improves. The winds calm earlier in some years than
others, and House Wrens advance the onset of breed-
ing by as much as one month in response to the fair
weather (Young 1994a).
Harsh weather can also affect bird foraging behav-
ior. Stripe-tailed Hummingbirds, which inhabit dis-
turbed habitats, forage at patches of flowering Hamelia
patens, a plant that produces copious nectar during
the wet season (Feinsinger 1976). During sunny
weather, many butterflies also visit Hamelia plants
and take enough nectar to deplete the resource to the
extent that the plants do not merit defense by hum-
mingbirds. On rainy days, however, the butterflies
are inactive and active defense becomes worthwhile
(Thomas et al. 1986).6.4.3. Timing of Molt
Molt is an energetically costly event in the annual
cycle of birds (Walsberg 1983). Whether birds time
annual molting periods to coincide with periods of
high food abundance or with reproduction is not well
studied. House Wrens molt during September and
October when their arthropod prey is at its annual
peak (Young 1994b). Similarly, frugivores molt dur-
ing periods of increasing fruit abundance on the Cor-
dillera Central (Loiselle and Blake 1991). Most birds
(other than hummingbirds) in Monteverde molt be-
tween July and November (Fig. 6.8), when arthropods
and nectar are moderately to very abundant (Buskirk
and Buskirk 1976, Feinsinger 1976, 1978, Feinsinger
et al. 1986, Young 1994b), but fruit abundance is low
(W. Haber, unpubl. data). All the hummingbirds that
have been studied (Green Hermit, Violet Sabrewing,
Green Violetear, Purple-throated Mountain-gem,Figure 6.8. Monthly change in the percentage of birds
(excluding hummingbirds) molting either flight or body
feathers in Monteverde (B. Young, unpubl. data).
Stripe-tailed Hummingbird, Coppery-headed Hum-
mingbird, Green-crowned Brilliant, and Magenta-
throated Woodstar) molt between December and April
when flower abundance is high (F. G. Stiles, unpubl.
data). Although many tropical species exhibit consid-
erable overlap between molt and breeding at the popu-
lation level (Foster 1974), little is known about molt-
breeding overlap at the individual level.6.5. Population BiologyAlthough the autecological approach taken by many
Monteverde ornithologists makes communitywide
comparisons difficult, the focus on individual species
has provided good data on the dynamics and demog-
raphy of certain populations of birds. The focus on
three species (Long-tailed Manakins, Brown Jays, and
House Wrens) has provided most of the data on the
population biology of Monteverde birds.6.5.1. DensityWe do not know the overall density of birds in Monte-
verde, which prevents comparison with other tropi-
cal sites such as Manii, Peru, where the densities of
many species are known (Terborgh et al. 1990). How-
ever, density estimates of some species that occur in
Monteverde exist. An estimated 50 pairs of quetzals
bred in or adjacent to the MCFP in 1980 (Wheelwright
1983). The MCFP had an area of 2700 ha at that time,
yielding one breeding pair per 54 ha. In 1978, the
home range of Brown Jay flocks averaged 20.9 ha,
with flocks averaging 10.2 birds/flock (calculated
from Williams et al. 1994). Flock size increased to
13.3 birds/flock by 1987, and by 1990, the home range
of a flock was 10.8 ha. House Wrens in the upper com-
munity (Zone 3) have territories that average 0.77 ha
(B. Young, unpubl. data).
Density is difficult to define for species that do not
maintain year-round territories. For example, Three-
wattled Bellbirds may occur at different densities
during the segments of their annual four-part migra-
tions. When they are in Monteverde during the breed-
ing season, males can form aggregations in which each
is in acoustical contact with the others (Snow 1977).
A 1974 study reported that 2.9 territorial males oc-
curred per 100 ha in Monteverde (Snow 1977); how-
ever, calling male bellbirds did not occur for many
kilometers in any direction outside of the study area,
even though the habitat there appeared to be similar
to the habitat within the study area. During the non-
breeding season, the same male bellbirds may be dis-
persed in forest patches separated by distances of over
100 km (G. Powell, unpubl. data). The density esti-195 Birds