytic moss in their nests (Table 6.3). Many hum-
mingbirds also use scales from tree ferns (Cyathea sp.)
in their nests (Stiles and Skutch 1989). Some species
use bulky masses of moss to insulate their nests; oth-
ers weave moss into the exterior of the nest for cam-
ouflage. That nests of several common species from
these life zones remain undescribed (Stiles and Skutch
1989) suggests that their nests are well camouflaged.
On the lower Pacific slope, moss grows much less
abundantly on trees and is correspondingly rarer in
bird nests. There, twigs, plant fibers, dry leaves, li-
chen, and fungal rhizomorphs are more typical con-
stituents of forest-bird nests. Pasture and pasture-edge
birds use grass in their nests (Table 6.3).
Some bird species build their nests in cavities of
trees (Table 6.3). In the MCFP (Zones 3 and 4), 23% of
142 resident birds nest in cavities (Gibbs et al. 1993).
Of these, nine species excavate their own cavities and
24 use previously excavated or natural cavities. Monte-
verde is typical of other tropical sites in having a greater
frequency of cavity nesters (1.5-2.0 times as many
species) and a higher percentage of its species depen-
dent on existing cavities than in temperate forest sites.
The density of standing dead trees (snags), however,
is only one-third to one-seventh as great in Monteverde
as in temperate sites. This paradox of more cavity-
nesting species in habitats with fewer snags may be ex-
plained by the lower breeding densities of tropical
cavity-nesters, extended breeding periods that permit
sequential use of cavities, existence of decay cavities
in live trees, and non-tree cavity sources such as ter-
mitaria and wasp nests (Gibbs et al. 1993).
63.3. Clutch Size
The clutch sizes of Monteverde's birds are small com-
pared to those of temperate birds, as is typical in the
humid tropics. The vast majority of forest interior spe-cies lay two-egg clutches; pasture and pasture-edge
species tend to lay larger clutches (Table 6.4). This
pattern of higher clutch sizes in birds of open areas than
in related forest birds may be related to the greater sea-
sonality or abundance of food availability in open
areas (Lack and Moreau 1965; see Winnett-Murray,
"Choosiness and Productivity in Wrens," pp. 208-210).
The question of why tropical birds lay smaller
clutches than their temperate relatives was ad-
dressed in an experimental study of House Wrens
(Young 1996). Tropical House Wrens lay about half
as many eggs per clutch as either south- or north-
temperate conspecifics (Young 1994b). In Monte-
verde, House Wrens were able to raise experimentally
enlarged broods in two out of three years. Enlarged
broods were preyed on at the same rate as smaller
broods, contrary to the hypothesis that tropical birds
lay smaller clutches to reduce predation risk (Skutch
1949). However, the enlarged broods produced more
variance in the number of independent young reared
than control broods. Given sufficient environmental
variability, a lower-variance strategy (such as that
observed in most tropical birds) can yield higher long-
term fitness; fewer young are produced during each
successful reproductive bout, but the chances of suc-
cessful reproduction are increased (Gillespie 1977,
Boyce and Perrins 1987). Long-term data on yearly
variation in conditions for reproduction are needed
to support this "bad-years effect" model as the best
explanation for the small clutches of tropical birds
(Young 1996).
Alternatively, parent birds in the tropics may have
small clutches to devote more time to their offspring.
Juvenile tropical birds may require more parenting than
young temperate birds to learn to distinguish among
the greater variety of toxic and nontoxic prey in the
tropics (see Chap. 4, Insects and Spiders) and to win
social contests for breeding opportunities, which are
more competitive due to higher survivorship in the
tropics (Fretwell 1969, Garnett 1981, Young 1996).
Tropical House Wrens have longer incubation, nest-
ling, and postfledging periods than temperate House
Wrens, indicating that tropical House Wrens invest
more time in reproduction (Young 1996). Further ex-
perimentation on whether offspring of small broods are
better foragers and socially dominant to offspring of
large broods will clarify whether offspring quality is a
factor leading to the small clutch sizes of tropical birds.
One anomalous case is that of the Yellow-throated
Euphonia, which lays unusually large clutches in
nests built in crevices in earthen banks (modal clutch
size of this species in Monteverde is five eggs; Sargent
1993), larger than any other neotropical passerine
(Skutch 1985). Larger clutches generally incur higher
rates of nest predation because the nesting attempt191 Birds