and dogs also take their toll on adult and nestling
birds. Hunting of birds in most of Monteverde has
stopped, but Black and Crested Guans were once
sought after for food, and other birds such as Macaws
and Cattle Egrets were shot out of curiosity or for
sport (M. Leiton, pers. comm.; see Chap. 7, Mammals).
Bird capture for the pet trade is not as rampant at
Monteverde as in other areas. For example, the Black-
faced Solitaire (Fig. 6.4) is common in Zone 4.
A singing male is worth $100 in San Jose, and the
species has consequently been driven close to extinc-
tion in many unprotected parts of the country. With
well-protected private reserves in Monteverde,
human-caused mortality of birds will remain mini-
mal (see Young, "How Have Humans Affected Bird
Populations?," pp. 433-434).
6.5.4. Among-Year Variation in Fertility
and Survivorship
Varying weather patterns cause changes in resource
levels, which affect the fertility and survivorship of
birds. The annual or stochastic variation that can re-
sult in these demographic variables should be incor-
porated into population models to portray popula-
tion dynamics accurately (Gillespie 1977, Boyce and
Perrins 1987, Caswell 1989, McDonald and Caswell
1993).
For House Wrens, female survivorship varied
among years from 0.42 to 0.57; male survivorship var-
ied from 0.40 to 0.66 (Young 1996). Fertility varied
little over the course of the three-year study. The abil-
ity of parents to raise artificially enlarged broods var-
ied from year to year, suggesting that in years more
extreme than the study interval, fertility could decline
(Young 1996). In Brown Jays, flocks inhabiting the
same territories varied fourfold in the number of fledg-
lings produced over a three-year period (Williams
etal. 1994).6.5.5. Population Dynamics
Population fluctuations may be the norm rather than
the exception (Pimm 1991). A dramatic example is the
Brown Jay, which first invaded Monteverde in the mid-
1950s and has continued to expand in range and in-
crease in population density to the present (Lawton and
Lawton 1985, Williams et al. 1994; see Williams and
Lawton, "Brown Jays," pp. 212-213). This range exten-
sion is not obviously tied to habitat alteration. The dis-
turbed habitats of Monteverde where Brown Jays oc-
cur (Zones 1-3) were deforested in the 1940s (10-15
years prior to the arrival of the jays), and the amount
of cleared land has probably decreased slightly since
then. Similarly, Great-tailed Crackles arrived in Monte-verde about 1980 and have since invaded most of the
farms of the Pacific slope, four decades after defores-
tation (B. Young and T. Guindon, pers. comm.; see
Chap. 12, Conservation Biology). Species such as the
Great Kiskadee and Eastern Meadowlark have also
expanded their ranges as habitats changed. Other range
expansions have been more subtle. Slate-throated Red-
starts slowly increased their range upward in Zone 4,
and Collared Redstarts retreated to the elfin forest along
the Continental Divide, although no obvious habitat
change occurred (Shopland 1985, B. Young, pers. obs.;
for examples of other apparently climate-driven range
changes, see Chap. 5, Amphibians and Reptiles).
These examples of population expansion and con-
traction result from changes in mortality and/or
reproduction. Hummingbird population density at
Monteverde also varies dramatically across years de-
pending on flower abundance, but the variation is due
to immigration and emigration rather than changes in
birth and death rates (Feinsinger 1980). Conversely,
population size may remain constant, but only be-
cause of constant immigration from other areas where
conditions are favorable for reproduction. Such
source-sink subsidy is also the case for House Wrens
in the upper Monteverde community (see Young,
"House Wrens," p. 448).
Changes in habitat via succession and land-clear-
ing also affect population dynamics. The squatters'
clearings in the Penas Blancas valley (Zones 5 and 6)
are the sites of the rise and fall of populations of sev-
eral species of open-country birds (e.g., Groove-billed
Ani, Common Pauraque, Tropical Kingbird, House
Wren). These species colonized and formed reproduc-
tive populations after deforestation in the 1960s and
1970s. After the Penas Blancas squatters were bought
out by the Monteverde Conservation League (MCL) in
the late 1980s (see Chap. 10, Conservation Institu-
tions), the clearings began to grow into secondary
forest; open-country birds disappeared except in a few
small clearings maintained for shelters (Fogden 1993).
The pattern of forest disturbance and regrowth in
Penas Blancas over the last 30 years probably mir-
rors the prehuman conditions in which these open-
country birds evolved. Large-scale disturbances such
as those caused by volcanoes, landslides, and river
course changes created isolated habitats for which
these species were adapted. All of the clearings in the
upper Penas Blancas valley were separated from more
extensive clearings on either the Pacific or Caribbean
slope by at least 5 km of forest. The observation of a
banded juvenile House Wren deep in Penas Blancas
(Zone 5), 12 km distant and 5 weeks after fledging
from a nest in Zone 3 in Monteverde (B. Young, pers.
obs.), demonstrates their ability to disperse across
substantial stretches of unsuitable habitat.198 Birds